Hybrid Concrete Buildings

CC

IP-030D

esign of Hybrid C

oncrete Buildings

R. W

hittle MA (Cantab) CEng M

ICE H. Taylor FREng, BSc, PhD

, CEng, FICE, FIStructE

Design of Hybrid Concrete Buildings

This design guide is intended to provide the structural engineer with essential guidance for the design of structures that combine precast and in-situ concrete in a hybrid concrete structure. It introduces the options available for hybrid concrete structures, and goes on to explain the key considerations in the design of this type of structure.

Bearings, interface details, consideration of movement, composite action, robustness and the effects of prestressing are all explained in this guide and design examples are included where appropriate. The importance of overall responsibility and construction aspects are also described.

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0Price Group P

© The Concrete Centre

Riverside House, 4 Meadows Business Park,Station Approach, Blackwater, Camberley, Surrey, GU17 9ABTel: +44 (0)1276 606 800 www.concretecentre.com

CI/Sfb

UDC624.072.33:624.012.3/.4

Robin Whittle has extensive knowledge and experience of designing all types of concrete buildings. He regular contributes to concrete industry publications and is a consultant to Arup. He was a member of the project team which drafted Eurocode 2.

Howard Taylor has extensive knowledge and experience of designing precast concrete elements and buildings, including developing alternative production methods. He is a past president of the Institution of Structural Engineers and is currently chairman of the British Standards Institution Building and civil engineering structures Technical Committee B/525.

Design of Hybrid Concrete BuildingsA guide to the design of buildings combining in-situ and precast concrete

A cement and concrete industry publication

R. Whittle MA (Cantab) CEng MICE

H. Taylor FREng, BSc, PhD, CEng, FICE, FIStructE

Hybrid cov-.indd 1Hybrid cov-.indd 1 29/01/2009 16:43:3729/01/2009 16:43:37


Published by The Concrete CentreRiverside House, 4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB Tel: +44 (0)1276 606800 Fax: +44 (0)1276 606801 www.concretecentre.com

CCIP-030Published January 2009 ISBN 978-1-904482-55-0Price Group P© The Concrete Centre

Cement and Concrete Industry Publications (CCIP) are produced through an industry initiative to publish technical guidance in support of concrete design and construction.

CCIP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777

All advice or information from The Concrete Centre is only intended for use in the UK by those who will evaluate the signifi cance and limitations of its contents and take responsibility for its use and application. No liability(including that for negligence) for any loss resulting from such advice or information is accepted by The Concrete Centre or their subcontractors, suppliers or advisors. Readers should note that the publications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Cover photo: Courtesy of Outinord International Ltd.Printed by Information Press Ltd, Eynsham, UK

AcknowledgementsThe authors would particularly like to thank the following people for their support in the development of this design guide:

Tony Jones ArupIan Feltham Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged:

John Stehle Laing O’RourkeGraham Hardwick John Doyle Construction LtdPeter Kelly Bison Concrete Products LtdAlex Davie ConsultantDavid Appleton Hanson Concrete ProductsKevin Laney Strongforce Engineering PlcNorman Brown British Precast Concrete Federation Ltd

Type 1Precast twin wall and lattice girder slab with

in-situ concrete

Type 2Precast column and edge beam with in-situ

fl oor slab

Type 3Precast column and fl oor units with cast in-situ

beams

Type 4In-situ columns or walls and beams with precast

fl oor units

Type 5In-situ column and structural topping with precast

beams and fl oor units

Type 6In-situ columns with lattice girder slabs with

optional spherical void formers

Typical hybrid concrete options.Please note this diagram is a repeat of Figure 2.1, page 8.



Contents

1. Introduction 5 1.1 Single point of responsibility 5 1.2 Design considerations 6 1.3 Best practice procurement guidance 6

2. Overview of hybrid solutions 7 2.1 Type 1: Precast twin wall and lattice girder slab with in-situ concrete 7 2.2 Type 2: Precast column with in-situ fl oor slab 9 2.3 Type 3: Precast column and fl oor units with cast in-situ beams 10 2.4 Type 4: In-situ columns or walls and beams with precast fl oor units 12 2.5 Type 5: In-situ column and structural topping with precast beams and fl oor units 13 2.6 Type 6: In-situ columns with lattice girder slabs with optional spherical void formers 14

3. Overall structural design 15 3.1 Robustness 15 3.2 Stability 18 3.3 Diaphragm action 18 3.4 Shear at interface of concrete cast at different times 19 3.5 Interface shear 22 3.6 Shear and torsion design 25 3.7 Long-line prestressing system 26 3.8 Secondary effects of prestressing and the equivalent load method 29 3.9 Temperature effects 29 3.10 Differential shrinkage 29 3.11 Designing for construction 33

Design of Hybrid Concrete Buildi1 1Design of Hybrid Concrete Buildi1 1 29/01/2009 16:47:3229/01/2009 16:47:32

4. Bearings and movement joints 34 4.1 Horizontal forces at bearings 34 4.2 Restrained bearings 35 4.3 Movement joints 36 4.4 Actions and restraints 36 4.5 Design considerations 37 4.6 Allowance for anchorage of reinforcement at supports 37 4.7 Bearings that allow limited movement 38 4.8 Connections between precast fl oors and in-situ concrete beams 42

5. Structural elements and connections 43 5.1 Twin wall construction (type 1) 43 5.2 Precast columns, edge beams and in-situ slabs (type 2) 52 5.3 Biaxial voided slabs 55 5.4 Prestressed hollowcore units 58 5.5 Double tee beams 68 5.6 Stairs 74 5.7 Corbels, nibs and half joints 82

6. Construction issues 87 6.1 Method of construction 87 6.2 Composite action between precast units and in-situ structural topping 89 6.3 Specially shaped standard units 89 6.4 Long and short units adjacent to each other 89 6.5 Differences of camber in double tees 91 6.6 Method of de-tensioning double tee units 91 6.7 Checking strand or wire pull-in for hollowcore units 91 6.8 Placing hollowcore units into the correct position 91 6.9 Production tolerances 92

7. Special structures - case studies 93 7.1 Lloyd’s of London 93 7.2 Bracken House 100

References 104


List of worked examples

Worked example 1 Hollowcore fl oor acting as a diaphragm 20Worked example 2 Interface shear between hollowcore slab and edge beam 23Worked example 3 Upwards camber on slab due to temperature gradient 30Worked example 4 Differential shrinkage 31Worked example 5 Bearing of a hollowcore unit 41Worked example 6 Vertical tie 56Worked example 7 Anchorage length of longitudinal tie bar 65Worked example 8 Dowel bar for connection of precast stairs 80Worked example 9 Corbel design 84



5

1. Introduction

Hybrid construction allows the most appropriate use of different materials and methods of construction to produce a pleasing and effective form of structure. The search for greater economy, in terms of material costs and reduced construction time, has resulted in innovative approaches that seek to combine construction materials and methods to optimum effect. Hybrid concrete construction (HCC) is one such development that combines in-situ and precast concrete to maximise the benefi ts of both forms of concrete construction. Further guidance on the benefi ts of HCC is given in Section 2.1.

This design guide is aimed at the designer and considers a range of hybrid concepts and the overall structural aspects. It provides design and detailing information for some of the common systems used and structural elements involved. Where applicable the information is in accordance with BS EN 1992-1-1 1, together with the UK National Annex (Eurocode 2 is used to refer to BS EN 1992-1-1 throughout this guide unless noted otherwise). This incorporates a section on the design of members by strut and tie methods, which is particularly useful when considering ‘hybrid’ design details. This guide also considers and refers to the following European Concrete Product Standards for precast concrete elements:

BS EN 133692 Common Rules for Precast Concrete Products

BS EN 11683 Precast Concrete Products – Hollowcore Slabs

BS EN 137474 Precast Concrete Products – Floor Plates for Floor Systems

BS EN 132245 Precast Concrete Products – Ribbed Floor Elements

BS EN 132256 Precast Concrete Products – Linear Structural Elements

BS EN 149927 Precast Concrete Products – Wall Elements

BS EN 148438 Precast Concrete Products – Stairs

The use of precast and in-situ concrete may well lead to the design of the individual elements by designers working for different companies. Therefore, it is essential that there should be a single named designer or engineer who retains overall responsibility for the stability of the structure and the compatibility of the design and details of the parts and components, even where some or all of the design, including details, of those parts and components are not carried out by this engineer. This is particularly important for the design of hybrid structures where misunderstandings as to who is responsible have occurred.

It is the responsibility of the designer, before incorporating any proprietary system as part of the structure, to ensure that the assumptions made in the design and construction of such are compatible with the design of the whole structure. This should include:

an adequate specification for that part. ensuring that any standard product designed and detailed by the precast

manufacturer, is suitable for that particular structure. the design of any such part is reviewed by the designer to ensure that it satisfies the

design intent and is compatible with the rest of the structure.

1.1 Single point of responsibility

Introduction 1


6

The design of each component should include consideration of: its performance in the permanent condition the construction method and loading any temporary supports required during construction.

The design should be carried out following the requirement of Eurocode 2, Cl. 1.3, which assumes:

Structures are designed by appropriately qualified and experienced personnel. Adequate supervision and quality control is provided in factories, in plants and on site. Construction is carried out by personnel having the appropriate skill and experience. The construction materials and products are used as specified in Eurocode 2 or in the

relevant material or product specifications. The structure will be adequately maintained. The structure will be used in accordance with the design brief. The requirements for execution and workmanship given in EN 136709 are complied with.

The design assumptions should generally include the following construction related information:

sequence of construction exposure requirements pour sizes assumed (if appropriate) concrete strength at time of striking formwork and back-propping requirements breakdown of loading including allowance for construction loads loading history assumed.

It should be noted that some of the advice given in this design guide is a result of failures that have occurred on completed structures.

Best Practice Guidance for Hybrid Concrete Construction10 looks at the procurement process from concept stages through to design and construction, suggesting processes that allow the capture of best practice. It is supported by a number of case studies. The guidance explains the benefi ts that result from:

early involvement of specialist contractors using a lead frame contractor using best value philosophy holding planned workshops measuring performance trust close cooperation – with an emphasis on partnering.

It is recommended that this guidance document is used to maximise the advantages of using HCC.

1.2 Design considerations

1.3 Best practice procurement guidance

66

1 Introduction


7

2. Overview of hybrid solutions

This section considers a range of possible hybrid concrete construction (HCC). The ideal combination of precast and in-situ is infl uenced by the project requirements. There is a wide range of possible options, a selection of which is presented here as representative of current UK practice. This is not intended to be exhaustive, but to refl ect the spectrum of possibilities. The planning and detailed design of hybrid structural systems will almost always require the involvement of specialist precast concrete manufacturers. These manufacturers are willing and able to assist early in the design process to produce an effi cient design.

There are advantages to using both precast and in-situ concrete summarised in Table 2.1; more detailed discussion on the benefi ts of concrete can be found in other publications11, 12, 13. The key to maximising the benefi ts of HCC is to use the most appropriate technique for each element to produce an economic structure.

Precast concrete Precast or in-situ concrete In-situ concreteEconomic for repetitive elements Inherent fi re resistance Economic for bespoke areas

Long clear spans Durability Continuity

Speed of erection Sustainability Inherent robustness

Buildability Acoustic performance Flexibility

High-quality fi nishes Thermal mass that can be utilised for fabric energy storage

Services coordination later in programme

Consistent colour Prestressing Locally sourced materials

Accuracy Mouldability Short lead-in times

Reduced propping on site Low vibration characteristics

Reduced skilled labour on site

Six of the most regularly used HCC options are shown in Figure 2.1 and are described in more detail in the remainder of this chapter. They will be referred to by type number throughout this guide where the detailed design of the various elements is discussed. Suggested span limits are given for each type of construction. Further guidance for initial sizing can be found in Economic Concrete Frame Elements14.

Hybrid concrete wall panels are increasingly being proposed on projects throughout the UK and are often known as ‘twin wall’. They comprise two skins of precast concrete connected by steel lattices, which are fi lled with concrete on site, see Figure 2.2. The external skins of the twin wall system are factory made, typically using steel moulds. This results in a higher quality fi nish than a typical in-situ wall. The panel surface quality is suitable to receive a plaster fi nish or wallpaper. The panel surface is not normally ‘architectural’ concrete and the colour may not be consistent or easy to specify. Joints are cast using in-situ concrete and either have to be expressed as a feature or concealed. This option offers potential advantages to the contractor in terms of speed of construction, as well as reducing the number of skilled site staff required to construct the walls.

Table 2.1Benefi ts of concrete.

77

2.1 Type 1: Precast twin wall and lattice girder slab with

in-situ concrete

Overview of hybrid solutions 2


8

Figure 2.1Typical hybrid concrete options.

Please note this diagram is repeated on the inside back cover for ease of reference.


in-situ concrete


fl oor slab


beams


fl oor units





Figure 2.2Type 1 construction,

twin wall erection.Photo: John Doyle Construction Ltd

2 Overview of hybrid solutions


9

Often the twin wall system is combined with the use of lattice girder precast soffi t slabs, with or without spherical void formers. These provide permanent shuttering for an in-situ slab that can be relatively easily fi tted to the wall system. Spans up to 8 m are common and spans up to 14 m are possible. (The manufacturer should be consulted early on to ensure the longer spans are viable.)

Potential structural uses of the twin wall system include: cellular type structures for residential use walls carrying vertical loads only shear and core walls; this has significant implications for the design, as discussed in

Section 5.1 retaining walls; this has significant implications for the design, as discussed in Section 5.1 ‘single sided’ formwork situations, where there is no access to one side of the wall to erect

formwork, for example wall construction on a party wall line against neighbouring buildings.

The major advantage is that it is an ‘in-situ structure’, fully continuous and tied together, but without the need for shuttering on site. Twin wall can also be cast with fully trimmed openings and with ducts for cables and other services.

Advantages: Quality finish for walls and soffits. No formwork for vertical structure and horizontal structure when lattice girder slabs are

used. Structural connection between wall and slabs is by standard reinforced concrete detail

and inherently robust. Reduced propping.

Disadvantages: Propping of precast required prior to sufficient strength gain of in-situ concrete. The smaller dimension of the precast units is typically a maximum of 3.6 m, so joints

in walls and soffits must be dealt with: expressed or concealed. Reduced flexibility of layout as this option requires walls rather than columns.

The combination of an in-situ slab, e.g. post-tensioned fl at slab, with precast columns can provide an economic and fast construction system. Precast concrete edge beams may also be used to avoid edge shutters on site and to allow perimeter reinforcement, cladding fi xings or prestressing anchorages to be cast in. This reduces the time required for reinforcement fi xing and erecting the formwork.

The maximum span for this form of construction depends largely on whether the in-situ slab is post-tensioned. For fl at slabs with spans greater than 10 m punching shear is likely to be a critical design issue.


2.2 Type 2: Precast column with in-situ fl oor slab


10


Where long-span thin slabs are used vibration limits should be checked, see A Design Guide

for Footfall Induced Vibration of Structures15.

This form of construction relies on the structure being braced. This is achieved by the lift core(s) or separate shear walls.

Advantages: Columns can be erected quickly. Quality finish for columns. Precast edge beam contains post-tensioning anchorages (if required), slab edge

reinforcement and cladding fixings, and avoids need for slab edge shuttering. Can be used with a variety of in-situ slabs, selected to suit individual project requirements. More flexible for late changes.

Disadvantages: In-situ slab requires falsework, formwork and curing time.

This form of construction allows a high proportion of the structure to be manufactured in quality controlled factory conditions off site leading to fast construction on site.

A variety of precast fl oor products could be used with this type of construction, including hollowcore units, double tees or lattice girder slabs (with or without spherical void formers) or bespoke cofferred fl oor units, see Figures 2.3a and 2.3b. The latter have successfully been used in high quality buildings designed for energy effi ciency, where the light fi ttings, architectural features and cooling systems have all been incorporated into the unit.

Advantages: Vertical structure can be erected quickly; no formwork required. Precast floor structure can be erected quickly; no formwork required. Quality finish for columns and soffits (although this is not always possible with

hollowcore units). Structural connection between precast elements is via standard reinforced or post-

tensioned concrete.

Disadvantages: Precast flooring must be temporarily propped. Sealing between precast units is required.

2.3 Type 3: Precast column and fl oor units with cast

in-situ beams


11

Figure 2.3aExample of type 3 projects.

Paternoster Square and offi ce building.Photo: John Doyle Construction Ltd


Figure 2.3bExample of type 3 projects.

Homer Road, Solihull. Photo: Foggo Associates


12


Figure 2.4Example of type 4 project, car park, West

Quay, SouthamptonPhoto: Hanson Concrete Products

This is a similar form to type 3 discussed above, the key difference being that the columns are cast in-situ rather than being precast, see Figure 2.4.

The advantage of this form of construction over a fully in-situ concrete structure is the ability to use long spans (up to 16 m) precast fl oor units, e.g. hollowcore slabs, double tees. These obviate the need for slab formwork and provide a relatively lightweight fl oor. This construction system does not require the involvement of a specialist subcontractor beyond the manufacture and supply of the standard precast units.

2.4 Type 4: In-situ columns or walls and beams with

precast fl oor units

Advantages: Precast floor structure can be erected quickly. Quality finish for soffits (although this is not always possible with hollowcore units). Short lead time for standard precast products.

Disadvantages: Precast flooring must be temporarily propped. Sealing between precast units is required.


13


In this form of construction the fl oor consists entirely of precast elements, which are tied together with an in-situ structural topping, see Figure 2.5. (A structural topping is now defi ned as wearing screed in BS 820416.) The column formwork can be designed as a temporary support for the precast beams and slabs to reduce the requirement for propping of the precast fl oor. The joint between the beam and columns and any structural screed is concreted with the columns to form a monolithic, robust structure.

This system requires particular attention to the connection details between the precast beam and fl oor units. It should be ensured that adequate structural ties are provided to achieve a robust structure.

Advantages: Precast floor structure can be erected quickly. Precast beams support precast floor units, minimising floor propping. Precast quality finish for soffits. Formwork for in-situ columns can be used to prop precast beams. Structural connection between precast elements is via standard reinforced concrete. In-situ structural topping to beam permits beams to be continuous over columns.

Disadvantages: Downstand beams need to be coordinated with the services distribution.

2.5 Type 5: In-situ column and structural topping with

precast beams and fl oor units

Figure 2.5Example type 5 project, Home Offi ce

Headquarters, London.Photo: Pell Frischmann Consulting Engineers Ltd and Bouygues (UK) Ltd


14


The main feature of this system is the use of the lattice girder panels to act as permanent formwork for a fl at slab. A variation is to include spherical void formers, which reduce the self-weight of the slab, for only a small reduction in fl exural strength and stiffness. Lattice girders and void former cages are cast into (usually class C40/50) concrete panels containing reinforcement in two directions, providing a precast panel that acts as the permanent formwork, see Figure 2.6. The slab may be designed as a fl at slab. If the spherical void formers are used, they are removed in areas of high shear where a solid section provides greater shear resistance.

The slab may be designed as a fl at slab, although propping of the panels will be required, to reduce the overall fl oor zone of the building and to simplify installation of services. The quality of the factory produced soffi ts provides the opportunity to take advantage of the thermal mass properties of the concrete slab by exposing them.

Advantages: Precast floor structure can be erected quickly; no formwork required. Structural connection between precast elements is via standard reinforced concrete. Quality finish for soffits. More flexible for late changes.

Disadvantages: Precast flooring must be temporarily propped.

2.6 Type 6: In-situ columns with lattice girder slabs

with optional spherical void formers

Figure 2.6Type 6: Lattice girder soffi t panels used as

permanent formwork.Photo: John Doyle Construction Ltd


15

Overall structural design 3

3. Overall structural design

This section gives specifi c guidance on the aspects of structural design that will apply to most forms of hybrid concrete construction (HCC). HCC requires special design care because the connections of elements within the structure are unlikely to use standard in-situ reinforcement details; more detailed guidance is given in Sections 4 and 5 on bearings, movement joints, various elements and their connections. The designer must be confi dent that the details will work satisfactory for all situations that the structure is likely to experience. The introduction to this design guide emphasizes the importance of a single named engineer responsible for the design of a hybrid concrete structure. This is particularly important in the design of the connection details.

The design and detailing advice provided in this guide assumes that the structure falls into Approved Document A17, class 2B (risk group 2B in Scotland) or above. It is essential to create a robust structure and this may require special details to be developed to allow the precast elements to be properly integrated.

The UK Building Regulations18 through Approved Document A refers to BS EN 1991-7, Actions on Structures – Accidental Actions19 and Eurocode 2. The full requirements are given in Eurocode 2, Cl. 9.10, its UK National Annex20 and PD 6687, Background Paper to

the UK National Annexes to BS EN 1992-121. The design of ties should take account of the minimum reinforcement requirements (related to the tensile strength of concrete) and the anchorage capacity of the bars.

Continuity of tiesA tie may be considered effectively continuous if the rules for anchoring and lapping bars given in Eurocode 2, Cl. 8.4 and 8.7 are followed and the minimum dimension of any in-situ concrete section in which tie bars are provided is not less than the sum of the bar size (or twice the bar size at laps), twice the maximum aggregate size and 10 mm.

The tie should also satisfy one of the following conditions: A bar or tendon in a precast member lapped with a bar in connecting in-situ concrete,

bounded on two opposite sides, by rough faces of the same precast member, see Figure 3.1.

A bar or tendon in a precast concrete member lapped with a bar in in-situ structural topping or connecting concrete anchored to the precast member by enclosing links. The combined ultimate tensile resistance of the links should be not less than the ultimate tension in the tie, see Figure 3.2.

Bars projecting from the ends of precast members joined by any method conforming with Eurocode 2, Cl. 8.7.

Bars lapped within in-situ structural topping or connecting concrete to form a continuous reinforcement with projecting links from the support of the precast floor or roof members to anchor such support to the topping or connecting concrete, see Figure 3.3.

3.1 Robustness


16

3 Overall structural design

Figure 3.1Continuity of ties: Bars in precast member

lapped with bar in in-situ concrete.

Figure 3.3Continuity of ties: Bars lapped within in-situ

concrete.

Tie

Tie Tie

Figure 3.2Continuity of ties: Anchorage by enclosing

links.

Tie

Peripheral tiesThe peripheral tie should be capable of resisting a design tensile force:

Ftie,per = (20 + 4n0) ≤ 60 kN

where n0 = number of storeys

Internal tiesThe internal tie should be capable of resisting a design tensile force:

Ftie,int = [(qk + gk)/7.5](lr /5)(Ft) ≥ Ft kN/m

where (qk + gk) = sum of the average permanent and variable floor loads (in kN/m2) lr = greater of the distances (in metres) between the centres of the columns,

frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration, and

Ft = (20 + 4n0) ≤ 60 kNMaximum spacing of internal ties = 1.5lr


17


Horizontal ties to columns and/or wallsEdge columns and walls should be tied horizontally to the structure at each fl oor and roof level. The tie should be capable of resisting a design tensile force:

Ftie, fac = Ftie, col = Maximum (Minimum (2Ft; lsFt/2.5); 0.03 NEd)

where Ftie,fac = in kN/m run of wall Ftie,col = in kN/column Ft = as defined in above ls = floor to ceiling height (in metres) NEd = total design ultimate vertical load in wall or column at the level consideredTying of external walls is only required if the peripheral tie is not located in the wall.

Vertical tiesFor class 2B and 3 buildings Approved Document A (and similarly the Technical Handbooks for Scotland for risk group 2B and 3 buildings) has the following requirements:a) Each column and each wall carrying vertical load should be tied continuously from the

lowest to the highest level. The tie should be capable of carrying a tensile force equal to the design load carried by the column or wall from any one storey under accidental design situation (that is loading calculated using BS EN 1990, Eurocode: Basis of Structural

Design22, Expression (6.11b)).b) Where ties described in a) are not provided a check should be carried out to show that

upon notional removal of each supporting column and wall, and each beam supporting columns or walls (one at a time in each storey of the building) that the building remains stable and that the area of floor at any storey at risk of collapse does not exceed 15 per cent of the floor area of that storey or 70 m2, whichever is the smaller, and does not extend further than the immediate adjacent storeys.

c) Where the notional removal of such elements would result in damage or is in excess of the limit above then these elements should be designed as ‘key elements’. A key element should be capable of withstanding a design load of 34 kN/m2 at ultimate limit state applied from any direction to the projected area of the member together with the reaction from the attached components, which should also be assumed to be subject to 34 kN/m2. The latter may be reduced to the maximum reaction that can be transmitted by the attached component and its connections.

Anchorage of precast fl oor and roof units and stair members PD 6687, Background Paper to the UK National Annexes to BS EN 1992-1-1 and BS EN

1992-1-221, Cl. 2.20.2 Anchorage of precast fl oor and roof units and stair members states that:a) In buildings that fall into class 2B and 3 as defined in Section 5 of Approved Document A

all precast floor, roof and stair members should be effectively anchored whether or not such members are used to provide other ties required in Eurocode 2, Cl. 9.10.2. (Similar requirements apply in Scotland.)

b) The anchorage described in a) should be capable of carrying the dead weight of the member to that part of the structure that contains the ties.


18


HCC frames may be designed as either braced or unbraced. The design of unbraced frames requires extra care to ensure that the joint details can resist the applied moments without excessive rotation.

Where fl oor diaphragm action is used in the design, type 3 and 4 structures have the precast elements carrying horizontal shears for diaphragm action to take place. Types 2 and 6 structures have the in-situ fl oor acting as a diaphragm, and type 1 and 5 structures can have the diaphragm action shared by the precast units and the in-situ structural topping.

Multi Storey Precast Concrete Framed Structures23 describes the design approaches for fl oor diaphragm action formed from different types of precast units supported by tests. One approach is the use of precast units, either alone or with a structural topping, having suffi cient horizontal shear capacity between them, such that together they can be considered as horizontal beams with longitudinal steel at each gable and tie steel across the unit-to-unit joints, see Figure 3.4a.

An alternative method, appropriate to hollowcore fl oors with no structural topping considers the hollowcore unit as a member in a virendeel girder and with reinforcement in the embedment zone in the edge beams acting as the stiffening component in the virendeel joints, see Figure 3.4b.

Figure 3.4Typical diaphragm action from precast fl oor

systems.

3.3 Diaphragm action

b) Floor carrying horizontal forces from wind by virendal actiona) Floor carrying horizontal forces from wind by beam action

3.2 Stability


19


BS EN 1168, Precast Concrete Products – Hollowcore Slabs3 has an informative annex that gives some advice on the design of horizontal diaphragms to carry lateral loads, usually wind loading. This, in turn, refers to Eurocode 2, Cl. 10.9.3 where the maximum longitudinal shear stress for grouted connections vRdi is limited to 0.15 MPa for smooth and rough surfaces, as found at the edges of hollowcore, and 0.1 MPa for very smooth surfaces as found in the ex-mould fi nish of bounding edge beams, see Figure 3.2.

A considerable amount of test work has also been carried out on hollowcore diaphragms and is discussed by Elliott23.

Eurocode 2, Cl. 6.2.5 also covers the design approach for shear at the interface between concrete cast at different times. A design example (worked example 1) is included here to illustrate the process, as it is required in many areas of hybrid design where precast and in-situ concretes are combined to produce composite sections. The example using hollowcore without structural topping is a useful one as it is more critical than diaphragms with any topping.

A further consideration is the shear connection between the hollowcore units and also between the end unit and the bounding beam. In this case, the connection to the main support beams and the longitudinal steel in the support beams is usually suffi cient to ensure that the hollowcore units cannot move apart and so the structural model used in worked example 1 remains valid.

3.4 Shear at interface of concrete cast at different

times


20


Project details

Worked example 1Hollowcore floor acting as a diaphragm

Calculated by Job No.

Checked by Sheet No.

Client Date

Check the design of the hollowcore diaphragm, without structural topping, carrying wind load to walls at each end, as shown below.

Plan: 15 m x 9 m with 250 mm thick hollowcore unit

Section A - A

vs - Very smooth surfaces - Smooth surface

vs

s

vs

vsvs

s

A

A

Edge beam

Hollowcoreunit

KEY

RW

OB

TCC

CCIP-030

WE 1/1

April 08


21


Project details




Client Date

Wind load: 2 kN/m2 (A high wind load)

Assume a 3 m high storey, calculate maximum moment, MEd, from the diaphragm edge wind load/m run. wd = 1.5 x 3 x 2 = 9 kN/m γQ is taken as 1.5 MEd = 9 x 152/8 = 253 kNm

Calculate shear reaction at the diaphragm edges, VEd. VEd = 9 x 15/2= 67.5 kN

Assume 2 No. hairpins (U bars), 12 mm diameter, in each 1.2 m wide hollowcore unit.

Check shear at interface: vEdi < vRdi

gives: vEdi = β VEd/(z bi) where β = 1 VEd = 67.5 kN at end of diaphragm d = 0.83 h and z = 0.67 h (assuming elastic stress distribution) Hence: z = 0.67 x 9 = 6 m bi = 250 – 50 (say) = 200 mm ∴ vEdi = 67.5 x 1000/(6000 x 200 ) = 0.056 MPa

rRdi is limited to 0.10 MPa (> 0.056 MPa → OK)

Check vRdi (which is unlikely to control); for this example the first and second terms are small and may be ignored as a first estimate.

vRdi = ρfyd (μ sin α + cos α) ≤ 0.5 υ fcd

where ρ = As/Ai μ = 0.5 (very smooth surface) fyd = the design yield strength of reinforcement As = the area of reinforcement crossing the interface Ai = the area of the joint α = 90 for reinforcement perpendicular to the joint υ = 0.6 (1 – fck/250)

Eurocode 2, Cl. 6.2.5

Eurocode 2, Exp.(6.24)

Eurocode 2, Figure 6.8

Eurocode 2, Cl.10.9.3(12)


Eurocode 2, Cl.6.2.5 (2)

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Project details




Client Date

For this example: As = 2 x 2 x 113 = 452 mm2

Ai = 1200 x 200 = 240 000 mm2

Hence: ρ = 452/240 000 = 0.00188 and: vRdi = 0.00188 x 500 x (0.5 x 1 + 0)/1.15 ≤ 0.5 x 0.6(1 - 25/250) x 1 x 25/1.5 = 0.41 ≤ 4.5 MPa Use 2 No. hairpins (U bars) - 12 mm diameter

This check demonstrates that Exp. (6.25) is not usually a limiting control.

The design would now normally continue to calculate the tensile steel required in the edge beam to carry the diaphragm tensile boom force, taking into account that this calculation must also consider the other actions for the appropriate combination of actions.

For many beams in HCC there is an interface between concrete cast at different times. The interface may be between precast and in-situ, two precast elements or in-situ concrete with a construction joint. All interfaces and critical sections in the composite section must be considered in accordance with Eurocode 2, Cl. 6.2.4 and 6.2.5 (see example in Section 3.4). Typical interfaces are shown in the Figure 3.5, and typical calculations are presented in worked example 2.

3.5 Interface shear

Interface 3 Interface 2

Interface 1

Interface 4

Figure 3.5Typical interfaces between precast and in-situ

joints.

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Project details

Worked example 2Interface shear between hollowcore slab and edge beam



Client Date

Consider Example 13.7 in the Precast Eurocode 2: Design Manual24. Interface shear check is between the edge beam and in-situ concrete provided in the joint (see figure). In this example the contribution of the horizontal surface is ignored. The shear resistance of the interface between the upstand of the precast unit and the main body below should also be checked.

The flange over each hollowcore is cut out and therefore the units should be temporarily propped. 1 No. H16 U-bar is placed in each void to interlock with projecting reinforcement in the edge beam as shown.

Assume that the compression flange of the edge beam is 600 + 175 + 110 = 885 mm wide.

Check shear at interface according to Eurocode 2, Cl. 6.2.5.

fck = 35 MPa fy = 500 MPa Maximum sagging moment, MEd = 267 kNmMaximum design shear, VEd = 223 kN bi = 200 mm d = 540 mm

MEd/bd2fck = 267 x 1000000/(885 x 5402 x 35) = 0.0296

600 175 110

200

In-situ concrete

Shear interface

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Project details

Worked example 2Interface shear between hollowcore slab and edge beam



Client Date

From Figure B1 of the Precast Eurocode 2: Design Manual24 find value of z (alternatively find z by calculation or with any suitable design aid): z = 0.97

vEdi = βVEd /z bi

where β = ratio of the longitudinal force in the new concrete and the total

longitudinal force = width of new concrete/total flange width = 775/885 = 0.88 bi = 200 mm

Hence: vEdi = 0.88 x 223 x 1000/(0.97 x 540 x 200) = 1.87 MPa

vRdi = c fctd + μ σn + ρfyd (μ sinα + cosα) ≤ 0.5 υfcd

where c = 0.35 and μ = 0.6 for a smooth surface σn = 0 α = 90º fctd = 1 x 2.2/1.5 = 1.47 MPa υ = 0.6(1 – 35/250) = 0.52

vRdi = 0.35 x 1.47 + 0 + ρ x 0.6 x 500/1.15 ≤ 0.5 x 0.52 x 1 x 35/1.5 (= 6.07 MPa)

vEdi ≤ vRdi ≤ 0.515 + 260.9 ρ

Hence: ρ ≥ (1.87 – 0.515)/260.9 = 0.005

Now: ρ = As /Ai

∴ As,req = ρ Ai = 0.005 x 1200 x 200 = 1200 mm2

Using 3 No. voids each containing 1 No. H16 U bar.

As,prov = 3 x 2 x 162 π/4 = 1210 mm2 OK

Eurocode 2, Exp (6.24)

Eurocode 2, Exp (6.25)

Eurocode 2, Exp (6.6N)

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Shear and torsion are predominately critical at the ultimate limit state and the composite sections can be considered to be monolithic if the interface shear calculations have been carried out appropriately, as discussed in Section 3.4 (see Eurocode 2, Cl. 6.2.4 and 6.2.5).

The variable strut inclination method used in Eurocode 2 is based on the shear load being applied at the top of the beam element. When it is applied near to the bottom, the load must be ‘carried up’ to the top with vertical reinforcement additional to the vertical reinforcement required by the shear calculation. This is sometimes called ‘hang up steel’, as its effect is to hang up the applied load to the top compression chord of the beam (Eurocode 2, Cl. 6.2.1(9)), see Figure 3.6.

3.6 Shear and torsion design

Figure 3.6‘Hang up steel’ requirement.

Slab shear strut

Beam shear strut

“Hang up steel” additional toreinforcement required to carryshear Eurocode 2, Cl 6.2.1 (9)

Slab shear strut

Types 2, 3 and 4 apply the fl oor permanent actions to the spine beams at the bottom of the section and this element of the load must be carried by hang up steel. Whether the subsequent variable actions should also be covered in this way depends on the form of the composite connection. In any event, the load only needs to be carried up once to the top of the truss and the extra link requirement is not onerous.

Where type 5 is used a further check is required for edge beams or where there is out-of-balance loading on an internal beam.

The edge beam and internal spine beam with unequal loading in this form of construction must be designed to resist the torsion set up by the eccentric loading. Both the transient situation during construction and the ultimate limit state must be considered. The joint between the beam and its support must also be designed to take this torsion, see Figure 3.7.


26


Figure 3.7Design for torsional restraint.

Centre of resistanceof column

Shear centre of beam

V

h1

h2

For the torsional design of the edge beam, the design torque is equal to the load multiplied by the distance from its line of action to the shear centre of the edge element Vh1. For the design of the temporary support system to give equilibrium, the overturning torque is equal to the torsional force multiplied by the distance from the line of action of the force to that of the restraining system Vh2.

Many prestressed precast elements are produced by the long-line pre-tensioning system on prestressing beds of up to 200 m in length with built-in jack heads at each end, see Figure 3.8. The normal construction procedure is as follows:

The moulds are placed in a continuous line along the bed (the number depending on the length of each unit) and end plates are fitted to the required dimensions of the units to be cast.

The tendons are laid out and stressed from fixed external jack heads. They pass through each unit as straight horizontal tendons.

The secondary reinforcement is then fixed within each mould. The concrete is poured into each mould. When the concrete reaches the required transfer strength (confirmed by test cubes),

the stress is gradually released from the jack heads and is transferred into the concrete by anchorage bond.

A typical detail of the placing of moulds on the long-line system is shown in Figure 3.9.

3.7 Long-line prestressing system


27


Gradual detensioningmechanism

Stressed strands

Unit moulds or continuouslyextruded units

Jack blocks and embeddedcantilever upright inconcrete strong floor

Figure 3.8The long-line pre-tensioning system.

Mould end plateStrand

Detail of gap between moulds

Unit in mouldFigure 3.9

Typical detail of placing of moulds on the long-line system.

Debonding tendonsThe position of the strands in the section is normally determined by the length of the unit and the design loading at mid-span. Stress limits are set for the serviceability limit state (for further information see Precast Eurocode 2: Design Manual24 and Post-tensioned

Concrete Floors Design Handbook25).

Since the tendons are straight the prestress is the same at the end of the units as it is at mid-span (apart from within the transmission zone), but there is little balance from the stresses due to permanent actions at the ends. This creates high-tension stresses at the top of the section that will be a maximum immediately after transfer of prestress. In order to reduce these stresses locally some of the tendons are debonded by placing tubing over them at the end of the unit for the required length, see Figure 3.10.

It should be noted that the bottom strand should not be debonded, as it ensures that the concrete near the end of the unit has less chance of being damaged. It is advisable to provide two links just beyond the debonding point in the beam span to restrain anchorage stresses. Two 10 mm diameter links, the fi rst at 100 mm from the debonding point and the second 40 mm beyond that, are typically suffi cient. The proximity of the links to the bonding position ensures suffi cient restraint to bursting even if the transmission zone is less than that assumed in design in accordance with Eurocode 2.


28


Figure 3.10Typical detail showing the debonding of a

strand.

Typically 7 - 8 protruding links

Extra links atdebonding point

Debonded strandFully bonded stressed strand

Debonding is used in double tee design because it is such a simple and cost-effective option. An alternative to debonding some of the tendons is to defl ect them at the ends of the unit. This method is very seldom adopted, as it requires special features to be built into the long-line system to take account of the vertical forces involved.

The difference between the effects of straight bonded and debonded tendons is shown in Figure 3.11.

Balance of moments

Unit with straight bonded tendons Unit with straight debonded tendons

Moments from quasi-permanent loading

Moments from prestress

Resulting camber

Figure 3.11Comparison between straight bonded,

debonded and defl ected tendons.


29


Prestressed units camber because of the hogging moment provided by the prestress. A pre-tensioned prestressed beam with no camber, unless it has a very short span or is debonded, should be viewed with caution. Camber is equivalent to the defl ection of a reinforced concrete beam; in fact for a permanent and variable action balanced by prestress, the upwards camber would be less than the downward defl ection of the reinforced section. This is because the prestressed section would be uncracked and stiffer than the cracked reinforced beam. Thus, camber should not be a problem but should be allowed for when setting fl oor levels. An estimate of camber should be obtained from the manufacturer of the prestressed unit. It will be affected by the strength of concrete at the time of transfer.

Debonding has the advantage of reducing camber, as the debonded prestressed moment diagram is closer to the permanent load diagram than the fully bonded one. The typical camber of a fully bonded 16 m double tee beam carrying car park loading is 35 to 45 mm and this can be reduced by debonding to the range of 10 to 25 mm. Debonding, however, reduces the net prestress at the support and this reduces the design shear strength, but for double tees this reduction is seldom a critical design issue.

The occasions where secondary effects (sometimes referred to as parasitic effects) need to be considered relate to indeterminate frames and continuous beams/slabs. The most likely example for HCC is where post-tensioned slabs are used. Section 5.6 of the Post-tensioned

Concrete Floors Design Handbook25 describes the phenomena and the use of the equivalent load method.

The defl ection of a fl oor in response to a temperature gradient can be large and this can result in rotational movements at supports, which can produce unwanted local damage such as cracking and spalling. This problem is particularly acute in uninsulated roofs, often found in car parks. The following simple calculation, worked example 3, gives an idea of the magnitude of the displacements. Further guidance can be found in Movement, Restraint and

Cracking in Concrete Structures26.

When an in-situ screed is added onto a fi rst stage cast fl oor of either reinforced or pre-stressed construction, the shrinkage of the screed after its initial hydration will develop a compressive strain in the top of the fi rst stage cast and will induce a downwards defl ection in the span of the composite unit and, if the fl oor is of continuous construction, a hogging moment at the supports. Note that these effects are of importance at the serviceability limit state only, as at the ultimate limit state these imposed strains will have little effect.

Figure 3.12 shows how the strains are built up through the height of the composite section for a given free differential shrinkage strain, εfds. The fi nal curvature, φ, is constant across the section. Design equations can be developed as follows:

3.8 Secondary effects of prestressing and the

equivalent load method

3.9 Temperature effects

3.10 Differential shrinkage


30


Project details

Worked example 3Upwards camber on slab due to temperature gradient



Client Date

Calculate the upwards deflection of a 16 m span 300 mm deep simply supported floor resulting from a temperature gradient of 20ºC with the upper surface being the hotter. Assume that the gradient is linear and steady state, and that the temperature coefficient for concrete, α, is 10 x 10-6.

The curvature, φ, from this temperature gradient is = 20 x α/300 = 20 x 10 x 10-6/300 = 0.67 x 10-6

The curvature is constant along the length of the unit.

From the second moment area theorem, the mid-span deflection:δ = φ x l2/8 = 0.67 x 8000 x 4000/1000000 = 21.4 mm

Force equilibrium:εi Ei Ai = εp Ep Ap (1)

εp = εi Ei Ai /Ep Ap

Section equilibrium (φEI = M):φ (Ei Ii + Ep Ip) = εi Ei Ai ( yi,b + yp,t) (2)

Strain equilibrium:εfds = εi + εci + εcp + εp = εi + φ yi,b + φ yp,t + εp

φ = (εfds - (εi + εp))/(yi,b + yp,t)

φ = (εfds - (εi + εi Ei Ai /Ep Ap))/(yi,b + yp,t) (3)

In-situ

Precast

yp,t

yi,b

εfds

εcp

εp

εci

ε i

φ

Figure 3.12The effect of differential shrinkage across a

section.

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Combining (2) and (3):φ{yi,b + yp,t + (εfds - (Ei Ii + Ep Ip)) (1/Ei Ai + 1/Ep Ap)/(yi,b + yp,t)} = εfds

φ = εfds /{yi,b + yp,t + (Ei Ii + Ep Ip) (1/Ei Ai + 1/Ep Ap)/(yi,b + yp,t)} (4)

εi = εfds /{1 + Ei Ai /Ep Ap + (yi,b + yp,t)2 Ei Ai /(Ei Ii + Ep Ip)} (5)

εp = εfds /{1 + Ep Ap /Ei Ai + (yi,b + yp,t)2 Ei Ai /(Ei Ii + Ep Ip)} (6)

From equations (4) to (6) all the strains, stresses and forces can be determined.

Worked example 4 describes the method for determining the effect of differential shrinkage where in-situ concrete is placed on a precast concrete T section.

Project details

Worked example 4Differential shrinkage



Client Date

Calculate the effect of differential shrinkage in a beam constructed in two stages as shown below. The element is simply supported and 20 m span. The free differential shrinkage strain is 0.0002.

B785 fabric in in-situ concrete B283 fabric in precast concrete flange2 x 2 No. 7.9 mm super strand in precast rib

In-situ concrete

Precast concrete

150

1000

100

50

300

B785 mesh

B283 mesh

2 x 2 No 7.9 super strand

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Project details

Worked example 4Differential shrinkage



Client Date

In-situ concrete fck,in = 25 MPa, fcm,in = 33 MPa, creep coefficient, ϕ = 1.5 Ec,in,long = 22 [fcm,in/10]0.3/(1 + ϕ) = 22 x (33/10)0.3/(1 + 1.5) = 12.59 GPa

Section properties, including the reinforcement, are as follows: Ain = 112 x 103 mm2

Iin = bd3/12 = 1000 x 1003/12 = 87.5 x 106 mm4

yinbar,b = 52.1 mm zin,b = 1680 x 103 mm3

Precast concrete fck,p = 50 MPa, fcm,p = 58 MPa, Creep coeficient, ϕ = 1 Ec,p,long = 22 x (58/10)0.3/(1 + 1) = 18.64 GPa

Section properties, including the tendons and reinforcement, are as follows: Ap = 101.5 x 103 mm2

Ip = 1220 x 106 mm4

ypbar,b = 237.4 mm ypbar,t = 112.6 mm zp,t = 10900 x 103 mm3

CurvatureUsing expression (4) above:Curvature:

φ = 1000 x 0.0002

52.1 + 112.6 + (12.59 x 87.5 x 106 + 18.64 x 1.22 x 109) x (1/(12.6 x 112 x 103) + 1/(18.6 x 101.5 x 103))

50 + 112.6

= 0.00058/m

Defl ectionDeflection from differential shrinkage δ = φ l 2/8 = 0.00058 x 202/8 = 29 mm

Eurocode 2, Table 3.1 and Cl.3.1.4

Eurocode 2, Table 3.1 and Cl.3.1.4

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Designers should take into account the stability of the structure during construction: Precast elements are heavy. Bearings must be adequate and be robust enough to

withstand normal unit fixing operations including landing and ‘barring’ (see Section 6.7). Beams must be securely fixed and have adequate safe bearing at each end to avoid

overturning, excessive deflection or collapse when the precast elements are placed. Consideration must be given to the unequal loading when precast elements are being

placed. Where precast elements are tilted or twisted to allow them to be placed in their final

position consideration should be given to ensuring there is sufficient clearance to place the unit and achieving the minimum end bearing required in the final position.

Special requirements, such as special fixing techniques, temporary measures or sequencing, should be clearly conveyed.

3.11 Designing for construction


34

4 Bearings and movement joints

4. Bearings and movement joints

The design of bearings and joints for hybrid concrete construction (HCC) is critical to the serviceability and lasting integrity of the structure. Careful design can avoid problems which lead to deterioration of joints, which ultimately compromise the whole safety of the structure.

Where a bearing is introduced between precast elements or between precast and in-situ elements great care is required to take account of all the forces and movements that may be imposed on the elements connected to the bearing. In addition consideration must be given to:

how the robustness of the structure is attained effects of composite action practical tolerances temperature changes shrinkage differential settlement effects of repeated changes in imposed deformations ensuring construction meets the assumption made in design.

The decision to design a full continuity joint or one that allows some movement is critical. The design must then follow the decision to reach a practical and lasting solution. The joint detail must be robust and must not deteriorate with time due to the effects of movement.

Joints that are designed to be monolithic are considered in Chapter 5.

Horizontal forces at a bearing can reduce the load carrying capacity of the supporting member considerably by causing premature splitting or shearing. The forces may be due to creep, shrinkage and temperature effects or may result from misalignment, lack of plumb or other causes. Allowance should be made for these forces in designing and detailing by the provision of:a) bearings that allow limited movement orb) suitable lateral reinforcement in both the supporting and supported members orc) sufficient continuity reinforcement through the joint to resist the lateral forces.

Where type a) bearings are used then conservatively the horizontal design force should be taken as 20 per cent of the vertical force. A more detailed assessment may show this force can be reduced. For type b) and c) bearings the design horizontal force should be not less than half of the design vertical force on the bearing.

Unless top and bottom continuity reinforcement is provided precast fl oor slabs, e.g. hollowcore slabs, spanning more than 8 m should be supported on elastomeric bearings, e.g. neoprene.

4.1 Horizontal forces at bearings


35

Bearings and movement joints 4

These can normally be attached to the support surface. They allow: the forces resulting from variation of bearing surfaces to be absorbed any small horizontal movements to be absorbed without causing cracking and limited rotation (as a result of cyclic upward and downward deflection) of the precast

slab.

Where top and bottom continuity reinforcement is provided, to make a homogenous joint it may be acceptable not to provide elastomeric bearings. In this case great care must be taken in construction to ensure that the precast element is not damaged during placing and that it can absorb the movements that take place during and after construction without damage.

For bearings that offer signifi cant restraint to sliding or rotation, e.g. dry bearing on concrete or mortar bedding, actions due to creep, shrinkage, temperature, misalignment, lack of plumb and other things must be taken into account in the design of adjacent members. Further guidance on creep, shrinkage and temperature effects can be found in Movement,

Restraint and Cracking in Concrete Structures26.

The effect of such actions may require transverse reinforcement in supporting and supported members, and/or continuity reinforcement for tying elements together. They may also infl uence the design of the main reinforcement in such members. Such joints are not con-sidered suitable for external situations or for spans greater than 8 m for internal situations.

It should be noted that it is unlikely that a dry connection without bedding material will have a uniform contact surface and that concentrated loading will result that may cause local cracking.

For joints with bedding material, e.g. mortar, concrete, polymers, relative movement between the connected surfaces should be prevented during hardening of the material.

The bearing width should not be greater than 600 mm unless specifi c measures are taken to obtain a uniform distribution of the bearing pressure.

In the absence of other specifi cations, the bearing strength, fRd, of a dry connection should not exceed 0.4 fcd and the average bearing stress between plane surfaces should not exceed 0.3 fcd.

The bearing strength for joints with bedding material should not exceed the design strength of the bedding material, fbed ≤ 0.85 fcd where fcd is the lower of the design strengths for supported and supporting members.

4.2 Restrained bearings


36


Expanding materialto plug gap

Friction cancause cracking

Movement

Rotation

If no plug, hard materialcan prevent rotation

Rotation

Rotation cancause spalling

Figure 4.1Examples of potential failures at movement

joints.

It is possible to deal with movement at bearings using movement joints, and care should be given to the design and construction, as for bridge decks, to minimise the risk of failures. In general it is recommended to seek solutions that do not require movement joints. Figure 4.1 describes potential failure mechanisms that can occur even with a structural topping.

4.3 Movement joints

If the bearing material creates large friction forces (use neoprene or similar to avoid this), this can lead to large tension stresses in both the support and the precast slab or beam.

If the space between the precast slab or beam and the face of the supporting member is not adequate for the required movement or if in time it it fi lls up with hard material, then cracking can occur.

If the effects of movement and/rotation cause the line of action to move too close to the edge of the support, local spalling can occur.

4.4 Actions and restraints4.4.1 Action effects In addition to the effects of direct loading (imposed variable and permanent actions) the

following action effects on the elements supported by the bearing must be considered: shrinkage (both long term and early thermal) temperature changes (both seasonal and short term) creep.


37


In addition to the above action effects the following restraints must be considered: internal, e.g. from reinforcement, differential shrinkage edge restraints end restraints.

For detailed consideration of these effects and restraints refer to Movement, Restraint and

Cracking in Concrete Structures26.

When designing bearings the following details should be checked: calculation of the bearing area bearing layout the detail of the reinforcement in the end of the supported member the detail of the reinforcement in the supporting member tolerances construction issues – especially any additional forces imposed on the bearing through

‘barring’ the units into final position, see Section 6.8.

The design and detailing of the reinforcement at supports is critical. The supported member has to be designed to bear safely onto the support without spalling of the end cover and also to sustain any forces that may come from shrinkage of the fl oor, through shortening of the fl oor, if prestressed, and from thermal, live and further dead load movements, see also Section 4.1.

Prestressed members used for fl ooring are commonly pre-tensioned and the main prestressed steel continues to the end of the member. Reinforcement in supporting and supported members should be detailed to ensure effective anchorage, allowing for deviations, see Figure 4.2.

di = ci + Δai with horizontal loop bars di = ci + Δai + ri with vertically bent barsci = nominal concrete coverΔai = a deviation (see Section 4.8)ri = radius of bend (see Table 4.1)

4.4.2 Restraints

4.5 Design considerations

4.6 Allowance for anchorage of reinforcement

at supports


38


c2 > +a a1 2� d3

r2

r3

d2 > +a a1 � 3 c3

Figure 4.2Effect of reinforcement on bearing

dimensions.

Table 4.1Minimum bend radii for reinforcement to

avoid damage to reinforcement.

Bar diameter Minimum radius of bendφ ≤ 16 mm 2 φ

φ > 16 mm 3.5 φ

Bearings that allow limited movement, e.g. neoprene pads, not only distribute the bearing forces over uneven supports but also allow limited rotational and longitudinal movement of the supported member to take place. The bearing pad also defi nes the area of load transfer and thus has a direct effect on the detailed design of the ends of the supporting and supported members.

In the absence of other specifi cations, the bearing strength, fRd = fbed ≤ 0.85 fcd where fbed is the design strength of the bearing material may be used.

The layout of a bearing is critical to its successful execution. The concrete surfaces must be separated in areas where load transfer is not intended and must be bedded appropriately where load transfer is required. To ensure that spalling does not take place in the contact area at the end of the supported and supporting concrete, the provision of suffi cient bearing length must be provided. This should allow for constructional tolerances and ensure the overlap of reinforcement between the supporting and supported concrete. The required allowances are shown in the Figure 4.3 and are described in Eurocode 2, Cl. 10.9.5.2. These will lead to the design of minimum bearing shelf and nib sizes.

4.7.1 Design of the bearing area

4.7.2 Bearing layout

4.7 Bearings that allow limited movement


39


The nominal length, a, of a simple bearing may be calculated as:

a = a1 + a2 + a3 + √(Δa22 + Δa3

2)

where a1 = net bearing length with regard to bearing stress = FEd /(b1fRd) but not less

than the values in Table 4.2 FEd = design value of the support reaction b1 = net bearing width fRd = design value of the bearing strength = 0.85fcd

a2 = distance assumed ineffective beyond outer end of supporting member (see Table 4.3)

a3 = distance assumed ineffective beyond outer end of supporting member (see Table 4.4)

Δa2 = allowance for distance between supporting members (see Table 4.5) Δa3 = allowance for deviation of the length of the supported member = ln /2500 ln = length of member in mm

b1

a1

> +� �a a2 3

a a3 3+ �a1

a

a a2 2+ �

Figure 4.3Critical dimensions for bearings.

Relative bearing stress, σEda/fcd ≤ 0.15 0.15 to 0.4 > 0.4

Line supports (fl oors and roofs) 25 30 40

Ribbed fl oors and purlins 55 70 80

Concentrated supports (beams) 90 110 140

Key:a σEd is the design bearing stress

Table 4.2Minimum value of a1 (mm).


40


Relative bearing stress, σEda/fcd ≤ 0.15 0.15 to 0.4 > 0.4

Reinforced concrete ≥ C30/37

LineConcentrated

510

1015

1525

Reinforced concrete < C30/37

LineConcentrated

1020

1525

2535

Keya σEd is the design bearing stress

Detailing of reinforcement Type of support

Line Concentrated

Continuous bars over support(restrained or not)

0 0

Straight bars, horizontal loops, close to end of member

5 15, but not less than end cover

Tendons or straight barsexposed at end of member

5 15

Vertical loop reinforcement 15 End cover + inner radius of bend

Support material Δa2

Precast concrete 10 ≤ l /1200 ≤ 30 mm

Cast in-situ concrete 15 ≤ l /1200 + 5 ≤ 40 mmNote:l is clear distance between supports in mm

An example calculation is shown in worked example 5.

Table 4.4Distance a3 (mm) assumed ineffective from

outer end of supported member.

Table 4.5Allowance for deviations for the clear distance

between the face of the supports.

Table 4.3Distance a2 (mm) assumed ineffective from

outer end of supporting member.


41


Project details

Worked example 5Bearing of a hollowcore unit



Client Date

A 1.2 m wide hollowcore slab seated on an in-situ concrete nib, treated as a non-isolated member. The length of hollowcore unit is 9 m. The in-situ concrete beam is class C35/45 concrete.

Actions Self weight = 3.33 kN/m2

Variable load = 4 kN/m2

Partitions = 1 kN/m2

Finishes = 0.7 kN/m2

Bearing stress FEd = 9 x 1.2 x {1.35 (3.33 + 0.7) + 1.5(4 + 1)}/2 = 69.9 kNAssume a 30 mm wide neoprene bearing. σEd = 69.9 x 1000/(30 x 1200) = 1.94 MPa σEd/fcd = 1.94/(0.85 x 35/1.5) = 0.098

GeometryMinimum value of a1 from Table 4.1 for a line support is 25 mm. Hence: a1 = 30 mm OK a2 = 5 mm a3 = 5 mm Δa2 = 15 mm Δa3 = 9000/2500 = 4 mm say 5 mmThe reinforcement in the in-situ concrete nib is assumed to be 20 mm vertically bent with a nominal cover of 20 mm. d2 = c2 + Δa2 + r2

= 20 + 15 + 3.5 x 20 = 105 mm a2 + Δa2 ≥ d2

∴ a2 + Δa2 = 105 mm

Allowance for clearance at end of unit Δa2 + Δa3 ≥ 15 + 4 mm = 19 mm say 20 mm

The bearing stress should also be checked for the hollowcore unit.

Table 4.2Table 4.3Table 4.4

20

10 30 105

20

H20 bar

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Figure 4.4Typical methods to avoid spalling of bearing

corners.

Chamfer option Lowered support area option

Spalling of the support is avoided if a large chamfer is provided on the outer corner or alternatively a local part of the bearing shelf is lowered, see Figure 4.4. This and the compressed thickness of any bedding material in the bearing must be suffi cient to avoid contact, taking into account any long-term movements, defl ection, hogging and if the fl oor is laid to a fall for any reason, the difference in angle of the fl oor soffi ts at its end and that of the bearer beam. Neoprene is recommended as a suitable material for bearings but other materials may be used (see also PCI Design Handbook27).

In an HCC situation, the bearing may be in a different state when it carries construction actions and when it is fully constructed and carries superimposed permanent actions and variable actions. These interactions should be considered and very soft bearing materials may be inappropriate if the fi nal objective is to have a fully continuous connection.

Type 3 and 4 systems that use precast fl oors with in-situ beams do not always have a direct bearing since the in-situ concrete is often cast against the precast unit. The fl oor is propped and the formwork for the edge beam is fi xed. The steel protruding from the fl oor units is incorporated into the reinforcement of the edge beam that is then cast. The con-tinuity steel must be fully anchored in both the in-situ and precast concrete. Consideration should be given to the possibility of tension occurring in the bottom steel at the support. This can be caused by temperature and shrinkage effects. The design of the interface for shear requires the provision of ‘hang-up steel’ as the shear load in the fl oor is concentrated near to the bottom of the section. This is described in Eurocode 2, Cl. 6.2.1 (9) and is also shown in Figure 3.6.

4.8 Connections between precast fl oors and in-situ

concrete beams


43

Structural elements and connections 5

5. Structural elements and connections

Twin wall panels comprise two skins of precast concrete, connected by steel trusses, which hold the precast skins apart at a constant spacing to produce a wall of a particular thickness. Figure 5.1 shows a typical view of a twin wall panel system.

5.1 Twin wall construction (type 1)

Figure 5.1Typical example of a twin wall panel.

Photo: John Doyle Construction Ltd

The panels are supplied to site, erected and then fi lled with in-situ concrete to form a solid concrete wall. The trusses, therefore, also act to hold the skins together against the pressure exerted by the in-situ concrete before this has cured. A typical layout is shown in Figure 5.2. The precast skins function as permanent formwork.

The precast skins contain the main horizontal and vertical reinforcement for the wall, in the form of a cross-sectional area of fabric or bars, which can be specifi ed by the designer. However, starter bars and continuity reinforcement must be provided within the in-situ concrete.

The precast skins areconnected and spacedby steel lattice

Main horizontal andvertical reinforcementfor the wall is fittedwithin the precast skins

Figure 5.2Simple layout of a twin wall system.


44

5 Structural elements and connections

The twin wall system is often combined with a precast concrete permanent shuttering system, e.g. lattice girder slabs. This allows the minimum use of temporary formwork on site. The wall system is ideally combined with a precast lattice or composite slab fl oor, as the in-situ element of both the wall and fl oor can be combined to produce a monolithic structure.

The precast skin on one side of the panel is cast horizontally on a steel mould, with the trusses projecting. After curing, the assembly is rotated so that the trusses face down and can be cast into the pour for the precast skin on the other side, see Figure 5.3.

5.1.1 Manufacturing process

Figure 5.3Precasting sequence for twin wall

manufacture.

a) One side of panel cast with outer face down with trusses projecting upwards

b) Assembly then turned over and the second side of panel cast with outer face down

Fabric reinforcement, which can be specifi ed by the designer, is cast into each precast skin, see Figure 5.4. A 60 mm thick precast skin could accommodate, for example:

25 mm cover to external face (or as appropriate to meet durability bond and fire requirements)

16 mm vertical bar 8 mm horizontal bar 10 mm cover to internal face (whilst not required for durability in the permanent

condition, some cover here is advisable).

Clearly, walls that require larger bar sizes to achieve required levels of reinforcement, or walls in exposed conditions, will in turn need thicker precast skins to achieve required covers.

Overall panel thicknessThe fi nal wall thickness can range typically from 200 to 350 mm in total width, although thicker walls are possible. A typical 250 mm panel thickness may comprise:

60 mm precast skin 130 mm gap for in-situ concrete, starter bars, continuity reinforcement 60 mm precast skin.


45


Precast concrete

Lattice reinforcement

In-situ concrete

Vertical reinforcement

Slab reinforcement

In-situ concrete


Tie reinforcement

Lap length

Figure 5.4Twin wall connection with a lattice girder slab.

With two layers of reinforcement an overall wall thickness less than 250 mm is diffi cult to achieve. This is because the precast skin thickness is typically 50-70 mm each side (plus tolerance), and the thickness of the in-situ concrete in between must accommodate starter and continuity reinforcement with suffi cient space for the concrete to fl ow around the bars, see Figure 5.5. With one layer of reinforcement it is possible to reduce the overall section thickness to 200 mm. It is worth noting that, due to the manufacturing process, tolerances on the inside faces of the precast skin are easily controlled and can reduce the space available for in-situ concrete or starter bars by 10-15 mm each side. Tolerance for the hybrid panel to be erected over the starters is a related issue and it is advisable to use a single row of starters, rather than one row each side as for a traditional in-situ wall.


46

Figure 5.5The available space for vertical continuity

reinforcement is restricted.Photo: Hanson Concrete Products

Overall panel sizesTypically the maximum panel dimensions are 10 m x 3.5 m as shown in Figure 5.6. These dimensions are often limited by the capacity of the lifting equipment, transportation or size of moulds. The minimum dimension of a panel is typically 1.20 m.

10 m max.

3.5 m max. 10 m max.

3.5 m max.

Figure 5.6Typical twin wall maximum panel dimensions.



47

A key impact of introducing twin wall panels, as with many prefabricated forms of construction, is to increase the amount of coordination required early in the programme. To assist in this planning it is advised that the points below are considered.

Which walls are twin walls?Agreement on the extent of twin walls is likely to involve architect, client, contractor, and structural and services engineers. Until fully defi ned, this decision may have an impact on the design programme – an issue that should be communicated to the proposer of the system (often the contractor).

Manufacturers’ requirements and their impact on co-ordinationOften the twin wall manufacturer will require the following to be fully defi ned before commencing manufacture:

dimensioned CAD wall elevations showing all walls to be manufactured locations of all cast-ins (e.g. junction boxes, conduit) locations and sizes of all holes and cut-outs (e.g. for services, drainage, builders work,

windows, downstand beams) reinforcement to be cast into the precast skins locations and details of any bend-out bars required information showing which side of the panel is to be propped (to permit the prop

attachments to be cast in).

To produce CAD elevations showing this level of detail, the design of the services must be well progressed (and any builders’ work holes assumptions agreed and recorded); the architect must have frozen the wall layout; all suspended and ground slab levels, soffi t levels and upstands/downstands must be fully defi ned and frozen; and the contractor must have defi ned a pour sequence so that the side to be propped can be identifi ed. The designer should allow for the additional time required to coordinate the work.

Detailing continuity rebar at jointsThe catalogues of the twin wall manufacturer often show a number of typical joint details where fabric or loose bars are used, within the in-situ concrete, to provide reinforcement continuity. It is important to realise that the designer is responsible for detailing and scheduling such bars despite what may be implied in the catalogues.

Checking of fabrication drawingsIt is important that the designer checks the key panel layout drawings. The twin wall manufacturer produces shop drawings for each panel. They are likely to be presented to the designer just as the project begins on site. A plan for checking these should be set up in advance to avoid the confl uence of site queries with panel drawing checking creating possible resourcing diffi culties at a key project stage.

5.1.2 Planning implications



48

The panels are typically propped on one side only, using typically two ‘push–pull’ props to achieve verticality. This requires a cast base slab on at least one side of the wall. If panels are being erected before the whole slab is cast, coordination with the contractor’s pour sequence will be required. Where no slab is adjacent, e.g. walls inside lift shafts, there should be a clear method statement on how these panels will be safely erected.

The methods of fi xing the continuity reinforcement, particularly if the walls are acting as shear walls, should be clearly stated. The contractor should provide a method statement for the following:

At panel base level, how the panel is fitted over the projecting reinforcement in the lower slab taking account of the accepted tolerances. Figure 5.7 indicates other points that should be considered.

At top of panel, how the vertical continuity reinforcement is fixed. One method may be to tie horizontal fixing bars onto the trusses (say two each side) and tie the vertical projecting bars onto those. The alternative proposal of pushing them into the wet in-situ concrete is not recommended. A template for the vertical bars should be considered to ensure that the next lift of wall panel will fit over them correctly.

For the fabric reinforcement at joints between adjacent panels at the same level, how this is held in position within the pour (see Figure 5.8).

5.1.3 Site erection and fi lling

Decide from which levelthe wall should spring

Consider tolerancesfor starter bars

Decide which side propsshould be positioned

Figure 5.7Typical issues to consider in the layout design.



49

Distance to 1st trussis typically 340 mm

Check sufficient lap lengthwith reinforcement in skin

Use of U-bars or links recommendedto ensure reinforcement remainsin correct position during concreting

Figure 5.8Plan view showing horizontal continuity

reinforcement.

As the precast skins take up a fair proportion of the overall width of the wall, the gap between them is often very narrow in comparison to their height. This may make it diffi cult to remove all the air when concreting (‘blowing out’) and the contractor should provide specifi c proposals for this. Due to the very low volume of in-situ concrete required to fi ll the walls on site, the contractor’s preference may be to erect a large number of wall panels at one level, before arranging a concrete pour to fi ll them. As the precast skins are functioning as permanent formwork, resisting the pressure of the wet in-situ concrete, the wall manu-facturer’s catalogue may have rate of rise limits – typically less than 1 m/hr. Coupled with the low fi ll volume, this leads to a relatively slow fi lling process on site, and one that the operatives may be tempted to speed up! The operatives should be made aware of and respect the wall manufacturer’s rate of rise limits. The panels are typically erected on chocks to leave a gap at the base of around 30 mm. This is the principal means of checking that the in-situ concrete reaches the base of the pour. Timbers acting as grout checks are placed along each side at the base of the panel.

Precast lattice girder slab unitsFigure 5.9 shows a typical section of a composite fl oor using precast lattice girder units. The lattice girder is cast into (usually class C40/50) concrete reinforced with high-yield reinforcement. The width of the precast slab is typically 2.4 m with a depth of 50 mm or 75 mm. They are used for spans of up to 10 m (larger spans are possible with careful planning).

Lattice

Precastconcreteslab

In-situconcrete

Distribution steel

Main steel

Figure 5.9Section showing typical lattice girder fl oor.



50

Top bar 8 mm to 14 mm dia.

Diagonal bar 4.5 mm to 7 mm dia.

Bottom bar typically 5 mm dia.

Figure 5.10Typical details of a lattice girder.

The design of the lattice girder is dependant on the thickness of the composite fl oor, fi nal loading and propping system. Typical details of a lattice girder are shown in Figure 5.10.

Figure 5.10 shows a typical layout of a 2.4 m wide unit containing four sets of lattice girders.

Propping to support the self-weight and in-situ concrete can be reduced or eliminated by increasing the stiffness of the slabs through increasing the diameter of the reinforcement to the top of the lattices and/or reducing the spacing of the lattices. Unpropped spans of up to 5 m can be achieved depending upon the design loads and the overall depth of the slab.

Temporary propping is required where the end bearing is small. An example of this is at end supports where the slab unit is seated on just one leaf of the wall.

Normally the minimum cover to the reinforcement will be 20 mm; however, the cover to the reinforcement can be adjusted to meet the specifi c bond, durability and fi re resistance requirements for individual contracts.

Design momentsDesign moments about the minor axis of a wall should be considered even where central bars are placed in the joint, as these do not represent a hinge.

Flexural, shear and axial designWhen checking the strength of a section of a wall more than a full lap length from a joint the full width of section may be included. Otherwise just the in-situ part should be considered. If the whole section is in compression, it is reasonable to assume that the full section can provide axial resistance.

Lap lengths At the top and bottom of the wall there will be a lap between the main vertical reinforcement and the vertical continuity reinforcement, see Figure 5.4. When the distance between these bars is greater than 4φ or 50 mm the lap length should be increased by a length equal to the space between the bars (Eurocode 2, Cl. 8.7.2(3)).

5.1.4 Design of panels



51

Minor axis bendingIf the decision to use a single row of starters has been adopted, minor axis bending on such walls should be checked. If signifi cant, the decision should be revisited – with potential impacts on wall thickness, as noted above.

Horizontal joint between panels stacked one above the other (no slab adjacent)Horizontal joints commonly occur, for example, in lift shaft walls, or in walls adjacent to risers, stairs, or double-height spaces. At the panel joint level, effective design to resist minor axis buckling moments would tend towards the use of two rows of vertical continuity reinforcement (one layer on each face) within the in-situ portion of the wall. Due to the position of the continuity bar within the in-situ portion, and the possible tolerance and positional control issues, a realistic effective depth should be used in assessing the moment capacity of the wall at this point, see Figure 5.11.

5.1.5 Concrete and fi nishes Concrete mixThe nature of the in-situ concrete mix used to fi ll the panels on site should be considered. As the gap between the precast skins may be as little as 100 mm for a 250 mm wall, and starters and continuity reinforcement may protrude into this gap, using a vibrator poker may be diffi cult or impossible. The use of self-compacting concrete should be considered. A smaller aggregate size, for example 10 mm, may also be appropriate.

Surface fi nishTypically, the use of steel moulds gives the external faces of the panels a smooth fi nish. The fi nish quality is suitable to receive a plaster fi nish or, on request, wallpaper. It should be noted, however, that the fi nish is not ‘architectural’ concrete as colour is not consistent or easily specifi ed.

Figure 5.11Detail at an unrestrained horizontal panel

joint in compression.

The tendency to buckleunder compression at anunrestrained horizontaljoint, is resisted by thevertical continuityreinforcement, acting ata reduced lever arm.

C

d



52

Horizontal continuity reinforcementlowered into position after placingtwin walls. This must be detailed tomiss the wall trusses

Figure 5.12Horizontal continuity reinforcement to fi t

with twin wall reinforcement.

Vertical joints between adjacent panels at same levelAt junctions between adjacent panels and at corner junctions, horizontal continuity reinforcement is recommended within the in-situ portion. Detailing this reinforcement in the form of fabric or prefabricated cages is likely to be the easiest way of fi xing it within the pour. As noted above, the designer should be responsible for detailing this reinforcement. It should be noted that the presence of the trusses at a typical distance of 340 mm from the ends of each panel effectively constrains the volume in which continuity reinforcement can be provided. If the forces applied to the wall are such that they cause signifi cant shear or tensile forces to develop at the vertical panel joints, the suitability of twin wall panels as a design solution may need to be revisited.

Interface with reinforced concrete ground slab.It is important to obtain the contractor’s pour sequence for the ground slab – at locations where the ground slab steps (changes level) this will often defi ne the panel base level. Agree with the contractor whether the panel will sit on the higher-level slab, or on the lower-level slab with the higher-level slab poured up against the wall. Also agree details at the edge of slabs or at lift pits. Agree from which side the panels are to be propped. It is likely that the twin wall panels will need to be installed over projecting starter bars cast into the foundations. As well as the use of a template and the consideration for using a single row of starters, as noted above, the starters will need to coordinate with the horizontal continuity reinforcement provided at locations of vertical joints between panels. This means fi ve or six layers of reinforcement locally overlapping within the gap between the panels – a potential congestion issue, see Figure 5.12.

5.1.6 Detailing

The type 2 system uses precast columns and edge beams, often with a prestressed in-situ fl oor slab. The complex fi xing of steel and anchorages in the edge strips is more safely and accurately carried out in the precast concrete factory. The use of precast concrete columns speeds up the time between the casting of the fl oor plates. The precast edge strip is supported on the same shutter system that is used for the fl oor.

5.2 Precast columns, edge beams and in-situ slabs

(type 2)



53

The column to fl oor joint in this form of construction is assumed to be semi-monolithic, i.e. the in-situ concrete is cast up to the surface of the column or a fully grouted connection is made.

It may be desirable that levelling devices, for example nuts and wedges, having no load bearing function in the completed structure should be slackened, released or removed as necessary. Where this is necessary, the details should be such that inspection (to ensure that this has been done) can be carried out without undue diffi culty.

The design of the vertical continuity or tying reinforcement requires careful consideration. Three examples are shown in Figure 5.13.

Where a central dowel bar, as shown in Figure 5.13, is also acting as a vertical tie, the load on the grouted connection between the slab and the dowel bar can be signifi cant. The designer should ensure that the detail can carry this load either by design or through testing.

Bearing under the precast columnIn the absence of more accurate information (derived from a comprehensive programme of suitable tests), the area of concrete that should be considered in calculating the strength of the joint should not be greater than 90 per cent of the area of column assumed to be in contact with the joint, unless specifi c means are taken to ensure that no voids exist in the grout. The strength of the concrete in the precast column may be taken as fcd (= 0.85fck/1.5). The area of any bar passing through the joint should be deducted from the bearing area. The design force of such a bar may be deducted from the applied force on the bearing when calculating the capacity of the concrete provided that the bar has suffi cient anchorage beyond the joint.

GroutingThe contractor should provide a method statement for the grouting work. This should ensure that no pockets of air are trapped in the ducts and that the interface between the base of the column and support is fully grouted. Trials may be necessary to demonstrate the method.

Maximum compression through fl oorFor axial load with moment transfer Eurocode 2, Cl. 6.7 limits the compression within the slab. Exp. (6.63) is modifi ed to:

FRdu = Ac0,eff fcd √(Ac1/Ac0,eff) ≤ 3.0 fcd Ac0,eff

where Ac0,eff = 0.9 x Ac0

Ac0 = area of precast column fcd = design strength of the slab Ac1 = (h/2 + b1) (h/2 + d1) h = depth of slab b1 = breadth of the precast column d1 = depth of the precast column

5.2.1 Column to fl oor joint



54

Figure 5.13Typical column fl oor connections.

a) Column shoe

b) Column bar coupler

c) Central dowel bar


Bars welded to doweltable and columnreinforcement

Grouting ringHole grouted beforeplacing column


55

Where moment is transmitted through the joint Ac0,eff should be reduced to 0.9 x the area of the stress block shown in Figure 5.14, where Ac0,eff = 0.9 b1 x 2(d1/2 - e).

2 x ( - )d e1/2

fcd

e M/N=

C

C

L

L

of column

of compressionblock

d1

Figure 5.14Stress block in slab where moment is

transmitted from column.

For class 2B and 3 buildings (risk group 2B and 3 in Scotland) the vertical tie must be designed to take the full fl oor load in tension under ‘accidental’ loading conditions. The partial factors for the accidental combination of actions are equal to 1 (see BS EN 1990, and UK National Annex, Table NA.A1.3), see worked example 6.

If a central dowel bar system is considered for such a fl oor, i.e. span > 7 m, it should be effectively continuous throughout the height of the building. Full tension mechanical couplers should be used where joints are required.

Figure 5.15 shows a typical section of a composite fl oor using precast lattice girder units with spherical void fomers (biaxial voided slabs). The lattice girder and the void former cages are cast into a (usually class C40/50) concrete panel containing reinforcement in two directions. The width of the precast slab is typically 2.4 m with a depth of 50 mm or 70 mm.

Normally the minimum cover to the reinforcement will be 20 mm; however, the cover to the reinforcement can be adjusted to meet the specifi c bond durability and fi re resistance requirements for individual projects.

5.2.2 Vertical tie

5.3 Biaxial voided slabs

Figure 5.15Typical layout of biaxial voided slab.

Photo: Cobiax Technologies Ltd



56

Project details

Worked example 6Vertical tie



Client Date

Consider a 9 m x 9 m flat slab floor 300 mm thick with imposed variable load of 3 kN/m2 and finishes of 1 kN/m2.

ActionsThe total design force in vertical tie FEd = Gk + Ad

= 9 x 9 x {(25 x 0.3 + 1.0) + (0.5 x 4) = 851 kN.

ResistanceUsing a column shoe system: 4 No 25 mm bars will provide a resistance of FRd = γs fyk As

= 1.0 x 500 x 252 x 4 x π/4/1000 = 981 kN FRd > FEd → OK

Use 4 No 25 mm bars

Eurocode, Table NA.A1.3

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Initial sizing can be determined from manufacturers literature. The manufacturer literature will also advise the size of the spheres available, the spacing requirements and the general confi guration of the slab.

The benefi t of the reduced self-weight should be taken into account in the design. The design may assume a fl at slab model, which has been demonstrated as appropriate through testing of the slabs. A check should be carried out to ensure that the concrete compression zone remains outside of the depth of spherical void formers. Where this is not the case, as in heavily loaded slabs, the manufacturers will be able to offer appropriate guidance on determining the permissible compression zone that can be used in the calculation of the fl exural strength.

Testing has been carried out to determine the shear strength of this type of slab, alongside a theoretical assessment of the reduction in the shear plane due to the inclusion of the voids. The manufacturers recommend that shear strength of a solid slab of the same depth should be reduced by a factor of between 0.55 and 0.6 to obtain the design shear resistance for the voided slab, see Figure 5.16.

For punching shear it is recommended that the void formers are left out where the design shear stress exceeds the reduced shear resistance of a voided slab, see Figure 5.16. Punching shear checks may then be carried out on the solid slab areas around the columns.

5.3.1 Slab geometry

5.3.2 Flexural design

5.3.3 Shear design



57

Figure 5.16Typical layout with fi nal reinforcement in

place.Photo: Cobiax Technologies Ltd

Manufacturers have carried out testing to determine the reduced stiffness of the slabs due to the voids. Conservatively, the stiffness of the voided slab may be taken as 0.87 times the stiffness of a solid slab, although in some confi gurations the factor may be increased to 0.96. The manufacturers have data available to take advantage of these situations.

When using a fi nite element analysis, the stiffness of the slab (by adjusting the modulus elasticity) can be reduced accordingly. The use of the span-to-effective depth rules of Eurocode 2 is not valid for this form of construction since it is not clear how the slab stiffness is incorporated in the manufacturers design expressions.

Splice bars are used across the panel joints so that the slab may be designed as a continuous member. Figure 5.16 shows a typical layout including the fi nal reinforcement.

Buoyancy of voidsWhilst the concrete is being place and vibrated, the buoyancy force can reach the displaced weight. The void formers are held in place by:

firm tying of the void former to the lower and upper reinforcement casting of concrete in several stages (normally two, but three may be required where

the voids are larger than 360 mm).

Slab edgesVoids are not normally provided near slab edges to ensure a robust and continuous edge detail.

5.3.4 Defl ection control

5.3.5 General considerations



58

Prestressed hollowcore units are produced by an extrusion or slipform process with a typical width of 1200 mm, in lengths of up to 200 m. Each length is prestressed before casting. After curing, the units are sawn to the required length. Figure 5.17 shows a typical production layout.

It should be noted that where the only reinforcement in the units is the prestressing strands, as is common, it makes the support zone particularly vulnerable since this is where the maximum stresses due to bearing, shear and anchorage occur. The design should be in accordance with Eurocode 2.

5.4 Prestressed hollowcore units

Figure 5.17Typical hollowcore unit production.

Hollowcore units have lateral edges provided with a longitudinal profi le in order to make a shear key for transfer of vertical shear through joints between contiguous elements. For diaphragm action these joints are designed to resist horizontal shear.

Hollowcore units are often specifi ed from manufacturers’ tables rather than designed from fi rst principles. These tables are based on assumed loading, support and reinforcement details, and where the actual situation varies from that assumed in the tables, e.g. the existence of concentrated loads or different fi re rating, detailed calculations should be made to verify such units are appropriate.

BS EN 11683 describes the requirements and the basic performance criteria and specifi es minimum values where appropriate. It covers terminology, performance criteria, tolerances, relevant physical properties, special test methods and special aspects of transport and erection. Reference should also be made to Precast Prestressed Hollowcore Floors28.

An example of the design of a hollowcore unit is given in Precast Eurocode 2: Worked

Examples29.

Resistance at the end of the hollowcore unit relies on the interaction of shear and bond, therefore it is very important to understand the end prestressing conditions of hollowcore units. Figure 5.18 shows how the stress in the prestressing wires or strands and the moment of resistance, builds up from the end of a unit and further guidance is given in Eurocode 2, Cl. 8.10.2.2.

5.4.1 Anchorage of prestressing tendons



59

The transmission length, lpt, for the prestressing wires or strands is that length required to transmit the full prestress, σp. lpt is defi ned in Eurocode 2, Cl. 8.10.2.3 where σpt1 and lpt1 are the values at ‘transfer’ and σpt2 and lpt2 are the values after all losses (as shown in Figure 5.18). The ultimate design strength of the tendon requires further anchorage length. The slope of the line between σpt2 and σpd is less than that for the transmission length, lpt2, because the tendon reduces in size as it is stressed. The reverse is true within the transmission length over which there is a wedging effect. One reason for assuming a linear build-up of stress is because any fl exural stress in this region will tend to reduce the section size and nullify the wedge effects.

5.4.2 Transmission length

Tendon stress

Distance fromend of unit

Ipt1

Ipt2

Ibpd

pd

pt1

pt2

Figure 5.18Build-up of stress in prestressing wires or

strands from end of unit.

The cracking length, lcr, is the distance from the end of the unit to the point where the bottom fi bre stress resulting from all actions (bending, prestress and horizontal forces at the bearings) equals fctd. Figure 5.18 shows the components of actions and the net effect on the bottom fi bre stress. Note that if lcr is less than lpt2, the prestress is reduced.

Figure 5.19 indicates the results from the example given in the Precast Eurocode 2: Worked

Examples29.

The following points are of particular note: Consider all action effects to determine where the unit is likely to crack. Where dry or mortar bearings are used large horizontal forces may arise from

temperature and shrinkage effects. In this example the horizontal force at the bearing may cause cracking close to the end

of the unit, before lcr is reached, see Figure 5.19(d).

If cracking does occur close to the support, the shear resistance is likely to be exceeded.

5.4.3 Cracking length

σ

σ

σ



60

a) Stress due to flexure b) Stress due to prestress

c) Stress due to horizontal force at support

d) Net bottom fibre stress showing cracking length, lcr

Support

CLof unit

Bottomfibrestress

CL

Support

of unit

Bottomfibrestress

CL

Support of unit

Bottomfibrestress

Bottomfibrestress

0

fctd

lcr

Tension

Compression

CLof unit

Possible overstress

near end of unit

fb,m

= Mx/Z

b

fb,P

= P/Ac + P

e/Z

b

fb,H

= H/Ac + Hy

b/Z

b

fb,Net

= fb, M

+ fb,P

+ fb,H

Figure 5.19Build-up of bottom fi bre stress in concrete

from end of unit.

The total anchorage length, lbpd, is the distance from the end of the unit to the point beyond which the full design resistance of the wires or strands can be obtained, as shown in Figure 5.18.

When the prestress is transferred from the anchor blocks to the hollowcore units, there is anchorage bond along the full length of the strand, apart from the transmission length at each end of the prestressing line. The concrete is then cut into the required lengths and at each end a further transmission length is introduced. Although expressions have been developed to determine the relationship between the end slip of the strands and the transmission length, it has been shown27 that, for hollowcore units that have been sawn, there is no simple relationship between transmission length and initial slip at these positions. This is discussed further in Section 6.6.

5.4.4 Total anchorage length

5.4.5 Tendon slip at ends of units



61

Figure 5.20 shows the three typical types of end failure that may occur. It should be noted that types a) and b) can interact, one reducing the resistance of the other.

Anchorage bond failure, see Figure 5.20a, may occur due to cracking close to the support which does not allow the full anchorage resistance to develop and strands start to slip. This causes the crack to grow until the unit fails. The most common cause of anchorage failure is when the end of the unit is subject to movement relative to its bearing. This may be the result of the effects of one or more of the following: shrinkage temperature changes humidity changes vertical loading.

It is important that the designer considers each of these possible effects. This is especially important for units with spans greater than 8 m. Reference should be made to Movement, Restraint and Cracking in Concrete Structures26.

Cracked sectionsThe cracked shear resistance should be checked at positions likely to be cracked at the ultimate limit state. The position at which this check should be carried out is at a distance lcr from the end of the unit, see Section 5.4.3. The shear tension resistance is calculated in accordance with Eurocode 2, Exp (6.2a and b) together with UK National Annex:

VRd,c = [0.12k(100ρl fck)1/3 + 0.15σcp]bwd

with a minimum of

VRd,c = (0.035k3/2 fck1/2 + 0.15σcp)bwd

5.4.6 Types of end failure

Large crackclose tosupport

Anchorageslip

Shear tensioncrack

Horizontal splitting cracks

a) Anchorage bond failure b) Shear tension failure c) Horizontal splitting cracks

Figure 5.20Types of end failure.

5.4.7 Anchorage bond failure

5.4.8 Shear resistance



62

where k = 1 + (200/d)0.5 ≤ 2.0 ρl = Asl /bwd ≤ 0.02 (normally = 0 since the distance to the end of the unit

< lbpd + d) σcp = NEd/Ac < 0.2fcd (NEd should be taken as γp times the prestress force)

Uncracked sectionsShear tension failure, see Figure 5.20, occurs when the tension in the webs of the slab becomes too high causing a sudden failure. For a circular core section the critical section for a shear tension failure is likely to be at h/2 from the inner face of the support, see Figure 5.21. For oval core shapes the critical section is likely to be closer to the bottom of the section.

Critical position

For circular core shapes = /2For oval core shapes say /3

hhs

Figure 5.21Critical section for shear tension failure.

The shear tension resistance is calculated in accordance with Eurocode 2, Exp (6.4):

VRd,c = I bw/S {(fctd)2 + αlσcp fctd }

0.5

where I = second moment of area bw = width of the cross-section at the centroidal axis S = first moment of area above and about the centroidal axis αl = lx/lpt2 ≤ 1.0 lx = distance of the section considered from the starting point of the transmission

length lpt2 = upper bound value of the transmission length of the prestressing element

according to Exp (8.18) of Eurocode 2 σcp = concrete compressive stress at the centroidal axis due to prestress (this

should include γp = 0.9)

For cross-sections where the width varies over the height, the maximum principal stress may occur on an axis other than the centroidal axis. In such cases the minimum value of the shear resistance should be found by calculating VRd,c at various axes in the cross-section.

(Note: At the time of writing a revision to this expression was being considered by the Eurocode 2 Committee in discussion with the Committee for BS EN 1168.)



63

BS EN 1168, Precast concrete products – Hollowcore slabs3 sets out further design checks that are required:

Prevention of horizontal splitting cracks (Cl. 4.3.3.2.1) Combined shear and torsion (Cl. 4.3.3.2.2) Shear capacity of longitudinal joints (Cl. 4.3.3.2.3) Punching shear capacity (Cl. 4.3.3.2.4) Transverse bending caused by concentrated loads (Cl. 4.3.3.2.5) Additional torsion where one long edge cannot deflect (Cl. 4.3.3.2.6)

It is also critical that the requirments for bearings, see Section 4, are fully satisfi ed, otherwise there is a danger of deterioration of the supporting nibs and ends of the hollowcore units that could lead to a shear and anchorage failure of the hollowcore units.

Floors are not always uniformly loaded; they often are required to carry point loads and line loads from partitions to supporting beams. BS EN 1168, Appendix C, Transverse Load Distribution, charts factors that can be used to determine the loads on units adjacent to the loaded unit. These charts are for use with units in fl oors with no or one free edge. They apply to units without structural topping and are therefore conservative for units with structural topping. BS EN 1168, Cl. 4.3.3.2 provides a method of assessing transverse tensile stresses in the hollowcore units that are un-reinforced in the transverse direction.

Longitudinal tie barsHollowcore units should be connected to the supports or to the adjacent fl oor bay by means of longitudinal tie bars. Tie arrangements should realise the structural integrity and meet the requirements with regard to:

diaphragm action transverse distribution of vertical loads differential settlements restrained deformation robustness (in accordance with Section 3.2).

The longitudinal tie bars should be equally distributed and their spacing should not normally exceed 0.6 m at edge supports and 1.2 m at intermediate supports.

The ties pass through grouted longitudinal joints between units, see Figure 5.22, provided that they are anchored into the members supporting those units (see also section 3.1), or in the concreted cores of the units, see Figure 5.23; in either case it important that the bars are fi xed in the correct position, as shown. If the latter method is used, note that it is essential that, after removing the top fl ange, the open core is thoroughly cleaned to allow good bonding of the new and old concrete.

5.4.9 Further design checks

5.4.10 Lateral distribution of vertical loads

5.4.11 Multi-span without structural topping



64

h/2

Limits to theplacing of tiebar

Limits to theplacing of tiebar

40 mm

a) With grouting key at top b) With grouting key at bottom

≤ 2φ,

≤ 25 mm

≤ 2φ,

≤ 25 mm

Figure 5.22Placing tie bars between hollowcore units.

h/2

Normal limits toplacing of tie bar

φ

Figure 5.23Placing tie bars in hollowcore unit.

The yield load of the tie bars anchored in any core of a unit or between units should not exceed 80 kN and the total yield load per unit should not exceed 160 kN. If the yield load for a tie bar between units is greater than 30 kN, hooked bars should be used. In such cases the anchorage length should not be less than 75φ, as shown in Figure 5.24. Otherwise straight bars may be used with a minimum anchorage length of 100φ.

h/2

≤ 75 φ ≥ lcr

Figure 5.24Minimum length of tie bar between units.

The anchorage length of a tie bar should not be less than lcr (see Section 5.4.3). The anchorage length should normally be suffi cient to anchor the yield load of the tie bar (see also Precast Prestressed Hollowcore Floors28). In order to prevent progressive collapse the anchorage length should be increased by Δlb in accordance with Table 5.1 (see worked example 7).

Concrete grade

C20/25 C30/37Grout 13φ 10φ

Concrete 11φ 9φ

Table 5.1Additional anchorage length, Δlb, for ribbed tie bars with regard to design against progressive

collapse.



65

Where further strengthening of the support zone is required the tie bars should be anchored to transfer their yield load at any cracked section within the critical support zone. In such situations the tie bar should be placed above the mid-height of the hollowcore unit to provide moment capacity and should be anchored with a hook. An additional anchorage length, ladd, should be provided to ensure the shear transfer between the in-situ concrete or grout and the hollowcore unit.

ladd = Fst/fctu

where Fst = tensile capacity of the tie arrangement in one core or joint fct = tensile strength of the in-situ concrete or grout u = perimeter is of the core or 2h for anchorage in joints (h is height of the

hollowcore unit)

Alternatively, straight bars may be used. In this case the anchorage length should be increased to lcr + lbd (see Eurocode 2, Cl. 8.4.4) (+Δlb) for anchorage in concreted cores and to lcr + 100φ for anchorage in grouted joints.

Connections to wallsIf the wall supports more than three fl oors, it is advisable to provide hollowcore units with slanted ends and for the ties to be anchored in the concrete cores (not between units) as shown in Figure 5.25. It is important that the reinforcement is detailed to interlock as shown. If the wall supports less than three fl oors, it will normally be satisfactory for the units to have a square cut, but the reinforcement details should be as shown in Figure 5.25. Details that do not provide a mechanical link should not be used.

Project details

Worked example 7Anchorage length of longitudinal tie bar



Client Date

Consider the use of 20 mm size straight bars with C30/37 grout. lcr = 1080 + 10 x 20 = 1280 mm

Minimum length of 10φ = 10 x 20 = 200 mm.

Use anchorage length = 1280 mm

For example 10 of Precast Eurocode 2: Worked Examples29

RW

OB

TCC

CCIP-030

WE 7/1

April 08



66

a) Edge support b) Intermediate support

Figure 5.25Connections to walls.

b) Internal supporta) Edge support

In-situ concrete

In-situconcrete

Figure 5.26Connections to beams.

Connections to beamsTypical connections to beams are shown in Figure 5.26.

Connections to ledge beamsThe continuity tie reinforcement should interlock with the reinforcement of the supporting beam. A typical detail is shown in Figure 5.27. The fl ange width of the supporting ledge beam should be limited to the continuous solid section at the ends of the hollowcore units or confi ned to the depth of their top fl anges.



67

Transverse tie bars

� � �/ (+ / ) /b b cr

Figure 5.27Typical detail for connections to ledge beam.

5.4.12 Multi-span with structural screed

Tie reinforcement within hollowcores

Tension lap length

Tie reinforcement within structural screed

Figure 5.28Typical detail showing the tie reinforcement

within the structural screed.

b) For Class 2B and over structuresa) For Class 2A structures

Tension lap length

Tying reinforcement within hollowcores

a) For Class 2A structures b) For Class 2B structures

Figure 5.29Typical detail showing connection of tying

reinforcement to an edge beam.

Figure 5.28 shows a typical detail where the tie/fl exural reinforcement is placed within the structural screed.

Where the structural screed is used to provide the tie and fl exural continuity reinforce-ment it should be adequately tied to the perimeter ties. Figure 5.29 shows a typical detail for this.

The permitted deviations are specifi ed in BS EN 1168, which are complementary to those given in Eurocode 2. BS EN 1168 provides further restrictions with respect to cover based on the geometry of the hollowcore units. The following are extracts from that standard.

5.4.13 Dimensions



68

The maximum deviations, unless declared by the manufacturer, shall satisfy the following: slab length ± 25 mm slab width ± 5 mm slab width for longitudinally sawn slabs ± 25 mm

The minimum cover cmin to the nearest concrete surface and to the nearest edge of the core, as stated in BS EN 1168 shall be:

for the exposed face, the one determined in accordance with Eurocode 2, Cl. 4.4.1.2; for preventing longitudinal cracking due to bursting or splitting and in the absence of

specific calculations and/or tests as follows: when the nominal centre to centre distance of the strands ≥ 3φ: cmin = 1.5φ; when the nominal centre to centre distance of the strands < 2.5φ: cmin = 2.5φ; cmin may be derived by linear interpolation between the values of above

where φ is the strand diameter (mm). In the case of different diameters of strand, the average value shall be used for φ.

Double tee beams are ribbed units, usually with two ribs in each 2.4 m wide unit. Other widths can be provided. It is also possible to obtain an inverted trough unit with the ribs at each unit edge. The double tee is the lightest precast unit for spans in the 9 to 20 m range thus requiring a lighter support structure than hollowcore, for example, see Figure 5.30. Alternatives to double tees exist in the form of multi-rib units, usually with three ribs.

5.4.14 Tolerances for construction purposes

5.4.15 Minimum concrete cover and axis distances of

prestressing steel

5.5 Double tee beams

a) Without structural screed b) With structural screed

Figure 5.30Typical double tee units.



69

The shape of the of the double tee unit is particularly suitable and economical for pre-stressing because of the high position of the neutral axis, which maximises the lever arm, and because the ratio of the top and bottom fi bre modulus is similar to the concrete to steel modular ratio.

Double tee units can be procured in a variety of depths, from 300 to 800 mm and even beyond, but the most common unit is 600 mm deep as this conveniently carries offi ce loading to 12 m and car park loading to 16 m. The most common application of double tee units is in car park structures. The top fl ange is usually 50 or 60 mm deep and the ribs taper from a minimum of 140 mm at the base, widening towards the underside of the top table, the taper of 1 in 20 each side allowing for easy lifting out of a fi xed mould. There are variations to dimensions as some manufacturers have fi xed moulds set for the full depth, e.g. 800 mm, and fi t pallets inside to make units of less depth; thus the shallower the unit is, the wider the bottom of the web. It is advisable to check what dimensions are available from the manufacturers at the time of design, although these variations are not usually critical.

In order to achieve maximum economy, grids should be at 2.4 m modules, 7.2 m being the most common. Specially shaped units, to cover irregular grid areas, narrow or tapering units, units with splayed ends and notched units to fi t round columns and others, can be supplied.

Double tee units are normally designed by the precast manufacturer and a typical example of this is given in the Precast Eurocode 2: Worked Examples29.

BS EN 13224, Precast Concrete Products – Ribbed Floor Elements5 provides the specifi cation for materials, production, properties, requirements and methods of testing for ribbed fl oor elements. This includes a section on permitted deviations and minimum dimensions.

A less common system for manufacturing double tees is by using self-stressing moulds. These can incorporate defl ected strands (see Taylor30).

Welding is commonly used in double tee construction as tests and experience show that the welded connection between fl anges is the only method of connection that is positive, taking account of differential camber, and that gives excellent long-term performance with respect to controlling cracking at the fl ange joints in car park construction from the rolling loads (see Figure 5.31).

In car parks it is common for the weld plates and the welded cross-bar to be in stainless steel with the anchor bars beneath the concrete surface in mild steel. Manufacturers have procedures for ensuring the stainless to mild steel welds are made correctly, and account for the higher temperatures required with stainless steel. This can result in more expansion of surface mounted plates and spalling on site.

5.5.1 Self-stressing moulds

5.5.2 Welded joints



70

Where welding is permitted it should be the responsibility of an erection subcontractor and carried out before the structure, including any areas immediately below, is released for access by other trades. Thus safety issues with respect to personnel (arc eye) and fi re in debris beneath are controlled.

It is essential to ensure that the erection subcontractor is experienced in welding work, that modern gas shielded weld procedures are used by trained and tested welders and that site procedures take account of welding hazards with respect to shielding from arcs and in the removal of any fl ammable material from the workplace.

Weld inspection procedures should be agreed with the welder. End connectors are critical and should all be de-slagged and inspected. The number of fl ange connectors usually allows inspection to be on an agreed statistical basis.

Double tee secondary reinforcement usually consists of end cages, which commonly pro-trude from the top surface of the unit to bond into the structural topping, and a special light fabric in the top table (fl ange), sometimes held in place by fi ve or seven stressed wires. To assist shear fl ow from the rib to fl ange at the ends, it is also usual to provide some transverse steel in the fl ange at the end, see Figure 5.32.

100 x 40 shear connector

with 25 φ bar in welded

connection

100 x 100 steel plate

anchored into double tee unit

(The plate should be welded

to the anchorage bar)

Figure 5.31Typical double tee connection details.

5.5.3 Structural topping



71

Where the designer has designed the double tee fl oor as a slab in accordance with Eurocode 2, Cl. 6.2.1 (4) minimum shear reinforcement is not required when VEd ≤ VRd,c. Apart from the main stressed strands, double tee beams often only have reinforcement in the form of a light fabric in the top fl ange to control shrinkage and transportation stresses and a light end cage in the web to control transfer transmission zone stresses.

Structural concrete screed with fabric reinforcement is often provided for the fi nal structure. This is also used to augment the welded connections between units. The lateral shear con-nectors, which should be welded, provide lateral continuity between the double tee units and can spread concentrated loads from one unit to another. The fabric size is defi ned by the need for transverse ties (they augment the welded shear connection tie capacity) and in some cases for load distribution of point loads on the fl oor to adjacent units.

Floors are often required to carry point loads and line loads from partitions to supporting beams. Eurocode 2, Cl. 10.9.3 (5) states that transverse distribution of loads should be based upon analysis or tests. The designer should check any test report carefully to ensure that it covers the specifi c design situation. It is not recommended that differences between the defl ection of units are removed by jacking and then welding the shear keys. Any shear forces resulting from such an operation or any other load variation should be considered in the design of the connections.

Lifter position decidedby supplier

May protrude intostructural topping

Steel end plate withinternal anchor fortie to support

Transverse bars forshear flow with flange

U-bars to close linksat end of web.

Upper strand layermay be debonded

Lower strand layershould never bedebonded

End U-bar or anchoredangle to restrain spallpotential at end of rib

Link cage nominally10 mm at 50, 100 &200 ctrs in end 2 ofunit to aid anchorageof strands

d

Figure 5.32Typical double tee end detail.

5.5.4 Transverse distribution of concentrated loads



72

It is recommended that the width of slab assumed to contribute to the support of concen-trated loads (including partitions in the direction of the span) should not exceed the width of three precast units and joints, plus the width of the loaded area, or extend more than a quarter of the span on either side of the loaded area. In some forms of construction, for example long span wide units, these limits may be inappropriate and more detailed con-siderations should be made. Where there is a reinforced structural topping the width of four precast units and joints may be allowed to contribute. Elliott23 gives further information. Double tee fl oors can be designed either to carry line partition loads by providing extra strength in the unit beneath or by a 2D elastic analysis. The double tee deck can be taken as being comprised of a two-way beam grillage with the beam stiffness in one direction and the fl ange stiffness from the full fl ange depth in the other, even where the fl anges between adjacent double tees meet.

Double tee beams can be provided with additional reinforcement, for example links and additional longitudinal steel for more than the normal one hour of fi re resistance, shear reinforcement for exceptionally heavily loaded cases and top steel for cantilever ends.

Typical end and side connections are shown in Figure 5.33; these connections can be part of the tying strategy of the complete design.

Free standing double tee beams with end and side shear connectors should always be put on elastomeric bearings. A mortar bed may only be used if suffi cient reinforcement is provided through the joint to ensure that it behaves monolithically as shown in Figure 5.33, see also Section 4.1. The welded connection in Figure 5.33a is formed from two surface plates with anchoring reinforcement welded to it cast into and anchoring around the beam longitudinal steel, and in the double tee rib anchoring to the end cage. A surface plate is then placed on top and welded down with fi llet welds. This anchorage can be used as part of the transverse tying of the structure.

5.5.5 Tying requirements

End connector withwelded tie bar

Structuraltopping

Double tee

a) Standard double tee supportwith welded connection

b) Support of double tee with full continuity.Note: Temporary support of beam may be required

End of web End of flange

Figure 5.33Typical double tee connection detail.



73

Double tees cast into in-situ edge beams should have protruding steel as a tie and this steel should be taken far enough into the double tee to ensure that it is fully lapped with the stressed reinforcement.

To create half joints the ends of double tee units may be scarfed, as shown in Figure 5.34. Ends should not be scarfed to more than two-thirds of their depth, for example a 600 mm deep unit may be scarfed to 400 mm. Scarfi ng allows the edge beams to still support the double tee in the temporary situation and be no deeper than the double tee itself. The scarf may also be extended so as to provide a convenient path for services between double tee ribs. Figure 5.34 shows typical reinforcement in a scarfed end. Debonding should never be applied to the bottom strands in the rib or to the strands immediately above a scarf.

5.5.6 Half joints

Strut and tie (1) Strut and tie (2)

Reinforcement and anchorage provided for struts and tie layouts 1 and 2

Chamfer allows inclined tie tobe in optimum position

Only additional reinforcement for the mechanism of strutThis figure is to be read with Figure 5.32

and tie is shown.

Strand must not be debonded

Strand must not be debonded

Strand must bepresent and mustnot be debonded

Figure 5.34Double tee with scarfed end.

A variant of the half joint support is to use a billet protruding from the rib at the end of the double tee at a high level, as shown conceptually in Figure 5.35.

This has the advantage that a nibbed bearer beam is not required and that the bearer beam does not need ‘hang up steel’. A disadvantage is that the bearer beam has no restraint to rotation from the bearing force of the double tee at its soffi t. This lack of restraint should be considered in the temporary condition, when there may be out-of-balance moments on the bearer beam and in the permanent condition for edge beams, or beams supporting

5.5.7 Billet support of double tee units



74

fl oor spans of varying length on each side. Where there is a permanent torsion applied to the beam, the connection to the supporting column should be capable of providing tor-sional fi xity. This should not be a problem if the bearer beam and column are in-situ concrete, but this would be an important design consideration if the bearer beams are precast.

The billet assembly can be purchased as a proprietary item. The designer should ensure that the fi tting is adequate, meets the specifi cation and is suitable for use in the UK. Galvanised fi ttings can have corrosion problems in chloride bearing environments so expert advice should be sought before the fi ttings are used in swimming pool roofs, car parks and exposed coastal locations.

Finally, the fi tting has to be incorporated into the double tee in such a way that it interacts with the other reinforcement in the unit to develop the strut and tie action, conceptually illustrated in Figure 5.35. Internal tie forces required for robustness may also have to be carried by the fi tting. These may not have been considered in the development of the fi tting, particularly if it was manufactured overseas where the traditions of tying structures may not be the same as in the UK.

Two beams are usually supplied in a load and should be secured in such a way that holding-down straps do not bear on the top fl ange edges. The site access must be fi rm without irregularity. Careless handling and the loading of the top fl anges with site construction material can crack the top fl ange of a unit, typically at the ends at the interface between the fl ange and web. Such cracking is unsightly rather than hazardous in the long term and the manufacturer can be consulted to suggest repair procedures that should be carried out before the structural topping is cast.

Precast concrete stairs are produced to be incorporated within many forms of construction. This section considers their use within in-situ and precast concrete frames. Their use has become common, especially within ‘design and build’ contracts, where the speed of construction is a benefi t.

Tie

Tie

StrutDouble tee

The tension in the vertical tie will be about double the value of the compression forces

Figure 5.35Support of double unit using billet connection.

5.5.8 Transportation of long double tee beams

5.6 Stairs



75

It should be noted that stairfl ights are the primary means of escape if a building is subject to fi re or explosion and thus the robustness of the structure is vital.

BS EN 14843, Precast Concrete Products – Stairs8 provides the specifi cation for materials, production, properties, requirements and methods of testing for precast stairs. This includes a section on production tolerances and minimum dimensions. It also describes terms and defi nitions that are used. With regard to detailing it requires the technical documentation to include the construction data, such as the dimensions, the tolerances, the layout of reinforcement, the concrete cover, the expected transient and fi nal support conditions, and lifting conditions. In particular, the technical documentation must include the maximum acceptable gap between components when erected to ensure the design overlap of the reinforcement is achieved, see Eurocode 2, Cl. 10.9.4.7.

When considering the use of any proprietary system it is essential to consider: how the stairflight is adequately tied to the adjacent parts of the structure sequence of construction temporary works involved chain of responsibility in achieving the final structure (often the temporary actions, say

due to props, are the critical design condition).

The following procedure and points should be followed. The working drawings should include complete propping instructions related to the

cube strength of the in-situ concrete (in any event a minimum of four floors should be propped). The sequence of construction and grouting-up instructions (if required) should be stated

on the drawings. The method of levelling should be determined and agreed with the contractor and the method stated on the drawing. The waist dimension should not be less than 100 mm. For a precast stair flight on an in-situ landing nib section the precast flight should be

positioned first before the in-situ landing is cast up against it.

Figure 5.36 shows the main features of a typical single stair fl ight.5.6.1 Single stair fl ights

Tread

Going

WaistRiser

Figure 5.36Typical single stair fl ight.



76

Production tolerancesThe tolerances are given in BS EN 13369, Common Rules for Precast Concrete Products2 and BS EN 148438, table 1, see Table 5.2. Unless stricter tolerances are given in the project specifi cation these should apply.

Target dimension of the cross-section in the direction to be checked

ΔLa (mm)

Δcb (mm)

L ≤ 150 mm +10

L ≥ 400 mm -5±15

± 5+15-10

Key:a The difference between two consecutive risers must not exceed 6 mm.b The minimum concrete cover defi ned in BS EN 14843, Cl. 4.3.7 must take into account the depth of any concrete removed by a fi nishing

process. The positioning of reinforcement shall ensure that the minimum cover defi ned in BS EN 14843, Cl. 4.3.7 is achieved.

Minimum dimensionsThe minimum dimensions given in Table 5.3 should apply.

Dimension Minimum dimension (mm)Thickness of a step or landing 45a

Thickness of a wall 80

Thickness of a parapet 60

Wall thickness of a hollow element 45

Plan dimension of a column 120

Key:a Special care should be taken to ensure the correct position of the reinforcement

Where precast stair fl ights are used supported on in-situ landings, the landings should be cast against the precast fl ight. This avoids the problems of tolerances where precast fl ights are placed on in-situ landings previously cast. Temporary propping will also be required for the precast stairs, see Figure 5.37. Figure 5.38 shows alternative preferred arrangements of the reinforcement at the joints.

Table 5.2Tolerances for stairs.

Table 5.3Minimum dimensions for stairs.

5.6.2 Top and bottom supports with in-situ

connections

In-situ concrete

Precast stair flight

Temporary 2 waybraced props

In-situ concrete

Precast stair flight

Temporary 2-waybraced props

Figure 5.37Temporary support of precast stairs.

Note: It is important that the temporary braced props are supported by a permanent structure.



77

H12 barScreed

Screed

ScreedH12 bar

H12 bar

a) Connection with dowel bar only

b) Connection with hanging and dowel bar

Figure 5.38Preferred arrangements of reinforcement for

connection with in-situ concrete.

5.6.3 Top and bottom supports with precast

concrete

LayoutFigure 5.39 shows the preferred dimensions for the detailing of the top joint between a precast stair fl ight and a precast support.

The design of the bearings shall be in accordance with Eurocode 2, Cl. 10.9.5 and due allowance shall be made for erection tolerances. For the application of this rule, two classes of stair nibs are defi ned:Class A: The stair nib is manufactured with the design end cover in accordance with BS EN 14843, Cl. 4.3.1.1.Class B: The stair nib is similar to Class A but with reduced end cover. In this case the full concrete cover is achieved on site with a non shrink mortar. The result shall be in accordance with Eurocode 2, Section 4.

Recommended bearing typeThe recommended bearing type for precast stairs to precast concrete supports is a 10 mm thick mortar bedding.



78

1590

100 min

9015

100 min

10

90 15

10

100 min

9015

90

90 min

15 90

100 min

15

10

a) Landing with sloping interface

b) Landing with square interface c) Wall with square interface

Figure 5.39Preferred dimensions for top joint between

stairfl ight and precast support.Design and supervision considerationsThe following should be considered during the design and construction process: an allowance for a very generous impact factor on self-weight (say 2 or 3) of the

precast flight checking the consequence if the support is assumed to be at the edge of the in-situ nib

(or designing seating layer to even out the loading) failure mode in shear and hanging tension behind the nib the construction procedure and temporary propping loads are properly understood ensuring that the concrete reaches the required strength no shims are included the reinforcement is checked prior to concreting.

Lapped horizontal connectionFigure 5.40 shows a preferred layout of reinforcement. This may not be the easiest way to construct an acceptable cage but ensures that the dimensions and the positioning of the loop and link reinforcement is correct.



79

Tie reinforcement in structural topping

Tie reinforcement in structural topping

375 15

15 375

Figure 5.40Preferred layout of reinforcement for precast

joints.

120

3540

Screed

70

120

375 15Screed

15 375

Dowel connectionTo provide suffi cient room for a dowel hole the dimensions of the nib need to be as shown in Figure 5.41.

Figure 5.42 shows the preferred layout of reinforcement for dowel connections, and worked example 8 shows a typical calculation.

Figure 5.41Dimensions to allow for dowel hole.

Figure 5.42Reinforcement arrangement for dowel

connection.



80

Project details

Worked example 8Dowel bar for connection of precast stairs



Client Date

Consider a 1.5 m wide stair flight spanning 4 m, with a vertical spacing between precast units of 10 mm and using a 20 mm diameter bar for the dowel. The tying force, FEd required should be at least the permanent action of the stair flight.

ActionsAssume average vertical thickness of stair flight is (150 + 100)√2 = 350 mmSelf weight of stair flight FEd = 25 x 0.35 x 1.5 x 4 = 52.5 kN

ResistanceIt can be shown that the maximum dowel force, FRd, is FRd = φb

2.√(fcd.fyd).{√(1 + ε2) - ε} ≤ Asfyd/√3 (shear resistance of the dowel) where ε = 3(e/φb) x √(fcd/fyd) e = equal to half the vertical spacing between the units Hence: e = 10/2 = 5 mm ε = 3 x (5/20) x √{(0.85 x 40/1.5)/(500/1.15)} = 0.171 and FRd = 202 x √(0.85 x 40/1.5 x 500/1.15) x {√(1 + 0.1712) – 0.171}/1000 = 33.5 kN ≤ (π x 202/4) x (500/1.15)/(√3 x 1000) = 78 kN ∴ FRd = 33.5 kN

No req’d = 52.5/33.5 = 1.57

Use 2 No. 20 mm dia. dowel bars

Steel angles are used to allow the stair fl ight to rest directly onto walls or fl oor units, see Figure 5.43.

5.6.4 Top and bottom supports using steel angles

Figure 5.43Support using steel angles.

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Structural topping

Structuralreinforcement

Full strength weld of reinforcement to angle.Lap with structural reinforcement.

Full strength weld toreinforcement to angle.Lap with structuralreinforcement.

Structural screed

Structuralreinforcement

In-situ structure

The angle provides the bearing onto the supporting structure but often does not have any joint continuity reinforcement. The designer should ensure that the design of the precast unit incorporating such a steel angle is adequate for the particular situation and provides an adequate tie to the structure. One method of achieving this is to weld reinforcement to the steel angle and anchor it to the structure through the screed. Typical top and bottom details are shown in Figure 5.44.

The tension forces transmitted from the angle to the reinforcement within the precast unit in the top joint requires links welded to the bottom of the top angle. These should be designed to resist the forces from the angle with the force of the support at the worst possible position, i.e. when the joint between units is the widest permitted by the tolerances.

The stability of the staircase before the screed has been cast is not normally considered by the manufacturer. It is essential to ensure that any temporary supports are provided and clearly identifi ed in the construction sequence. One example is to provide a positive tie between the fl ight and the landing by reinforcement welded to the bearing angle (at the precast factory) to lap with the fabric in the structural topping.

Stair fl ights can be provided with an upper or lower integral landing as shown in Figure 5.45. It is important that an insert (typically 50 mm) is provided on the top surface of the landing. This allows the top fi nish to be laid uniformly over the whole of the landing surface, avoiding any steps, due to construction and installation tolerances. In order to establish an adequate tie to the supporting structure the reinforcement projecting from the precast unit should interlock with that of the support.

Figure 5.44Bottom and top support details using steel

angles.

5.6.5 Stairs with integral landings



82


50 mm recess forfinish surface

In-situ wall

Bar inserted to lap withwall reinforcement

Two horizontal bars insertedwithin ‘U’ bar to lap withwall reinforcement

b) Stair flight with integral upper landing

Bar inserted to lap withwall reinforcement

50 mm recess forfinish surface

In-situ wall

Two horizontal barsinserted within‘U’ bar to lap withwall reinforcement


a) Stair flight with integral lower landing

Figure 5.45Stairs with integral landings.

Corbels, nibs and half joints are common to many forms of hybrid concrete construction. The correct position of and cover to the reinforcement is critical to the performance of this type of element. The design should carefully specify the requirements through the layout and reinforcement detail drawings.

Corbels should be designed using strut and tie models when 0.4hc ≤ ac ≤ hc or as cantilevers when ac > hc, see Figure 5.46 for defi nitions of hc and ac. Unless special provision is made to limit the horizontal forces on the support, a minimum horizontal force of HEd should be combined with the vertical force FEd. Reference should be made to Section 4.1 concerning the value of HEd.

Corbels, nibs and half joints are examples where non-linear strain distribution exists. For such situations design using strut and tie models is appropriate. Eurocode 2, Cl. 6.5 provides advice and stress limitations for the struts and nodes.

5.7 Corbels, nibs and half joints

5.7.1 Design by strut and tie model



83

Strut and tie model for a corbelFigure 5.46 shows the layout of the strut and tie layout for a corbel.

The following procedure may be adopted to check the strength of the corbel: The stress σ in the strut of width x should be limited to σRd,max = 0.34fck(1-(fck /250)),

see Eurocode 2, Exp (6.56). The value of x effects the angle of the strut and hence the force in the strut. The position of the top of the strut should be determined by the resolution of FEd and

HEd, and the depth to Ftd (aH), as shown in Figure 5.46. The angle and width of strut may be found by iteration or by use of the charts given in

Figure 5.36 of the Manual for the Design of Concrete Building Structures to Eurocode 231. It is recommended that z0 should not exceed 0.75d. The bearing stress under the load should not exceed 0.48 fck(1- (fck /250)), see Euro-

code 2, Exp (6.61). Check the tie force, Ftd = Ftd’ + HEd where Ftd’ is the horizontal component of the strut

force caused by FEd. The total area of secondary links should be at least 0.5 area required to resist Ftd, see

worked example 9.

hcdz0

aHHEd

FEd

ac

Ftd

ac’

x

�

Figure 5.46Layout of strut and tie for a typical corbel.



84

Project details

Worked example 9Corbel design



Client Date

Design ultimate load FEd = 300 kN, fck = 40 MPa, distance to the centre of the tension reinforcement is assumed to be 45 mm. The width of the corbel is 300 mm, other details are as shown below.

Actions HEd = 0.2 FEd = 0.2 x 300 = 60 kN

Geometry y = 175 + 60/300 x 45 = 184 mm z = √(1842 + 6052) = 632.4 mm α = sin -1 (120/(2 x 632.4)) = 5.44º β = tan -1 (184/605) = 16.92º γ = 90 – 5.44 – 16.92 = 67.6º z0/d = (184 tan 67.6)/605 = 0.73 < 0.75 ➝ OK

Strut designMaximum stress in the strut is: σRd,max = 0.34 fck(1-(fck 250)) = 0.34 x 40 x (1 – 40/250) = 11.4 MPa For an angle of strut to the horizontal of 67.6º and strut force is: FEd = 300/sin 67.6º = 325 kNHence the stress: σEd = 326 x 1000/(120 x 300) = 9.1 MPa σRd,max > σEd ➝ OK

Note: Further iteration could be carried out to maximise the strut efficiency.


300 kN

175160

60 kN

bc = 300

45

650

400

300

x = 120

Z0

Ftd

67.64o 605

a) Chosen solution b) Geometry of solution

�

� �

605

yx/2

Z

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Worked example 9Corbel design



Client Date

ReinforcementThe tension force in the reinforcement: Ftd = Ftd’ + HEd = 300 cot 67.6º + 60 = 183 kNArea of reinforcement required: As,req = 183 x 1000/(500/1.15) = 421 mm2 Try H20 bars: No. req’d = 421/(π x 202/4) = 1.34

Use 2 H20 bars

Area of secondary links required = 421/2 = 211 mm2 Try H8 links: No. req’d = 211/(82 x π/4) = 5.2

Use 5 H8 links

See the figure below for layout of reinforcement in accordance with The Standard Method of Detailing Structural Concrete32.

2 H20 bars

H32 bar

5 NoH8 links

Strut and tie model for nibsWhere a nib is connected to the bottom of a beam, Figure 5.47 shows the arrangement of strut and ties for a given arrangement of reinforcement.

The angle of the strut should be determined by the position of the centre of the bottom corner bar of the beam, up to the point of intersection of the resultant of the applied forces and the centre of the tension bar in the nib.

It should be noted that the reaction, Ft2d in the link bar is FEd (zb + ac)/ zb. The value of zb may be taken as 0.8 db. Note this force is in addition to any shear force in the beam link.

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db

ac

FEd

HEd

Ft1d

Ft2d

Zb

Zn

Figure 5.47Nib connected to bottom of beam.

Strut and tie model for half jointsFigure 5.48 shows the arrangement of struts and ties for a typical half joint. The addition of a diagonal bar is not considered essential but does provide a more direct route for the forces and better crack control (see also PD 668721 and The Standard Method of Detailing

Structural Concrete32).

Distance between edge of bearingand inside of bar to be a minimumof the bar diameter or 0.75 x cover,whichever is greater

Cranked bars improve crack control

Horizontal ‘U’ bar withstandard mandrel size

Tension lap

Nominal links at 150

Tension anchorage hh

Full depth links to resist totalreaction equally spaced

a) Section

b) Plan

Figure 5.48Layout of strut and ties for a typical half joint.



87

Construction issues 6

6. Construction issues

The performance of an HCC structure may be affected signifi cantly by the construction method. In order to achieve consistency between design and construction of structures it is important for the designer to include a method statement as part of the project specifi cation indicating the assumptions regarding construction. This will bring clarity to the project and set a benchmark for pricing. The contractor is, of course, free to submit an alternative price based on different assumptions, if any, from the original design. In this process, the performance criteria agreed with the client should not be compromised.

Although precast elements generally require less propping than in-situ elements, it is important to note that the forces in the props are also generally higher and therefore more care is required when considering the temporary works.

Static equilibrium during constructionBS EN 1991-1-6 Eurocode 1: Actions on Structures – Part 1-6: General Actions – Actions

during Execution33 and BS 5975, Code of Practice for Formwork34 provide information on the design of temporary works. The designer should also consider transient situations, for example the effect of temporary overturning forces during construction. BS EN 1990, Eurocode: Basis of Structural Design22, Table A1.2(A) describes the load factors that should be used. Figure 6.1 shows the single arrangement that includes both equilibrium (EQU) and structural resistance (STR).

6.1 Method of construction

Overturning 1.35 + 1.5* G Qk,f k c,

Resisting 1.15* Gk,b

Gk,b

Qk,c Construction

Gk,f floor

Resistancebeam

* Check that using a factor of 1.0 for both favourable and unfavourable

does not give a more unfavourable effect

Figure 6.1Temporary loading during construction.


88

6 Construction issues

For type 5 form of construction (see Figure 2.1) the in-situ concrete is used to knit the precast fl oor and beams together. The support of the precast fl oor should be designed as a bearing in the temporary case and, even though the bearing will eventually be part of an integral system, it will benefi t from neoprene pads beneath the fl oor elements. The outer edge of the supporting beam should include a chamfer to eliminate spalling when the fl oor is loaded onto the precast beam and the full load comes onto the combined system. The chamfer also gives a visually clean edge to the joint between the precast and in-situ concrete if the precast unit is ribbed – a double tee for example. The support and restraint of the beam onto the column should also be considered in the temporary situation, as this connection may not be fully made before the in-situ composite concrete is placed.

For construction types 3 and 4, see Figure 2.1, the precast fl oor is supported on some form of propped system before the in-situ edge beam is poured. The props should be designed for the construction loading and a means to gradually release the supported load onto the composite fl oor should be devised with back propping if necessary to support the fl oors above.

The defl ection of the shuttering of the in-situ edge beam during casting should be considered. If the fl oor and edge beam shuttering are supported from separate propping systems during the pouring of the in-situ concrete the support struts of the in-situ area will take up load and may shorten slightly. The fl oor is on a different set of props and will not shorten as no extra load is applied to it. This can result in cracking of the top of the fl oor near to the support as the moment from the wet concrete is applied. To avoid this risk entirely, the same support system should be used for the fl oor and edge beam shutter, see Figure 6.2.

A neat lower end to an embedded fl oor unit can be achieved by forming a small groove in the in-situ concrete. This allows the edge of the in-situ to fi ll properly, avoids the likelihood of spalling and masks any slight difference in the soffi t level, see Figure 6.3.

a) Separate support systems can cause cracking in precast unit. b) Common support of precast floor and insitu beam.

Figure 6.2Support for connection.


89

Construction joint

Groove detail

Figure 6.3In-situ/precast joint showing groove detail.

The preparation of the base is of paramount importance. The surface of the precast units should be left rough during production and contained shot blasting equipment (which will avoid damaging the unit) used to prepare the surface, unless it can be shown that there will be adequate bond. All loose debris should be removed. Where required, the joints between the units should be grouted at least one day before the screed is placed.

Hollowcore units are manufactured to a 1200 mm module and double tees are normally to a 2400 mm module. It is possible to introduce narrow units into a layout or units tapered in plan if the building layout requires it. In the case of hollowcore, these are cut after manu-facture, but double tees are cast to the required dimensions. In such cases, the manufacturer will be able to advise on how to detail the special units so that they are suffi ciently robust to be delivered and incorporated into the building successfully and to ensure that exposed soffi ts look acceptable. In the case of long span units, for example, it may be preferable to take up a required taper in the last two units rather than have the last unit tapering excessively. Double tees can also be cast as single tees allowing a greater taper in plan than can be provided in a double tee unit, see Figure 6.4.

In situations where long and short units are side by side, for example where lift and stair cores shorten spans, differential cambers can produce diffi culties. This is particularly the case with long span double tees, for example in car parks. A clear span double tee car park unit may have a camber of 30 mm whereas the unit next to it, spanning from a common bearing position at one end to a ramp or stair core, may be 12 m long and have a camber of 10 mm. This difference in level is usually accommodated in practice by bearing the non-common end of the shorter span at a higher level than the long span unit, as shown in Figure 6.5. The designer should consult with the manufacturer to obtain an estimate of these cambers and mark the drawings accordingly.

6.2 Composite action between precast units and

in-situ structural topping

6.3 Specially shaped standard units


6.4 Long and short units adjacent to each other


90

Webbeneath

Key

Figure 6.4Specially shaped standard units.


15 mm camber

30 mm camber

Outer supports at same level

Inner supports set approx. 15 mmhigher to reduce camber stepbetween long and shorter unit

Figure 6.5Long and short unit adjacent to each other.


91


In some countries, it is considered good practice to jack double tee fl anges at mid-span to even out camber differences. This is sometimes carried out by casting loops of reinforcement that protrude from the fl anges, vertically at mid-span, which are then used as purchase points for a crowbar or jack. While the built-in stresses from this process do not affect the ultimate strength of the structural system because of plasticity at the ultimate limit state, it is not recommended, as it can induce local cracking in the fl anges, see also Section 5.5.4.

Double tee units should always be de-tensioned using release jacks that release all of the tendons simultaneously and gradually. This is standard practice in the UK, but not throughout the world. Engineers should be aware of the different practices and ensure that gradual release is specifi ed and carried out. Otherwise bond checks should be carried out.

Hollowcore units are almost unique in that they are manufactured in a continuous length and are sawn to the required length only after the concrete has reached the appropriate strength. The de-tensioning process only de-tensions the strands at each end gradually whereas at the saw cuts a gradual release of tendon stress is not possible. The integrity of the anchorage bond of the tendons can be checked by examining the ‘pull-in’ of the strands at the ends of the unit. Assuming that the anchorage length is in the order of 1000 mm and that the build-up of strain is linear in that length, as stated in Eurocode 2, a pull-in design value of 2 mm can be calculated. However, this does not allow for the thickness of the saw-cut and in practice the measured pull-in is normally less than 1 mm. Manufacturers should check pull-in on units routinely and reject any with excessive pull-ins.

On site, hollowcore units are often lifted into their fi nal position using clamp lifting devices that clamp onto the sides of the unit near to each end. The clamp arms are of such a width that a unit cannot be placed exactly next to an already erected adjoining unit; thus, when the lifting device is removed, the unit has to be moved laterally to close up the gap. This is often accomplished by moving the unit, or ‘barring’ it with a crowbar. While this may not cause damage to a short span light unit, there is a risk of breaking a corner of a long span unit.

Manufacturers recognise that barring of long span and heavy units is not good practice and provide other means of lifting hollowcore units for this situation, e.g. ‘L’ shaped lifting arms or lifting loops cast into the hollowcore units. Lifting loops should be used for the last unit that has to fi t into an exact space. If lifting clamps are used, the unit would have to be placed at an angle, resting on the edge of the previously placed unit, while the clamps are removed and then barred until it drops into place.

Guidance on the safe practice of barring is given in Code of Practice: For the Safe Erection

of Precast Concrete Flooring and Associated Components35.

6.5 Differences of camber in double tees

6.6 Method of de-tensioning double tee units

6.7 Checking strand or wire pull-in for hollowcore units

6.8 Placing hollowcore units into the correct

position


92


Production tolerances are specifi ed in BS EN 13369, Common Rules for Precast Concrete

Products2, Cl. 4.3.1.1. For cross–sectional dimensions L, the permitted deviation is ΔL, and for position of reinforcing steel, prestressing steel and for the design cover c the permitted deviation is Δc. The permitted deviations of cross-sections for structural elements are reproduced in Table 6.1.

Target dimension of the cross-section in the direction to be checked

ΔL(mm)

Δc(mm)

L ≤ 150 mm + 10- 5

± 5

L = 400 mm ± 15 + 15- 10

L ≥ 2500 mm ± 30 + 30- 10

Notes:1. Linear interpolation may be used for intermediate values.2. ΔL and the positive values of Δc (upper permitted deviation) are given to ensure that deviations in cross-sectional dimensions and in position

of the reinforcement do not exceed values covered by the relevant safety factors in the Eurocodes.3. The negative values of Δc (lower permitted deviation) are given for durability purposes.4. In particular, functional specifi cities of the products may require tighter tolerances.5. The given values may be modifi ed by product standards.

The upper permitted deviation for the location of the reinforcement may be determined as the mean value of the bars or strands in a cross-section over 1 m in width, e.g. slabs and walls.

The design cover c of the reinforcement shall be at least the minimum cover, cmin, plus the permitted deviation , Δcdev, or the producer’s guaranteed deviation, whichever is lower.

For principal dimensions other than cross-sectional dimensions:

ΔL = ± (10 + L/1000) ≤ ± 40 mm

whereL is the target size of the linear measure expressed in millimetres

Other types of tolerances may be given by product standards together with the values of the related permitted deviations, e.g. camber of beams. These values will not include the deformation effects of any applied load or of prestressing. In the verifi cation of the measured deviations, such deviations shall be taken into account by computing their value for the test situation, including all the relevant time-related effects.

6.9 Production tolerances

Table 6.1Permitted deviations of cross-section.


93

Special structures - case studies 7

7. Special structures - case studies

This chapter describes two projects that relied upon hybrid concrete construction to realise an architectural requirement: Lloyd’s of London36, 1986, and Bracken House37, 1992.

Both these buildings were constructed within a traditional contract procedure led by the architect. The design engineers and contractors found solutions to ensure that the architect’s intent was achieved with the most suitable use of materials. This required very close cooperation between engineer and architect, with a particular contribution from the specialist precaster. The input of the contractor to the design solution was small on these projects.

One of the most important themes common to both projects related to the design of the structural joints. These were designed either to be: made of in-situ concrete that connected precast elements to in-situ elements or other

precast elements, allowing for reasonably large construction tolerances or made with close tolerance templates that ensured that great care had to be taken to

construct them correctly.

In 1977 the Committee of Lloyd’s decided to redevelop their site located either side of Lime Street, London. Architects Richard Rogers & Partners, with Ove Arup & Partners as structural and service engineers, won a competition by defi ning a design strategy rather than a building. The key points were that it: allowed for maximum flexibility of use gave continuity of trading and preserved the Lloyd’s tradition did not rely exclusively on providing a new ‘Room’ as quickly as possible but gave

Lloyd’s a means of maintaining expansion of business in the short term. The Room is the heart of Lloyd’s and is where the underwriters work.

Two important architectural features included in the design brief were: to show the columns cleanly throughout their height both on the external face and

within the atrium as shown in Figure 7.1. to show an exposed soffit of diagrid beams at 1.8 m centres.

The resulting design produced a rectangular ring fl oor with a central atrium. The span of the fl oor was 16.2 m (9 x 1.8 m) with a fl oor-to-fl oor height of 4.5 m. The fl oor depth was 1500 mm of which 1150 mm was structural. Prestressed in-situ beams span between external columns and those at the atrium as shown in Figure 7.2. Further prestressed beams were required in the corner areas of the building and precast concrete was used for the column brackets, bearing yokes and stub columns.

7.1 Lloyd’s of London

The design included in-situ columns with precast brackets to support the fl oors, see Figure 7.3a and 7.3b.

7.1.1 Achieving a clean column appearance


94

7 Special structures - case studies

Figure 7.1aLloyd’s of London redevelopment, external view.Photo: Copyright Arup

Figure 7.1bLloyd’s of London redevelopment, internal view.Photo: Copyright Arup


95


Precast yoke

Structural topping Prestressed inverted U beam

Stub column

Steel permanent formwork panels

Precast bracket

Figure 7.2Layout of the fl oor components.

Steel dowel

Tolerance pocket withsteel inserts

Precast yoke

Precastbracket

Dip groove

Stainless steelflange

Steel plate withshear studs under

Elastomeric bearings

Waterproofing detail

In-situ node

Prefabricated bracing

Figure 7.3a leftPrecast concrete bracket connection.Schematic layout of brackets.

Figure 7.3b abovePrecast concrete bracket connection.Precast bracket and yoke.Photo: Copyright Arup


96


The design of the bearing had to fulfi l a number of functions: to carry the vertical load from the floor while allowing for relative rotation as the floor

and bracket deflected, see Figure 7.4a to transmit the wind and stability forces from the main building into the bracing

system via the bracket, see Figure 7.4b to restrain the bracket from rotating in plan because this provided stability restraint to

the column at each level, see Figure 7.4c to allow construction tolerance.

7.1.2 Precast brackets

a) Bearing allowing rotation between filterand bracket

b) Bearing restrains column c) Bearing transmits shear from buildinginto bracing

Figure 7.4Design of bearings.

It was decided that all the forces should be carried on the top face of the bracket. The vertical loads were transmitted through elastomeric bearings. The bearing was bonded to a plate that was screwed down on an epoxy levelling bed and so could be replaced if necessary. The horizontal forces were transmitted through four steel dowels. The load on the dowels was too great to transmit directly into the concrete, so steel bearing-plates were cast into the top surface of the bracket with welded shear studs projecting down to transmit horizontal load into the brackets, see Figure 7.3a.


97


Connection of precast bearing to columnThe way the bracket was connected to the column was one of the key points in its design. It was essential that this provided a straightforward construction operation, and the details had to have a proper architectural quality. The solution chosen was to make the bracket an extension of a ring that would be formwork for the column at that point. The ring would identify the bracket on the column, visually, and express the connection. The main bracket reinforcement passed into the column zone within the ring where it turned up and down, while the ring itself contained nominal reinforcement.

The ring gave two possible sequences of construction: The bracket could be placed on the column formwork and the columns and the bracket

filled together. The column could be cast first up to the soffit of the bracket, then the bracket placed

and concreted.

The second solution was chosen because it was thought that it would be too diffi cult to hold the fi ve tonne bracket and column form in place with suffi cient accuracy, since this took place outside the slab. The details of the bracket and column profi le were worked out with the contractor to give grout tight joints while having the necessary visual articulation. The top of the column was slightly tapered to draw the bracket into the correct position on a sealing strip.

Because the brackets and some of the columns were heavily reinforced, great care had to be taken in the design and detailing to ensure that there was no clash. The fact that the columns were circular made the problem worse. The steel was detailed and fi xed, with templates, to precise dimensions that gave a clearance of a few millimetres. As is often the case with such a sensitive and potentially disruptive detail, so much care was taken that all went well.

A precast yoke was designed to transmit the loads from the in-situ prestressed beam to the precast bracket, see Figure 7.3b. The bearing and pockets in the precast bearing were designed to allow the elastic shortening of the prestressed U-beams to take place before grouting the precast yoke. However, the action of prestressing relieved the props of some of the load of the beam grid and transferred it to the bracket. This applied a moment to the column that caused an inward horizontal displacement. It was found to be better to grout the dowels before prestressing, which restrained the column against this displacement. The columns were pre-cambered outwards to allow for the prestress shortening of the U-beam. When a fl oor was cast it was propped down through two levels to limit the amount of load applied to the bracket and hence rotation.

7.1.3 Connection of precast bearing to in-situ prestressed

inverted U-beams


98


The main building contains none of the usual lift, stair and riser cores that can be used for stability, as these were provided through the satellite towers. A form of bracing between some of the columns was chosen, see Figure 7.5.

Where the bracing was required extra connections were built into the precast brackets.

Floor grid constructionThe fl oor-to-fl oor height is 4.5 m, of which 1.5 m is the fl oor itself. Both the structure and the services are exposed, with no false ceilings. Air is supplied through the raised fl oor and extracted at high level through the light fi ttings. The return air is taken out through ducts at stub column level. The permanent formwork panels were made of profi led metal sheets welded to pressed channels, see Figure 7.6. The channels were lipped on the underside to support the anchors for service hangers in the zone of the stub columns. A typical section through the fl oor is shown in Figure 7.7.

7.1.4 Stability

T3 T4

T5

T6

T1

T2

Figure 7.5Main building stability system.


99


Stub columns

Diagrid beams

1800

300

550

440 1500

60100

300

50Floor finishStructural slab

Permanent formwork& acoustic insulation

Figure 7.6Permanent formwork panel with acoustic trays.

Photo: Copyright Arup

Figure 7.7Typical section through fl oor.

The subcontractor developed a formwork system to produce the diagrid beams, see Figure 7.8. Their design was based on folded and welded steel frames with ply faces. Neoprene gaskets were built into the metal sections that also formed rebates at joint lines. The components were fi xed together with bolts and wedges with adjustment for tolerance. The reinforcement cages were supported on purpose-made plastic cradles bolted down to the soffi t form. These ensured accurate cover, and the threaded insert could be used later to restrain the top of partitions. This formwork was excellent; it gave a fi rst-class fi nish and could be put together and taken apart very quickly. It was the key to success of this subcontract.


100


This was a bespoke building and although time of construction was of the essence, the budget was generous. The interaction between client, architect and engineer was crucial and favoured the

‘traditional’ form of contract. Much time and effort was spent to provide the most suitable form of construction and

materials, but the contractor provided very little input to the development. The subcontractor developed a very efficient formwork system. Precast and in-situ concretes were used appropriately to ensure maximum benefit to

the aesthetics, speed of construction and accuracy of construction. Considerable effort and money was spent on setting up mock-ups and prototypes to

identify the most appropriate form of construction. Where it was made clear that great accuracy was required in construction it was

achieved without fuss.

Bracken house is on Cannon Street close to St Paul’s Cathedral, London. In 1986 Obayashi appointed Michael Hopkins as architect and Ove Arup as structural and service engineers to redevelop a building designed by Sir Albert Richardson. When this was listed it was decided to retain the two wings of the building and rebuild the centre block. From an engineering point of view one of the main features of the design was the integration of the structure and services in the centre block and the way this linked to the construction of the facade. The design was based on the principle of a wheel in which circumferential primary services routes around the outside of the building and inside the atrium connect to radial secondary routes running between radial beams, see Figure 7.9.

7.1.5 Points of interest

7.2 Bracken House

Figure 7.8Diagrid beam formwork system.



101


Structural organisation

Ceilingextract

The concept wheel

Extract

Supply

Floor services supplybetween structure

Figure 7.9Combined structure and services concept.

The outer circumferential route is supplied from risers located in the cores between the wings and the centre block. The inner circumferential ring connects to air exhaust risers contained within quadrant shaped columns in the corners of the atrium, see Figure 7.10.

For speed of construction the beams were precast, whereas the columns were cast in-situ because their construction had no time penalty. Alternating in-situ and precast permitted a very simple connection detail; the beam swelled out at the column position and a pocket was left out at this point: the column reinforcement passed through the pocket (see Figure 7.11), which was concreted up before casting the next lift of column. The beams are 650 mm deep and span 12 m from a column at the atrium to a column that is set back 4.2 m from the facade, and then continue with a reduced depth of 350 mm on to a support at the facade. In each of the quadrant corners, eight radial beams are supported on a continuous corbel that springs from the quadrant columns. There are no circumferential beams. The structural slab is in-situ concrete placed on metal decking permanent formwork between the precast beams.


102


Above: Figure 7.10Radial beams and quadrant-shaped column at

atrium corner.Photo: Copyright Arup

Above right: Figure 7.11Column beam connection.


The soffi t of the slab is above the soffi t of the beam and this zone is used for false ceiling, sprinklers, lighting, and the extract air plenum, see Figure 7.12. The zone above the 150 mm slab is used for the fl oor-based air supply, electrical power and communications. The raised fl oor is 300 mm above the beam.

Figure 7.13 shows the fl oor layout during construction.

Floor finish

False ceiling

Lighting, sprinkers and air extractionPrecast concrete radial beams

Air supply, electrical services and communications

950

300

650150

250

In-situ concrete slab on metal decking

Figure 7.12Typical section through fl oor zone.


103


Figure 7.13Floor layout during construction.


Apart from the plan of the site and the retention of the wings, the most important factor governing the design was the St Paul’s height rule, which restricted the height of the building to that of the wings to avoid obstructing the view of the cathedral. To fit six floors within the superstructure height available, while maintaining the clear heights and raised floor depth required of a modern City office, the depth of the floor zone had to be as small as possible. The result is a 12 m clear span, with a 950 mm overall, which provides a clear zone of 300 mm for telecommunications and small power. By placing the slab towards the middle of the beams the benefit of T-beam action is

lost, but it is this, combined with the radial interleaving of structure and services, that leads to the minimum possible depth of the structural and services zone. The financial benefit of the extra floor that this allowed far outweighed the reduction in structural efficiency. Similar to the Lloyd’s contract, the interaction between client, architect and engineer

was crucial and favoured the ‘traditional’ form of contract. Precast and in-situ concretes were used appropriately to ensure maximum benefit to

the aesthetics, speed of construction and accuracy of construction. Metal decking permanent formwork for the slab was chosen for its simplicity and ease of construction. As the structural slab was in the middle of the floor zone, the metal decking was hidden by the false ceiling. There was a strong belief that the joints between precast concrete units should be in

in-situ concrete and that the architecture should reflect this principle.

7.2.1 Points of interest


104

References

References

1 BRITISH STANDARDS INSTITUTION, BS EN 1992-1-1, Eurocode 2: Design of concrete structures-

Part 1-1: General rules and rules for buildings, BSI, 2005.

2 BRITISH STANDARDS INSTITUTION, BS EN 13369, Common rules for precast concrete products, BSI, 2004.

3 BRITISH STANDARDS INSTITUTION, BS EN 1168, Precast concrete products - Hollowcore slabs, BSI, 2005.

4 BRITISH STANDARDS INSTITUTION, BS EN 13747, Precast concrete products - Floor plates for

fl oor systems, BSI, 2005.

5 BRITISH STANDARDS INSTITUTION, BS EN 13224, Precast concrete products - Ribbed fl oor

elements, BSI, 2004.

6 BRITISH STANDARDS INSTITUTION, BS EN 13225, Precast concrete products – Linear structural

elements, BSI, 2004.

7 BRITISH STANDARDS INSTITUTION, BS EN 14992, Precast concrete products – Wall elements:

Production properties and performances, BSI, 2007.

8 BRITISH STANDARDS INSTITUTION, BS EN 14843, Precast concrete products – Stairs, BSI, 2006.

9 BRITISH STANDARDS INSTITUTION, BS EN 13670, Execution of concrete structures, BSI, due 2008.

10 GOODCHILD, C. and Glass, J. Best practice guidance for hybrid concrete construction. The Concrete Centre, 2002, Ref. TCC/03/09.

11 THE CONCRETE CENTRE. Hybrid concrete construction. The Concrete Centre, 2005, Ref. TCC/03/010.

12 THE CONCRETE CENTRE. Precast concrete in buildings. The Concrete Centre, 2007, Ref. TCC/03/031.

13 THE CONCRETE CENTRE. Concrete framed buildings. The Concrete Centre, 2006, Ref. TCC/03/024.

14 GOODCHILD, C.H. Economic concrete frame elements. The Concrete Centre, 2008, Ref. CCIP-025.

15 WILFORD, M. and YOUNG, P. A design guide for footfall induced vibration of structures. The Concrete Centre, 2006, Ref CCIP-016.

16 BRITISH STANDARDS INSTITUTION, BS 8204, Screeds, bases and in-situ fl oorings, BSI, 2003.

17 DEPARTMENT FOR COMMUNITIES AND LOCAL GOVERNEMENT, Building regulations (England

and Wales) Approved document A (2004). DCLG, revised 2006.

18 THE BUILDING REGULATIONS 2000 (Amended), Statutory Instrument 2000 No 2531 Building and Buildings, The Stationery Offi ce, 2000.

19 BRITISH STANDARDS INSTITUTION, BS EN 1991-1-7, Eurocode 1: Actions on structures – Part 1-7:

General actions – Accidental actions, BSI, 2006.

20 BRITISH STANDARDS INSTITUTION, UK National Annex to Eurocode 2: Design of concrete structures

– Part 1-1: General rules and rules for buildings, BSI, 2005.

21 BRITISH STANDARDS INSTITUTION, PD 6687: Background paper to the UK national annexes to BS EN 1992-1, BSI, 2006.

22 BRITISH STANDARDS INSTITUTION, BS EN 1990, Eurocode: Basis of structural design, BSI, 2002.

23 ELLIOTT, K S. Multi storey precast concrete framed structures. Blackwell Science, 1995.

24 NARAYANAN, R. Precast Eurocode 2: Design manual. British Precast, 2007.

25 CONCRETE SOCIETY. Technical Report 43: Post-tensioned concrete fl oors design handbook, second edition. CS, 2005.

26 CONCRETE SOCIETY. Technical Report 67: Movement, restraint and cracking in concrete structures. CS, 2008.

27 MARTIN, L. and PERRY, C. PCI design handbook, sixth edition. Precast/Prestressed Concrete Institute, 2004.


105

References

28 INTERNATIONAL FEDERATION FOR PRESTRESSING. FIP Recommendations: Precast prestressed

hollowcore fl oors. Thomas Telford, 1988.

29 NARAYANAN, R. Precast Eurocode 2: Worked examples. British Precast, 2008.

30 TAYLOR, H. Strand defl ection systems in pretensioned, prestressed concrete. The Structural

Engineer, Vol. 70, No. 5, March 1992.

31 INSTITUTION OF STRUCTURAL ENGINEERS. Manual for the design of concrete building structures

to Eurocode 2. IStructE, 2006.

32 INSTITUTION OF STRUCTURAL ENGINEERS/CONCRETE SOCIETY. The Standard Method of

Detailing Structural Concrete, third edition. IStructE, 2006.

33 BRITISH STANDARDS INSTITUTION, BS EN 1991-1-6, Eurocode 1: Actions on structures – Part 1-6:

General actions – Actions during execution, BSI, 2005.

34 BRITISH STANDARDS INSTITUTION, BS 5975, Code of practice for formwork, BSI 1996.

35 PRECAST FLOORING FEDERATION. Code of practice: For the safe erection of precast concrete

fl ooring and associated components. PFF, 2007.

36 RICE, P. and THORNTON, J. Lloyd’s redevelopment. The Structural Engineer, Vol. 64, No. 10, October 1986.

37 UNKNOWN. Inside job: Bracken House. Architects’ Journal, 27 May 1992, pp. 26–37. Anon.











CCIP-030Published January 2009 ISBN 978-1-904482-55-0Price Group P© The Concrete Centre




Cover photo: Courtesy of Outinord International Ltd.Printed by Information Press Ltd, Eynsham, UK

AcknowledgementsThe authors would particularly like to thank the following people for their support in the development of this design guide:

Tony Jones ArupIan Feltham Arup

The contributions and comments from the Concrete Society Design Group and also from the following people are gratefully acknowledged:

John Stehle Laing O’RourkeGraham Hardwick John Doyle Construction LtdPeter Kelly Bison Concrete Products LtdAlex Davie ConsultantDavid Appleton Hanson Concrete ProductsKevin Laney Strongforce Engineering PlcNorman Brown British Precast Concrete Federation Ltd


in-situ concrete


fl oor slab


beams


fl oor units





Typical hybrid concrete options.Please note this diagram is a repeat of Figure 2.1, page 8.


CC

IP-030D

esign of Hybrid C

oncrete Buildings

R. W

hittle MA (Cantab) CEng M

ICE H. Taylor FREng, BSc, PhD

, CEng, FICE, FIStructE


This design guide is intended to provide the structural engineer with essential guidance for the design of structures that combine precast and in-situ concrete in a hybrid concrete structure. It introduces the options available for hybrid concrete structures, and goes on to explain the key considerations in the design of this type of structure.

Bearings, interface details, consideration of movement, composite action, robustness and the effects of prestressing are all explained in this guide and design examples are included where appropriate. The importance of overall responsibility and construction aspects are also described.

CCIP-030 Published January 2009 ISBN 978-1-904482-55-0Price Group P



CI/Sfb

UDC624.072.33:624.012.3/.4

Robin Whittle has extensive knowledge and experience of designing all types of concrete buildings. He regular contributes to concrete industry publications and is a consultant to Arup. He was a member of the project team which drafted Eurocode 2.

Howard Taylor has extensive knowledge and experience of designing precast concrete elements and buildings, including developing alternative production methods. He is a past president of the Institution of Structural Engineers and is currently chairman of the British Standards Institution Building and civil engineering structures Technical Committee B/525.

Design of Hybrid Concrete BuildingsA guide to the design of buildings combining in-situ and precast concrete


R. Whittle MA (Cantab) CEng MICE

H. Taylor FREng, BSc, PhD, CEng, FICE, FIStructE


CC

IP-032R

esidential Cellular C

oncrete Buildings

O.B

rooker BEng CEng MICE M

IStructE R.H

ennessy BEng(Hons) CEng M

ICE MIStructE

Residential Cellular Concrete Buildings

This design guide is intended to provide the structural engineer with essential guidance for designing cellular-type structures. It is written for the structural engineer who has knowledge of building structures in general but who has limited or no experience of cellular structures. This guide highlights areas that require close coordination between the structural and services engineers and the architect.

Guidance is provided on selecting an appropriate solution, sizing the structure and carrying out detailed design. Detailing issues are covered, some of which should be considered at the early stages of a project to achieve an effi cient building confi guration.

CCIP-032 Published September 2008 ISBN 978-1-904482-46-8Price Group P



CI/Sfb

UDC69.056.5

Owen Brooker is senior structural engineer for The Concrete Centre where he promotes effi cient concrete design through guidance documents, presentations and the national helpline. A consultant by background, he is also author of a number of guides on the application of Eurocode 2.

Richard Hennessy is structures knowledge manager working in the structures discipline development group of Buro Happold. Richard is a structural engineer and was able to bring his fi rst-hand project experience and also Buro Happold’s collective experience of the tunnel form technique to this publication.

Residential CellularConcrete BuildingsA guide for the design and specifi cation of concrete buildings using tunnel form, crosswall or twinwall systems


O.Brooker BEng CEng MICE MIStructE

R.Hennessy BEng(Hons) CEng MICE MIStructE

Conc cellular build cov-v2.indd 1Conc cellular build cov-v2.indd 1 04/09/2008 10:01:3704/09/2008 10:01:37



CCIP-032Published September 2008 ISBN 978-1-904482-46-8Price Group P© The Concrete Centre




Cover photo: Courtesy of Outinord International Ltd.Printed by Alden HenDi, Witney, UK.

AcknowledgementsThe authors would like to acknowledge the input, comments and advice from the following people:

Mike Brown Precast Cellular Structures LimitedHussein Chatur Outinord International LimitedPeter Dunnion Malling Products LimitedKim Elliott The University of NottinghamGraham Hardwick John Doyle Construction LimitedPeter Kelly Bison Concrete Products LimitedAndrew Sims Outinord International LimitedRoy Spurgeon Bell and Webster Concrete LimitedGeorge Tootell PCE LimitedRod Webster Concrete Innovation and Design


Residential cellular concrete buildings

Contents

1. Introduction 3 1.1 What are cellular structures? 3 1.2 Why cellular structures? 4 1.3 What options are available? 4

2. Cellular concrete construction 7 2.1 Maximising the benefi ts 7 2.2 Balconies 10 2.3 Bathroom pods 10 2.4 Early coordination of services 11 2.5 Servicing routes 12 2.6 Screeds and toppings 13 2.7 Cladding 17 2.8 Internal walls 17 2.9 Stability 17 2.10 Ground insulation 18 2.11 Airtightness 18 2.12 Movement joints 18 2.13 Coordination of design 18

3. Performance of concrete in buildings 19 3.1 Fire resistance 19 3.2 Acoustics 20 3.3 Thermal mass 25

4. Structural support 26 4.1 Foundations 26 4.2 Transfer structures 26 4.3 Options for transfer structures 26 4.4 Robustness 30

Cellular Buildings.indd 1Cellular Buildings.indd 1 09/09/2008 11:54:3909/09/2008 11:54:39

5. Crosswall construction 31 5.1 Site 31 5.2 Initial sizing 31 5.3 Structural support at openings 33 5.4 Concrete 34 5.5 Finishes 34 5.6 Screeds 35 5.7 Design details 35 5.8 Construction 38 5.9 Tolerances 39 5.10 Robustness 41

6. Tunnel form construction 45 6.1 Site 45 6.2 Initial sizing 45 6.3 Concrete placing and curing 48 6.4 Finishes 48 6.5 Design checks required 49 6.6 Design 49 6.7 Construction 50 6.8 Robustness 53 6.9 Health and safety 53 6.10 Alternatives to tunnel form 53

7. Twinwall 54 7.1 Site 54 7.2 Initial sizing 54 7.3 Concrete 56 7.4 Finishes 56 7.5 Design details 56 7.6 Construction 58 7.7 Tolerances 59 7.8 Robustness 60

Appendix A. Volumetric precast concrete prison cells 61Appendix B. Crosswall worked example 63Appendix C. Tunnel form worked example 66References 69

2


3

Introduction 1

1. Introduction

Concrete cellular structures are used extensively for residential buildings. In concept they are structurally simple but they require attention to detail to realise the benefi ts of ease of construction and economy.

This guide is written for the structural engineer who has knowledge of building structures in general but who has limited or no experience of designing concrete cellular structures. It highlights areas that require close coordination between the structural and services engineers, the architect and importantly the system supplier.

It also provides guidance on selecting an appropriate solution, sizing the structure and carrying out detailed design. Detailing considerations are explained, some of which have to be considered at the early stages of a project to achieve an effi cient building confi guration.

Imagine some boxes, stacked upon one another, to gain a good impression of a cellular building (see Figure 1.1). Each box can be considered to be a cell with walls, a soffi t and a fl oor. The term cellular structures refers to cellular buildings where the walls of the cells are structural elements.

1.1 What are cellular structures?

Figure 1.1Example of a cellular building.

Photo: Outinord International Ltd


4

Cellular buildings are particularly effi cient for residential sectors such as: apartments hotels student residences key-worker accommodation prisons military barracks.

Where the building use leads to clearly defi ned, permanent walls, cellular structures are very effi cient. In addition to carrying the vertical and horizontal loads the concrete walls can meet the following requirements:

provision of fire resistance and compartmentation provision of acoustic separation concealed electrical services distribution minimal finishes to walls thermal mass, which can be used as part of a fabric energy storage (FES) design.

In addition, the systems in this guidance document have been refi ned to provide the following benefi ts:

fast construction thin structural zone because the floors span on to line supports (150 to 250 mm

depending on floor span) party walls as slim as 150 mm (depending on the solution adopted).

There are three main systems of concrete construction available for cellular structures: tunnel form, crosswall and twinwall. With all these systems the early involvement of the specialist manufacturer or supplier will bring benefi ts in the form of expert advice and experience. They will be able to maximise the effi ciency, productivity, buildability and cost-effectiveness of their systems for your project. The various systems are all described below and further expanded in subsequent chapters.

An alternative system available for prisons comprises four individual cells cast as one volumetric unit complete with all furniture, sanitary ware and services. This is a specialist product, for which all the design and detailing is undertaken by the supplier. Further information is provided in Appendix A.

Tunnel form is a formwork system used to form cellular structures from in-situ concrete (see Figure 1.2).

The system consists of inverted L-shaped ‘half tunnel’ forms which, when fi tted together, form the full tunnel. The system also incorporates gable-end platforms and stripping platforms for circulation and to strike the formwork. The cellular structure is formed by

1.2 Why cellular structures?

1.3 What options are available?

1 Introduction

1.3.1 Tunnel form


5

pouring the walls and slab monolithically. The system uses a 24-hour cycle. The formwork from the previous day’s pour is struck fi rst thing in the morning, as soon as the minimum concrete strength has been reached. The forms are then lifted into position for the next pour, the reinforcement fi xed and the concrete poured that same day.

Introduction 1

Figure 1.2Tunnel form project, University of East Anglia.

Photo: Grant Smith Photography

Figure 1.3High-rise tunnel form project,

Paramount, Atlanta.Photo: Outinord International Limited

Tunnel form buildings have been built up to 44-storeys high (see Figure 1.3), but the system is often used for low-rise housing as well. It is widely used across mainland Europe and in other parts of the world.

Strictly, all forms of cellular construction could be referred to as crosswall; however, in recent years the term has been used specifi cally to refer to precast concrete crosswall and for ease of reference this meaning has been adopted in this design guide.

Crosswall is a modern and effective method of construction that employs factory precast concrete components (see Figure 1.4). Each component is custom designed and manufac-tured to suit the specifi c project. Load-bearing walls across the building provide the means of primary vertical support and lateral stability, with longitudinal stability achieved by external wall panels or diaphragm action taking the load to the lift cores or stair shafts.

Structures up to and including 16 storeys have been completed in the UK using crosswall construction. Projects up to 48 storeys high have been built in mainland Europe (see Figure 1.5).

The construction method incorporates a series of horizontal and vertical ties, designed to comply with the Building Regulations, which specify that a building should not be

1.3.2 Crosswall


6

1 Introduction

Right: Figure 1.4Typical crosswall project, University of

East London.Photo: Bell and Webster Concrete Limited

Far right: Figure 1.5High-rise crosswall project, Strijkijzer,

the Netherlands.

susceptible to progressive collapse. The precast units, which are designed for ease of construction, fi t together with the minimum of joints to enable rapid sealing. Units are temporarily propped and then stitched together using a series of hidden joints that are grouted as the works progress.

Twinwall construction is a combination of precast and in-situ concrete construction. Each wall panel consists of two skins of precast reinforced concrete which are temporarily held in position by lattice girder reinforcement. The concrete skins are effectively permanent formwork, with the benefi t that they are used structurally in the fi nal condition (see Figure 1.6). The weight of a panel the same size as a fully precast panel is therefore reduced, permitting the use of larger panels or smaller cranes.

The wall panels are placed in position using similar methods to the crosswall elements. For the fl oors, lattice girder slabs are generally used. These have a thin precast concrete soffi t often called the ‘biscuit’, which includes the bottom reinforcement and acts as permanent formwork. Once the walls and fl oor units are positioned, reinforcement for the slab and to tie the walls and slabs together is fi xed. In-situ concrete is then poured into the void in the twinwall panels and on top of the biscuit of the lattice girder slabs.

1.3.3 Twinwall

Figure 1.6Typical twinwall project.

Photo: John Doyle Construction Limited


7

2. Cellular concrete construction

This section outlines how to maximise the benefi ts of concrete cellular structures and highlights some of the typical considerations that arise during design and construction. Early discussions with the system suppliers and their continued input during development will enable the benefi ts to be maximised.

Speed of construction and tight construction programmes are primary considerations in most building projects. Understanding the manufacturing and construction processes is essential to producing a structure that is simple to fabricate and erect. Tunnel form, cross-wall and twinwall offer signifi cant advantages for speed of construction and their main benefi ts include:

Systemised construction gives certainty to construction works, enabling a regular rhythm in the construction cycle.

Systemised construction reduces labour dependencies. Careful planning results in the majority of party walls being integral to the final structure

with minimal need for infill partitions. The cellular concrete walls do not require a full coat of plaster. Good workmanship can

also avoid the need for a plaster skim coat, hence less follow-on trades (especially ‘wet’ trades) compared to columns with infill walls.

Flush walls and ceiling are possible due to lack of columns. Electrical service distribution can be built into the structure.

High construction tolerance enables use of prefabricated façade units, fit-out units and flooring.

Repetition of the elements reduces costs, and even complex panels can be cost-effective if the moulds are reused a sufficient number of times (see Figure 2.1).

2.1.1 Speed of construction and buildability

Cellular concrete construction 2

2.1 Maximising the benefi ts

Figure 2.1Repetition can make complex projects cost-

effi cient.Photos: Bison Concrete Products Limited

The continuous cellular walls and fl oors are inherently robust and can easily meet the requirements for design against disproportionate collapse with appropriate reinforcement detailing. This is easily achievable with tunnel form and twinwall construction due to the use of in-situ concrete. Precast crosswall construction can also achieve this level of robustness if attention is paid to the detailing and construction of the joints between panels.

2.1.2 Robust structure


8

Cellular methods will almost certainly use good-quality facing to the formwork, giving high-quality smooth fi nishes. The quality of fi nish can be good enough to accept direct fi nishes, although often it is desirable to prepare the surface – at worst a skim coat is all that is required.

The surface is very hard-wearing, reducing maintenance, especially compared with stud walls. This also gives signifi cantly improved security for party walls for luxury apartments or secure accommodation.

Concrete has its own inherent fi re resistance which is present during all construction phases, and is achieved without the application of additional treatments and is maintenance free. As concrete is non-combustible and has a slow rate of heat transfer, it is therefore suitable as a separating material as well as maintaining its structural resistance during a fi re. In a cellular structure the use of concrete walls and fl oors is ideal for providing compartmen-tation where required to comply with the Building Regulations. Further details are given in section 3.1.

The use of solid concrete walls and fl oors gives an inherent acoustic performance. High levels of performance are achieved with tunnel form because of the monolithic nature of in-situ concrete. Precast crosswall and twinwall construction can also achieve high per-formance through correct detailing and construction of the joints between panels. Further details are given in section 3.2.

Effi cient cellular construction requires the principal load-bearing walls to be aligned vertically between fl oors. The locations of these walls are usually governed by the need for solid party walls between apartments or between bedrooms in a hotel (see Figure 2.2).

Party walls that are not vertically aligned will not be part of the load-bearing structure. However, they can still be formed using the same cellular construction techniques to give acoustic, fi re and security benefi ts.

There are sometimes exceptions. Solid load-bearing walls can be used for internal walls in large apartments, either to reduce the fl oor span, or to allow a consistent grid determined by other constraints. Similarly, the cellular units for a hotel unit could be constructed as a double room width which is then subdivided with a studwork or masonry party wall.

As with all forms of construction, repetition gives signifi cant savings in time and material cost. Repetition and systemisation need not be aesthetically dull. Complex layouts can be achieved through careful use of three or four modules in varied combinations. However, developing highly varied layouts without regard to buildability will result in high labour costs and material wastage, regardless of the technique or material used.

2.1.3 Hard-wearing quality fi nishes

2.1.5 Acoustic performance

2 Cellular concrete construction

2.1.4 Fire

2.1.6 Constraints to layouts


9

Bathroomarea

Services riser

Corridor

a) Linear arrangement b) Circular arrangement

c) Arrangement around a core d) Curved arrangement

Figure 2.2Typical layouts for cellular structures.


The site location and layout will infl uence the form of construction chosen. Tunnel form construction requires the formwork to be moved out horizontally from the building before moving along the building or lifted to the fl oor above. The tunnel form elements can be split down into a number of smaller units where space is restricted or where there is limited crane capacity. Maximum effi ciency is achieved using single units for an entire cell.


10

2.3 Bathroom pods


Crosswall construction requires space for deliveries and unloading of the units. Suitable cranage is also required and this can be in the form of tower cranes for high-rise construction or mobile cranes for low-rise construction. Both types of crane will require detailed plan-ning to ensure that all the necessary lifting operations can be performed. The precast units used in crosswall construction can be relatively heavy and therefore the crane may need a higher capacity than for other forms of construction.

Cantilevered balconies can be incorporated into the design for most of the systems. This is simplest to achieve with two-way spanning fl oors such as with tunnel form and lattice girder fl oors, which provide a backspan to the cantilever. Precast solid slabs also provide a backspan but the length is limited to the panel width and this may restrict the cantilever length.

Bathroom pods are commonly used in this type of building and their use has implications for the design. Bathroom pods are preassembled and self-contained (see Figure 2.3). They include all the bathroom furniture, services and fi nishes. All that is required on site is to put them in position and to connect the services.

2.2 Balconies

Figure 2.3Typical bathroom pods.

Photos:Outinord International Limited& Buchan Concrete Solutions

To make effi cient use of service risers, bathroom pods are usually located back to back around the service riser. This usually results in four pods concentrated around one area of the slab. This may occur at the walls (supports) or mid-span if alternate walls have been omitted. This later option should be given careful consideration at design stage because there will be a concentrated load as a result of the weight of the bathroom pods.


11


In crosswall and twinwall construction, it is possible with coordinated deliveries to place the pods directly onto the fl oor units while the cell is open, before the next fl oor slab is placed to close the cellular unit.

In tunnel form construction, the formwork is in place on the working fl oor and it is there-fore only possible to place the pods once the formwork has been struck. Hence the pods are lifted to a loading platform and manoeuvred into position (usually with a winch). This is the same installation method as for conventional concrete structures.

Various materials can be used for the structure of the pods including precast concrete, cold-formed steel and composite materials. They usually have a fl oor complete with fi nishes but increasingly are supplied without a fl oor. Where the pod is provided with a fl oor it introduces a confl ict in that, ideally, the fi nished fl oor surface of the pod should match the surface level of the surrounding fl oor. In some situations it is acceptable to have a step from the general fl oor level into the bathroom.

For tunnel form construction the recess can be formed in the slab below the position of the pod to maintain a level access. The method for precast structures is to use a thinner fl oor unit for the span under the pod, and to place a screed over the remaining area of the thinner unit to bring the adjacent fl oor up to the same level as the pod fl oor.

The pods are also the heaviest load on the fl oor slab (see Table 2.1), so the thickness of the slab beneath the pod will govern the design. Therefore the minimum thickness of the supporting slab is usually 150 mm.

Type of bathroom pod Typical loading (kN/m2)Lightweight steel frame 2 to 3

Composite materials 3 to 4

Precast concrete 5 to 8 Notes1. The weight of bathroom fi ttings has been ignored (can be considered to be imposed loads).2. An appropriate wall fi nish has been assumed to the external faces of the pod.3. No allowance has been made for heavy fi nishes such as wall and fl oor tiles.

The early coordination of services is key to achieving an effi cient design: The services in a residential building are widely distributed – every unit requires plumbing,

heating and lighting. This is in contrast to other uses for buildings where the plumbing is located in discrete areas.

The residential sector requires each unit to have independent, metered supplies. Horizontal services distribution is only possible along corridors, as opposed to the

flexibility of a commercial project with a suspended ceiling. Every unit has vertical distribution of waste pipes which have to be coordinated with

the structural frame.

Table 2.1Typical loads from bathroom pods.

2.4 Early coordination of services


12


As well as the need to coordinate, as noted above, this form of construction also enables the fi rst fi x for services to be incorporated within the concrete construction (see Figure 2.4a & b). This is more durable and aesthetically pleasing compared to surface-mounted distribution.

This does require a different approach to procurement. An earlier start to detailed design and setting out of the services is required. Hence the mechanical and electrical engineering contractor should be engaged suffi ciently early in the project to allow embedded services to be detailed before work on the structure starts on site.

This also allows the opening sizes to be suffi cient for the needs of the project without being made unnecessarily large to allow for all eventualities. Overly large openings increase costs in terms of structure, fi reproofi ng the openings, and the resulting loss of usable fl oor area.

For most cellular structures the water and waste services will be distributed vertically to each unit or pair of units. A vertical riser is usually located in the bathroom area in the corner adjacent to the corridor (for maintenance access) and an adjoining unit. A key decision to be made early on is whether to have a riser for each room or for there to be one riser for each pair of rooms (see Figure 2.5).

Having one riser for each room has a number of advantages: The wall dividing the units is taken through to the corridor and avoids flanking noise. The wall provides a vertical support adjacent to the corridor, simplifying the floor

structure. Plan layouts are more flexible: the bathrooms do not need to be located back to back.

However, providing one riser for a pair of units also has advantages: There are less risers required: one supply pipe can service two units, hence the cost of

services is reduced. The floor area required for servicing the building is reduced. Less fire resistance is required.

Right Figure 2.4a)Electrical services distribution cast into wall.

Photo: Outinord International Ltd

2.5 Servicing routes

Above Figure 2.4b)Detail showing electrical services distribution

cast into wall.Photo: Buro Happold


13

Services riser

Structural concrete wall

Non-structural walls

Services riser

Structural concrete wall

Non-structural walls

Figure 2.5Alternative arrangements for services risers.


Typically, both solutions will be adopted to service a varied fl oor layout. For example, the suites in the prime corner location on a hotel fl oor plan will have a single riser, while pairs of standard rooms with back-to-back bathroom pods will share a riser.

Specifying the correct depth and type of screed starts early in the design process. Ideally, the use of a screed should be avoided by fi nishing in-situ concrete so that it is suitable to receive the fl ooring.

Levelling screeds are likely to be used with solid precast units whereas it is more likely that a structural topping (wearing screed) will be used with hollowcore units and lattice girder slabs. With hollowcore fl oors the screed may form part of the design against dispro-portionate collapse and may also be part of the composite fl oor. In lattice girder fl oors, the structural topping (wearing screed) will always act compositely with the precast concrete and it is more appropriate to consider the cast-in-situ portion of the slab as structural concrete. In tunnel form construction the requirement for a screed is usually avoided, but smoothing compounds using latex or synthetic polymer may be required prior to laying the fl oor fi nishes.

2.6 Screeds and toppings2.6.1 Specifying a screed


14

The screeds can be either a traditional cement–sand screed or more recently developed proprietary pumpable self-smoothing screeds. The appropriate uses for these different types of screeds are explained below.

There are particular defi nitions which it is important to understand when specifying screeds. The latest versions of Parts 1, 2 and 7 of BS 82041,2,3 adopt the European defi nitions which can cause some confusion. The defi nitions are given below with clarifi cations where necessary.

Screed Types Defi nitions

Bonded Screed laid onto a mechanically prepared substrate with the intention of maximising potential bond.

Cement–sand screed Screed consisting of a screed material containing sand up to a 4 mm maximum aggregate size.

Fine concrete screed Screed consisting of a concrete in which the maximum aggregate size is 10 mm.

Levelling screed Screed suitably fi nished to obtain a defi ned level and to receive the fi nal fl ooring. It does not contribute to the structural performance of the fl oor.

Pumpable self-smoothing screed

Screed that is mixed to a fl uid consistency, that can be transported by pump to the area where it is to be laid and which will fl ow suffi ciently (with or without some agitation of the wet material) to give the required accuracy of level and surface regularity. It should be noted that pumpable self-smoothing screeds are often known as self-levelling screeds.

Unbonded Screed intentionally separated from the substrate by the use of a membrane.

Wearing screed Screed that serves as fl ooring. This term was formally known as high-strength concrete topping. It is also used to refer to structural toppings as well as wearing surfaces.

Cement–sand screedsThese are traditional screeds and are suitable for all applications, provided they are specifi ed correctly. The biggest drawback is the drying time; BS 82034 estimates the drying time for a sand–cement screed as one day for each millimetre of screed thickness up to 50 mm thick. Further guidance on drying times can be found in the code.

Calcium sulfate pumpable self-smoothing screedsThese screeds can be laid as bonded or unbonded. They can be laid in much larger areas than cement–sand screeds, at a rate of around 1000 m2/day. However, they must not be used with reinforcement because the calcium sulfate is corrosive to steel in damp conditions. They are also generally not suitable for use in damp conditions or where wetting can occur. These screeds are all proprietary products and therefore vary from one supplier to


2.6.2 Defi nitions

2.6.3 Which type of screed?


15

another; the guidance given here is therefore generic and the manufacturer should be consulted before specifying. If they are intended to be used as a wearing screed (structural topping) then the manufacturer should be consulted.

Bonded cement–sand screedRecommendations for levelling screeds are given in BS 8204 Part 11, which recommends that the minimum thickness of a bonded levelling screed should be 25 mm. To accommo-date possible deviations in the fi nished levels of the structural concrete, the specifi ed thick-ness should normally be 40 mm (with a tolerance of ±15 mm); this ensures a minimum screed thickness of 25 mm. However, CIRIA report 1845 recommends that a tolerance of ±10 mm be adopted with a nominal depth of 35 mm. This minimises the risk of debonding, but it should be noted that the tolerances specifi ed for the top surface of the base concrete should be compatible.

Where the bonded screed needs to be greater than 40 mm the following options are available to reduce the risk of debonding:

Use modified screed or additives to reduce the shrinkage potential. Use fine concrete screed, which reduces the shrinkage potential; this has been used

successfully up to 75 mm.

Bonded calcium sulfate pumpable self-smoothing screedRecommendations for pumpable self-smoothing screeds are given in BS 8204 Part 73, which recommends the minimum thickness of a bonded screed should be 25 mm. Manufacturers quote maximum thicknesses of up to 80 mm and therefore there are less restrictions on the overall thickness. A nominal depth of 40 mm with a tolerance of ±15 mm can be comfortably specifi ed.

Unbonded cement–sand screedThe screed thickness should not be less than 50 mm, therefore to allow for deviations in the fi nished levels the specifi ed design thickness should be a minimum of 65 mm for a tolerance of ±15 mm.

Unbonded calcium sulfate pumpable self-smoothing screedThe screed thickness should not be less than 30 mm, therefore to allow for deviations in the fi nished levels the specifi ed design thickness should be a minimum of 45 mm for a tolerance of ±15 mm.

Bonded screedRecommendations for wearing screeds are given in BS 8204 Part 22, which recommends the minimum thickness of a bonded wearing screed should be 20 mm (in contrast to the 25 mm given for a levelling screed in Part 1). To accommodate possible deviations in the fi nished levels of the structural concrete, the recommended thickness is 40 mm. However, the guidance in CIRIA Report 1845 recommends that a tolerance of ±10 mm is adopted


2.6.4 Thickness of levelling screed

2.6.5 Thickness of wearing screed (structural topping)


16

with a nominal depth of 30 mm. The specifi cation for the base concrete surface should be compatible. In some circumstances the design thickness will have to be increased above 40 mm, but it should be noted that there is an increased risk of debonding.

For hollowcore units, which often have an upwards camber, especially for longer spans, a nominal thickness of 75 mm, rather than 40 mm, should be specifi ed. The risk of debonding is mitigated because it is usual to use a concrete of class C25/30 or above and mesh rein-forcement. Using concrete rather than sand–cement screed reduces the shrinkage potential and the reinforcement in particular controls the drying shrinkage. This should ensure there is suffi cient depth at mid-span (i.e. the point of maximum camber) to allow for lapping the reinforcement while still maintaining cover to both surfaces. Even so, loose bars or mesh reinforcement with ‘fl ying ends’ may be required to allow lapping of the reinforcement near the point of maximum camber.

Unbonded screedThe wearing screed should be at least 100 mm thick, but to minimise the risk of curling, consideration should be given to increasing the depth to 150 mm.

Other criteria may have an impact on the design including: slip, abrasion and impact resistance type of traffic on the floor levels and flatness appearance and maintenance type of flooring to be used or applied drying out moisture in screed location of movement joints.

There is insuffi cient space to give any further details here, but BS 82041,2,3 and CIRIA report 1845 give ample guidance and should be referred to.

For all types of bonded screeds (both sand–cement screeds and calcium sulfate screed) preparation of the base is of paramount importance. The structural concrete base should be at least class C28/35 concrete with a minimum cement content of 300kg/m3.

For precast units the surface of the units should be left rough during production and should be thoroughly washed and cleaned, for example by wire brushing, to remove all adhering dirt. Where required, the joints between the units should be grouted at least one day before the screed is placed. Where the levelling screed is designed to act compositely with the units and additional preparation of the units is required, contained shot-blasting equipment should be used to avoid damaging the units.


2.6.6 Other design criteria for screeds

2.6.7 Base preparation


17

Where any bonded screed is required over in-situ concrete then all contamination and laitance on the base concrete should be entirely removed by suitable mechanised equipment to expose cleanly the course aggregate. All loose debris and dirt should be removed preferably by vacuuming.

With tunnel form construction the ends of each ‘cell’ are open, with no structure. The cladding system therefore requires some form of support. This is the same situation as for any other framed building and the same cladding systems are available. The gable ends of tunnel form are solid concrete walls and therefore the cladding can be fi xed directly to it.

For twinwall and crosswall construction it is more usual to close the end of each cell with a precast panel. These panels can be plain concrete and when they have been placed, the cladding can be fi xed directly to them. Alternatively, the cladding material can be prefi xed to the precast concrete in the factory and the completed panel is then brought to site. There are many materials that can be fi xed to the cladding panel including bricks, brick-slips, tiles and stone facings such as granite, limestone and slate. Alternatively, sandwich panels can be used, where insulation is fi xed between two concrete layers. This avoids the requirement to place insulation within the building footprint, thus saving internal space and removing an additional trade. Further information can be found in Precast Concrete

for Buildings6.

Corridor walls and dividing walls between rooms are usually constructed using concrete block walls or dry lining. It is particularly important to consider the detailing of the corridor walls to avoid fl anking noise.

In general, cellular structures are good at resisting lateral loads. The number of structural walls make these types of structure very stiff. However, there are occasions when the stability design requires further consideration. Examples are as follows:

Where lateral loads act perpendicular to the walls in tunnel form construction, or in crosswall/twinwall construction where there are no structural panels at the ends of each cell.

Where shear walls are placed around the lifts and/or stairs, the floor must act as a diaphragm. If precast floor units are used, they should be adequately tied together. Further guidance can be found in Multi-storey Precast Concrete Framed Buildings7.

For taller buildings using precast wall panels the bearing interface between the panels should be checked. Further guidance can be found in Multi-storey Precast Concrete

Framed Buildings7.

Temporary stability has not been considered explicitly in this document but should be considered during construction. The designer should make the contractor aware of the permanent stability system and request method statements demonstrating temporary stability.


2.7 Cladding

2.8 Internal walls

2.9 Stability


18

Residential structures may require insulation to the ground fl oor to meet the requirements of Approved Document L8. The amount of fl oor insulation required is dependent upon the size, shape and the ratio between the perimeter and area (P/A) of the building footprint. A com-petent person should determine whether a specifi c project requires ground-fl oor insulation.

Where insulation is required it can be accommodated with both suspended and ground-bearing slabs. In both cases the insulation can either be placed beneath or above the slab. Where insulation is required beneath a ground-bearing slab, there are insulation products available to transfer the loads from the slab to the ground without crushing.

Approved Document L8 requires pre-completion pressure testing. Failing these tests means a time-consuming process of inspecting joints and interfaces, resealing where necessary. All the systems in this publication have fl at soffi ts and simple edge details which are easy to seal, and consequently have a low risk of failure.

For structures over 30 m in length movement joints may be necessary. It is not within the scope of this publication to provide guidance on this subject; however, detailed advice can be found in Movement, Restraint and Cracking in Concrete Structures9.

The system supplier will often undertake the design of the system components but there should be one engineer who takes overall responsibility for the structural design. This engineer should understand the principles of the design of the system and ensure it is compatible with the design for other parts of the structure, even where some or all of the design and details of those parts and components are made by others.

When the specialist system supplier is appointed, the roles and responsibilities of the designers should be clearly set out, especially when the specialist is taking signifi cant design responsibilities.


2.10 Ground insulation

2.11 Airtightness

2.12 Movement joints

2.13 Coordination of design


19

Performance of concrete in buildings 3

3. Performance of concrete in buildings

As noted in section 2.1.4, concrete is inherently fi re resistant. The design standards for concrete provide guidance to enable the designer to ensure suitable performance in fi res of varying duration.

Eurocode 2, Part 1–2: Structural fi re design10 gives a choice of advanced, simplifi ed or tabular methods for determining the fi re resistance. The simplest method to use is the tabular method and a summary of the appropriate tables are presented here in Tables 3.1 and 3.2. The term axis distance is explained in Figure 3.1.

3.1 Fire resistance

Standard fi re resistance Minimum dimensions (mm)

One-way spanning slaba,b Two-way spanning slaba,b,c,d

ly/lx ≤ 1.5e 1.5 <ly/lx ≤ 2e

REI 60 hs =a =

8020

8010f

8015f

REI 90 hs =a =

10030

10015f

10020

REI 120 hs =a =

12040

12020

12025

REI 240 hs =a =

17565

17540

17550

Notes1. This table is taken from BS EN 1992-1-2 Tables 5.8 to 5.11. For fl at slabs refer to Chapter 7.2. The table is valid only if the detailing requirements (see note 3) are observed and in normal temperature

design redistribution of bending moments does not exceed 15%.3. For fi re resistance of R90 and above, for a distance of 0.3leff from the centre line of each intermediate

support, the area of top reinforcement should not be less than the following: As,req(x) = As,req(0) (1 –2.5(x/leff)) where: x is the distance of the section being considered from the centre line of the support. As,req(0) is the area of reinforcement required for normal temperature design. As,req(x) is the minimum area of reinforcement required at the section being considered but not less

than that required for normal temperature design. leff is the greater of the effective lengths of the two adjacent spans.4. There are three standard fi re exposure conditions that need to be satisfi ed: R Mechanical resistance for load bearing E Integrity of separation I Insulation5. The ribs in a one-way spanning ribbed slab can be treated as beams and reference can be made to Chapter 4,

Beams. The topping can be treated as a two-way slab where 1.5 < ly/lx ≤ 2.

Keya. The slab thickness hs is the sum of the slab thickness and the thickness of any non-combustible fl ooring.b. For continuous solid slabs a minimum negative reinforcemebt As ≥ 0.005 Ac should be provided over

intermediate supports if: 1) cold-worked reinforcement is used, or 2) there is no fi xity over the end supports in a two-span slab, or 3) where transverse redistribution of load effects cannot be achieved.c. In two-way slabs the axis refers to the lower layer of reinforcement.d. The term two-way slabs relates to slabs supported at all four edges. If this is not the case, they should be

treated as one-way spanning slabs.e. lx and ly are the spans of a two-way slab (two directions at right angles) where ly is the longer span.f. Normally the requirements of BS EN 1992-1-1 will determine the cover.

Table 3.1Minimum dimensions and axis distances for

reinforced concrete slabs.

a

b

asd

h b�

Figure 3.1Section through structural member, showing

nominal axis distances a and asd.


20

3.2 Acoustics

3.2.1 Robust Details

3 Performance of concrete in buildings

Table 3.2Minimum reinforced concrete wall dimensions

and axis distance for load-bearing for fi re resistance.

Standard fi re resistance

Minimum dimensions (mm)Wall thickness/axis distance, a, of the main bars

Wall exposed on one side (μfi = 0.7)

Wall exposed on two sides (μfi = 0.7)

REI 60 130/10a 140/10a

REI 90 140/25 170/25

REI 120 160/35 220/35

REI 240 270/60 350/60

Notes1. The table is taken from BS EN 1992-1-2 Table 5.4.2. μfi is the ratio of the design axial load under fi re conditions to the design resistance of the column at normal room temperature conditions. μfi

may conservatively be taken as 0.7.Keya Normally the requirements of BS EN 1992-1-1 will determine the cover.

The predominant uses for cellular structures are residential, and therefore the requirements of Approved Document E (AD E)11 apply in England and Wales. There are two approaches to compliance with AD E: either by using Robust Details or through Pre-completion

testing. Robust Details only apply to purpose-built dwelling houses and fl ats. Buildings incorporating ‘rooms for residential purposes’ (hotels, student accommodation etc.) are subject to pre-completion testing.

Robust Details are sets of construction specifi cations which, if applied to specifi c purpose-built houses and fl ats, and if constructed with care, will meet the level of sound insulation as specifi ed in the performance tables of Approved Document E. Robust Details aim to provide a consistent level of performance with an in-built safety margin, at least 5dB better than the AD E requirements.

Each separating wall or fl oor Robust Detail includes the required junction detailing, ceiling and fl oor treatments and general guidance notes. The details and guidance given must be strictly followed for approval to be given.

The complete set of Robust Details is presented in the Robust Details handbook12, published by Robust Details Limited, which manages its use, monitors existing performance and approves new details. In order to avoid the need for pre-completion testing, every dwelling using Robust Details must be registered with Robust Details Limited and a plot fee paid.

Where a plot is not registered with Robust Details Limited or is not for purpose-built dwelling houses and fl ats (i.e. hotels, student accommodation etc.), pre-completion testing is required. The performance standards are given in Table 3.3, where it should be noted that there is a higher standard for walls in dwelling houses and fl ats than for rooms for residential purposes.

3.2.2 Pre-completion testing (PCT)


21


Airborne sound insulationDnT,w + Ctr

Impact sound insulationL�nT,w

Purpose-built dwelling houses and fl ats

Walls ≥ 45dB

Floors and stairs ≥ 45dB ≤ 62dB

Purpose-built rooms for residential purposes

Walls ≥ 43dB

Floors and stairs ≥ 45dB ≤ 62dB

Sections 2 and 3 of AD E provide examples of construction types which, if built correctly, should achieve the performance standards set out in Table 3.3 for purpose-built dwelling houses and fl ats. Details of junctions between separating walls and fl oors are also given in AD E.

Concrete’s inherent qualities make it good for acoustic performance. It is a good sound insulator, even when the source of the sound is impact on the concrete itself. A number of results from pre-completion testing are given in Table 3.4 for concrete fl oors with a variety of fi nishes. Table 3.5 gives test results for walls, again with a variety of fi nishes. These results give an indication as to the level of sound resistance that can be achieved.

3.2.3 Acoustic properties of concrete

Table 3.4Results from pre-completion testing of

concrete fl oors.

Table 3.3Performance standards for separating walls

and fl oors.

Finish Structure Finish Airborne result

Impact result

None 175 mm in-situ concrete 12.5 mm Soundshield board 125 mm channel 52 >45 Pass 60 ≤ 62 Pass

Bonded carpet 200 mm precast concrete ‘Artex’ plaster 47 > 45 Pass 34 ≤ 62 Pass

50 mm screed bonded 6 mm carpet 250 mm in-situ concrete Painted 57 > 45 Pass 39 ≤ 62 Pass

Bonded 5 mm carpet 225 mm in-situ concrete 15 mm polystyrene on aluminium grids 59 > 45 Pass 42 ≤ 62 Pass

65 mm screed on resilient layer 200 mm precast hollowcore concrete 12.5 mm plasterboard on channel support 50 > 45 Pass

Tiled fi nish with resilient backing 250 mm in-situ concrete slab Metal framing system, 15 mm plasterboard with 13 downlighters

55 > 45 Pass 55 ≤ 62 Pass

NoteThis table is based on data from test results available on The Concrete Centre website, www.concretecentre.com. New data are being added as and when available.

Finish Structure Finish Airborne result

2 mm plaster skim 180 mm in-situ concrete 2 mm plaster skim 47 ≥ 45 Pass

None 180 mm in-situ concrete None 48 ≥ 45 Pass

Paint fi nish 150 mm solid precast concrete Paint fi nish 45 ≥ 45 Pass

Two layers of 12.5 mm plasterboard supported by channel system with 70 mm Isowool in cavity

150 mm precast concrete 12.5 mm plasterboard on 38 mm × 25 mm battens 51 ≥ 45 Pass

NoteThis table is based on data from test results available on The Concrete Centre website, www.concretecentre.com. New data are being added as and when available.

Table 3.5Results from pre-completion testing of

concrete walls.


22

FloorsThere are three robust details for fl oors that are relevant to cellular concrete structures. Detail E-FC-2 (see Figure 3.2) is suitable for in-situ concrete slabs and requires a 200 mm thick concrete fl oor slab and 40 mm of screed or 250 mm of concrete fl oor slab and no screed. This detail can be combined with a drywall separating wall (ref. E-WS-2). Where an alternative wall specifi cation is used PCT should be a carried out on the wall.

3.2.4 Typical details


Ceiling treatment *

Floating floor *

250 mm (min) in-situ

concrete floor slab, or200 mm (min) in-situ

concrete floor slab and

40 mm (min) bonded

screed *

* See Robust Details handbook12 for full details

Figure 3.2Robust Detail for in-situ solid slab (E-FC-2).

Two separating fl oors (E-FC-1 and E-FC-4) use precast concrete units. For both options the units should be 150 mm thick and have a mass of 300 kg/m2. This means that a minimum 150 mm solid unit can be used or a minimum 200 mm hollowcore unit can be used (depending on the supplier). All the options require additional fl oor and ceiling treatments; further details can be found in the Robust Details handbook.

These details are quite onerous and, if tested, are almost certain to pass the performance standards and probably by some margin providing they are well constructed. The examples given in AD E may also be referred to. These give examples for in-situ and precast concrete fl oors. Floor type 1.1C (see Figure 3.4) can be used for in-situ concrete, with or without a permanent shuttering (so it is suitable for twinwall options). The minimum mass per unit area is 365 kg/m2, so a 160 mm thick slab can be used. It must be combined with a soft fl oor covering (i.e. carpet) or better (see AD E) and plasterboard ceiling with either timber battens or proprietary resilient channels. Hollowcore units spanning perpendicular to the wall have been used and have achieved positive test results.


23

Figure 3.3Robust Details for precast concrete fl oors slab

(E-FC-1 and E-FC-4).Floating floor *

40 mm (min) bonded

screed *

150 mm (min) precast

concrete floor plank

(minimum 300 kg/m2)

Ceiling treatment

65 mm (min) cement-sand screed

Proprietary resilient

layer *

150 mm (min) precast

concrete floor plank

(minimum 300 kg/m2)

Ceiling treatment *

* See Robust Details handbook12 for full details


Separating floor type 1.1Ccarried through Timber batten

Screed

Fill gap between headof wall and undersideof floor

Precastconcrete

Right: Figure 3.4Floor type 1.1C (Approved Document E).

Far right: Figure 3.5Floor type 1.2B (Approved Document E).


24

Floor type 1.2B (see Figure 3.5) is suitable for precast concrete fl oors; again the minimum mass is 365 kg/m2. A minimum of 160 mm solid slab or a minimum of 200 mm hollowcore slab with a 50 mm bonded screed will be suitable. A regulating fl oor screed should be used; the joints must be fully grouted and soft fl oor covering (i.e. carpet) or better used (see AD A). The ceiling treatment should be plasterboard on proprietary resilient bars with absorbent material.

The above examples are not prescriptive and the performance requirements can be met with alternative details with advice from acoustic specialists.

WallsAlthough there are Robust Details using concrete blocks, there are no Robust Details for solid concrete walls. The only example detail available is wall type 1.2 in AD E (see Figure 3.6). The requirements for this detail are a minimum mass of 415 kg/m2 and plaster on both room faces. A 180 mm wall with 2 mm skim coat of plaster on each face should achieve a density of 415 kg/m2 and is usually the minimum used for houses and fl ats, where the airborne sound insulation requirement is 45dB. Walls between rooms for residential purposes have a lower requirement of 43dB and therefore a narrower wall could be justifi ed if necessary. Indeed a series of tests on 150 mm-thick walls with just a paint fi nish had the following results: 43, 44, 45 and 50dB.


In-sit

≤180

u concrete

Wall finishes

Figure 3.6Wall type 1.2 (Approved Document E).

Other considerationsWhere it is required to form recesses in the walls (e.g. for electrical sockets) they should be offset to minimise the passage of sound.

An important part of meeting the performance requirements is the junctions between elements and good detailing in these locations is required. Both the Robust Details hand-book and AD E give guidance. In particular, fl anking noise should be minimised. While an element may be a good sound insulator, noise may still be transmitted via other routes such as through junctions between elements, through services risers, through corridors linking rooms or through the cladding. All these potential routes should be considered and addressed in the detailing.


25

Whatever the material used for stairs, they can transmit impact sounds to the adjacent dwelling. Therefore, staircases should be isolated from the adjacent rooms and supported on elastomeric bearings or similar.

Concrete has a high thermal mass, which makes it ideal to use as part of a fabric energy storage (FES) system. FES utilises the thermal mass of concrete to absorb internal heat gains during a summer’s day to help prevent overheating and providing a more stable internal temperature. Night cooling purges the accumulated heat from the slab, preparing it for the next day. FES can be used on its own or as part of a mixed-mode system to reduce the energy requirements. The important requirement is to expose the walls and soffi t of the slab, or at least allow the air from the room to fl ow in contact with the concrete. This impacts on the structural solution and should be considered at the early stages of a project. Thermal mass can also be used to maintain warmth in a building during the winter, particularly if part of a passive solar design system.

Further guidance can be found in Thermal Mass13,Thermal Mass for Housing14 and Utilisation of Thermal Mass in Non-residential Buildings15.

3.2.5 Sound transmission via stairs

3.3 Thermal mass



26

4. Structural support

There are two foundation confi gurations which generally occur for cellular frames.Cellular structures can be supported by transfer structures above open-plan areas (i.e. a hotel accommodation above with public areas on lower fl oors), which result in concentrated loads in columns. Foundations are likely to be pile caps or large pads located under the columns.

Where the cellular structure continues to the foundation level, the options are wide strip footings or piled ground beams. Here the ground beam acts as a pile cap, i.e. the interface between the piles and the wall. It is worth examining several piled options. Using many, smaller piles means using a smaller ground beam with short spans between each pile. Using fewer, larger piles requires the introduction of pile caps (or a very wide ground beam) and signifi cant spanning of the wall between piles. Where there are few piles, the wall and ground beam design is similar to a transfer structure.

A transfer structure occurs where the load-bearing walls stop before they reach the foun-dations and the load path needs to be supported at discrete column or beam locations. This can be an expensive part of the structural frame and care is needed to ensure that effi ciency savings of the cellular construction are retained. A complex transfer structure to support cellular construction could prove less effi cient than using a conventional fl at slab and column solution. This is where high-quality engineering can result in signifi cant savings for a client, or even enable unviable schemes to become commercially viable.

Common situations requiring transfer structures are: hotel rooms above column-free function rooms mixed-used developments with residential units above open-plan offices or shops residential developments with basement car parking.

There are a number of transfer beam options that can be used. The choice will depend on constraints placed upon the design. Clearly, completely column-free spaces require heavier transfer structures than for layouts that can include intermediate columns.

This is a tried and trusted method and should be familiar to all reinforced concrete designers (see Figure 4.1). The beam could span from one side of the building to the other, or have intermediate supports. The latter will produce a smaller beam.

4.1 Foundations

4 Structural support

4.2 Transfer structures

4.3 Options for transfer structures

4.3.1 Option 1: Transfer beam


27

Structural support 4

First floor

Transfer beam

Ground to first-floor column

Figure 4.1Transfer beam under wall.

4.3.2 Option 2: Lowest level of wall acts as transfer beam

Where the columns can be placed near to the ends of wall panels, or where an interstitial plant zone would not need a corridor opening, the resulting solid panel can act as a storey deep transfer beam, sometimes with a thicker section. The wall acts as a deep beam to spread the loads from above to the supporting columns (see Figure 4.2). There are particular

Figure 4.2Lowest level of wall used as transfer beam.

First floor

Wall acts astransfer beam

Ground to first-floor column


28

The third option is to use the strut and tie design method to reduce the depth and/or width of the transfer beam. This technique is not widely used in the UK, but Eurocode 216 offers more guidance than was provided in BS 811017. It is outside the scope of this guide to explain the principles and application of this method.

The fi gures below show how the strut and tie method could be used in a variety of situations to produce an economic design. However, many buildings do not have lintels across the corridors due to low ceiling height or services distribution along the corridor ceiling void; a lintel or beam is critical for adopting this stability concept.

Figure 4.3 shows lintel beams across the corridor together with a strong transfer beam. In many cases this can provide the required lateral stability.

Figure 4.4 shows no lintel beams and no transfer beam. The pinned struts across the corridor result in a mechanism, hence stability cores are required. Checks are also required to ensure the strut action has a valid load path; for example, vertical service risers often punch through the strut load path.

Figure 4.5 shows the consequences of offset openings. In these cases, the out-of-balance forces require an additional column and/or stability cores.

4.3.3 Option 3: Strut and tie design


design considerations to consider in this option, as listed below: At 200 mm or less the ‘deep beam’ will be particularly narrow, therefore the layout of

the reinforcement should be considered at the early stages to ensure that the required reinforcement can be fitted within the element.

Eurocode 216 includes some particular rules for the design of deep beams which should be followed.

There will be some load from the adjacent floor that is carried at the lowest part of the beam and which may require additional link reinforcement (often referred to as ‘hang-up steel’) to resist the tension forces this imposes at the bottom of the beam.

The bearing area at the supports will be small and consequently the local stresses will be high and should be considered in more detail.

The construction sequence is important and should be clearly conveyed to other members of the team, especially the contractors.

The additional reinforcement will slow down the first lift of the construction using tunnel form. However, overall it is probably less time-consuming than constructing a transfer beam before starting the tunnel form construction.

With crosswall, this method is only practical when one panel can span between the sup-ports; even then careful consideration as to how the lower floor is supported is required.

It is unlikely that this method can be used with twinwall construction due to the fact that there is insufficient space for the flexure reinforcement.


29

Structural support 4

Compression in columns

Tension in beam

Vierendeel actionabove openings

Compression in wallevenly distributedto beam

Figure 4.3Transfer structure using strut and tie with

beam under wall.

Tie

Strut

High compression

in narrow width

Structure above openings

acts as prop/tie, i.e. pinnedBalancedcompressionforces

Low compressionin narrow width

Significant vierendeelaction above openings

Strong couple toprovide lateralstability

Poor lateral stabilityrequires strong cores

Below: Figure 4.4Design using strut and tie to minimise transfer

structure.

Below right: Figure 4.5Transfer structure – infl uence of door

openings.


30

Where a column or wall is supported at its lowest level by an element other than a foun-dation, alternative load paths should be provided in the event of the accidental loss of this element. In in-situ reinforced concrete the reinforcement can generally be used to tie the structure together. Where ties are not or cannot be provided, either:

the vertical member should be demonstrated for ‘non-removability’. Non-removability may be assumed if the element and its connections are capable of withstanding a design action at a limit state of 34 kN/m2 in any direction over the projected area of the member together with the reactions from attached components, which themselves are subject to a loading of 34 kN/m2. These reactions may be limited to the maximum reaction that can be transmitted; or

each element should be considered to be removed one at a time and an alternative load path demonstrated.

Further guidance on designing the ties for crosswall construction is given in section 5.10.

4.4 Robustness



31

Crosswall construction 5

5. Crosswall construction

The site layout, location and boundary conditions may impact on the design and construc-tion of a crosswall project. The particular design considerations to consider for a crosswall project are arrangements for unloading the units. It is far more effi cient to use a ‘just in time’ delivery system, where the units are lifted from the lorry into their fi nal position. In this case an unloading area that can be used throughout the working day is required.

The location and size of the crane are also important considerations, especially as precast units tend to require a crane with a higher lifting capacity. In particular the need to oversail beyond the site, especially public highways or railways (note that Network Rail will not allow oversailing) may well infl uence the crane location or perhaps even the structural solution.

The preliminary sizes given in this section are focused on strength requirements; other requirements such as acoustics (see section 3.2) may also determine the minimum require-ments. Manufacture, transportation and placing of the units impose limits on the maximum sizes (see section 5.8). More detailed worked examples are provided in Appendix B.

The initial sizing of solid concrete fl oors and hollowcore can be undertaken using the data in Figures 5.1 and 5.2. Solid units can be cast up to 3.6 m wide and generally span 2.5 to 4.0 m. Hollowcore units are cast 1.2 m wide and can span up to 16 m.

5.2 Initial sizing

5.1 Site

5.2.1 Slabs

350

300

250

200

150

1005 6 7 8 9 10 11 12 13 14

Span (m)

Slab

dept

h(m

)

KeyCharacteristicimposed load

1.5 kN/m2

(Ψ2= 0.3)

2.5 kN/m2

(Ψ2= 0.3)

5.0 kN/m2

(Ψ2= 0.6)

7.5 kN/m2

(Ψ2= 0.6)

Design assumptionsReinforcement fpk = 1770 N/mm2, stressed to 70%.

Loads A superimposed dead load (SDL) of 1.50 kN/m2 (for fi nishes, services, etc.) is included. BS EN 199018, Expressions (6.10a) and (6.10b) have been used.

Concrete Grade C45/55, density 25 kN/m3, 20 mm gravel aggregate.

Fire and durability Fire resistance 1 hour; exposure class XC1.

Figure 5.1Initial sizing of hollowcore fl oor

units, non-composite.


32

Figure 5.2Initial sizing of solid fl oor units, one-way

spanning

5.2.2 Walls

5 Crosswall construction

Key

Characteristicimposed load

200

150

1002.0 2.5 3.0 3.5 4.0 4.5 5.0

Span (m)Sl

abde

pth

(mm

)

1.5 kN/m2

(Ψ2= 0.3)

2.5 kN/m2

(Ψ2= 0.3)

5.0 kN/m2

(Ψ2= 0.6)

7.5 kN/m2

(Ψ2= 0.6)

Single span, m 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Overall slab depth, mm

IL = 1.5 kN/m2 115 115 115 115 120 134 149

IL = 2.5 kN/m2 115 115 115 115 127 142 158

IL = 5.0 kN/m3 115 115 115 126 141 159 175

IL = 7.5 kN/m2 115 115 119 135 153 170 190

Reinforcement, kg/m² (kg/m³)

IL = 1.5 kN/m2 3 (29) 3 (28) 4 (31) 4 (35) 6 (50) 7 (55) 7 (50)

IL = 2.5 kN/m2 3 (29) 3 (29) 4 (33) 5 (46) 6 (47) 7 (52) 8 (54)

IL = 5.0 kN/m3 3 (30) 4 (34) 5 (40) 6 (47) 7 (53) 8 (53) 11 (62)

IL = 7.5 kN/m2 4 (32) 5 (40) 6 (50) 7 (55) 8 (54) 10 (60) 11 (57)

Design assumptionsReinforcement fyk = 500 N/mm2.

Cover cnom = 20 mm; Δc dev = 0 mm.

Loads A superimposed dead load (SDL) of 1.50 kN/m2 (for fi nishes, services, etc.) is included. BS EN 199018 , Expressions (6.10a) and (6.10b) have been used.



Generally the walls are sized to be as narrow as possible to increase the net fl oor area. There may be occasions, such as in tall buildings where there are high compressive loads, or adjacent to long fl oor spans where there are high bending moments due to the notional eccentricity of the wall, when a thicker wall is required.

Each precast manufacturer will have their own minimum wall thickness that they are pre-pared to use and this will usually be in the range 150 to 175 mm. Where the walls are party walls a thickness of 180 mm is generally used, for acoustic reasons. These thicknesses are all for walls with two layers of mesh reinforcement.


33


Where a wall element has a lot of openings or the openings are close to the end of the wall then care should be taken to ensure that the unit has suffi cient strength during lifting operations.

Generally, the precast fl oor units will span onto the precast walls, but there will be occasions when there are openings in the walls and an alternative structural support is required. One method is to design the precast fl oor units to span in two directions; more usually steel sections are used to support the ends of the units. A variety of options are shown in Figure 5.3.

5.3 Structural support at openings

e) Precast units supported on inverted T-section

Grout

Steel T-section

Precast unitGrout Precast unit

Steel angle section

d) Precast units supported on back-to-back angles

b) Precast units supported by UB or UC steel section

Precast unit

Steel channel section precastto concrete unit

c) Integral steel channels

Concrete or grout infill

Precast unit

Concrete or grout infill

Precast unit

a) Two-way spanning precast units

Figure 5.3Structural support at openings (e.g. spanning

corridors).


34

To make effi cient use of the moulds, it is important to strike the elements in the factory as quickly as possible. For this reason precast concrete manufacturers prefer to use higher-strength concrete than is generally used for in-situ concrete. The typical class of concrete used for crosswall panels is C35/45. Self-compacting concrete is also increasingly used in the precast factory to reduce the use of vibrators to compact the concrete. This improves working conditions as it reduces an operation which in an enclosed environment is noisy and which also causes vibration to the user.

Generally high-quality fi nishes are achieved with precast concrete. This is due to a combi-nation of high-quality formwork, an internal working environment, use of self-compacting concrete and consistent workmanship. Precast concrete should achieve a Type B fi nish according to BS 811017. If required, a Type C fi nish can be achieved, but there is likely to be a premium to pay for this and it should only be specifi ed where it is needed. An alternative to specifying Type C is to use a suitable paint or skim coat of plaster. Alternatively there are other systems available, such as fi llers, which can be used instead of gypsum plaster, and which can prove to be more cost-effective.

If a quality fi nish is required on both sides of the wall then it should be cast vertically so that both faces are cast against a shutter. A set of battery moulds enables the vertical casting of many wall units simultaneously (see Figure 5.4).

Hollowcore units are usually cast by extruding the concrete and therefore high-quality fi nishes are not possible. A skim of plaster or other alternative can be used to achieve a suitable fi nish.

For exposed concrete, designers need to be clear that it is not possible to specify unequi-vocally the visual quality of fi nish required, for example colour and consistency. The way to achieve the required fi nishes is through communication between the design team and the precast concrete manufacturer. In this way there will be common understanding of the look required and what can be achieved.

Figure 5.4Battery moulds.

Photos: Bell and Webster Concrete Limited and Bison Concrete Products Limited

5.5 Finishes


5.4 Concrete


35


The precast concrete panels should be designed to make the casting, striking and erection as simple as possible. A brief outline of some of the design considerations to consider and typical details are given in this section.

Wherever possible a mesh should be used to reinforce the section; it is quicker and simpler to fi x a mesh than loose bars.

5.6 Screeds

KEY

Precast wall

225 mm-thick slab

150 mm-thick slab

with 75 mm screed

150 mm-thick slab

supporting bathroom pod

Steel lintel

Figure 5.5Plan showing use of screed in crosswall

construction.

5.7 Design details

Levelling screeds are likely to be used with crosswall and solid precast units, but only in the corridors and entrance ways to each bedroom/dwelling. This is shown in Figure 5.5, where the use of a screed is avoided over the deep fl oor units. A screed is used over the shallow fl oor units, which support the bathroom pods, to bring the general fl oor level up to the level of the bathroom fl oor. This has the advantage of being able to take up any tolerances in the screed which can be tied into the level of the bathroom pod and the adjacent solid precast concrete units.


36

Where there are openings within a panel, consideration should be given to striking the panel. A chamfer is usually needed to allow striking (see Figure 5.6).

On site, wall panels will be joined together. Figure 5.7 shows typical details for joining two or three panels, including a vertical tie if required for robustness. Figure 5.8 shows an example of a connection during construction.

Figure 5.7Plan details showing typical wall panel

connection details.

Opening

Casting bed

Chamfer to allowstriking of unit

Precast concreteunit

Figure 5.6Detailing of unit to allow striking of unit.

Vertical tie

Grout

Wire loop

Wire loop

Vertical tie

Grout

Vertical tie

Wire loopGrout



37


Figure 5.9Wall to solid fl oor connection details.

Figure 5.8Panel-to-panel connection.

Photo: Bell and Webster Concrete Limited

Slab panels should also be fi xed together and typical details are shown for the junction between solid slab units and wall panels (see Figure 5.9) and hollowcore units and panels (see Figures 5.10 and 5.11).

Horizontal tie

Grout

Wire loop

Vertical tie

b) External wall to solid slabs

Grout Wire loop

Horizontal tieVertical tie

a) Internal wall to solid slab

Shims fortolerance

Concrete infill

Grout

Horizontal tie placedin open trough

Mortar bed

Vertical tie

a) Internal wall to hollowcore slab

Horizontal tie

Grout

Horizontal tie placedin open trough

Vertical tie

b) External wall to hollowcore slab

Horizontal tie

Figure 5.10Wall to hollowcore fl oor connection details.


38

5.8 Construction


Right: Figure 5.11Wall to hollowcore fl oor connection.

Photo: Bison Concrete Products Limited

Far right: Figure 5.12Use of A-frame to transport panels to site.

Photo: Bell and Webster Concrete Limited

Construction is a critical aspect on a crosswall project and the precast concrete manufacturer will be well versed in achieving a solution that is fast to erect and will be able to give advice. It is not the intention in this short section to cover all the design considerations, rather to highlight those that should be considered in the early stages of the project. Consideration of the site layout has already been discussed in section 5.1.

Design considerations in the sizing of panels are: crane capacity at the precast yard crane capacity on site, which reduces with increasing radius layout configuration to minimise number of units, e.g. maximise external panel length

by enclosing more than one room access to site maximising the number of units on a wagon to minimise journeys transportation places limitations on the size and weight of the units. Wall panels are

usually transported on A-frames (see Figure 5.12) because they are easier and safer to lift in a more upright position. Slab panels will be transported in a horizontal position and they should be limited to 3.5 m width to avoid additional transportation costs.

Once the wall units have been lifted into position they must be temporarily propped until the fl oor above has been placed and grouted. Usually push–pull props are used and these are fi xed to wall panels and fl oor units via cast-in fi xings which have to be made good once the props are removed. Perimeter walls can be erected with edge protection attached to remove the need for scaffolding.

When everything is positioned correctly, and any tie reinforcement placed, the junctions are grouted up. To avoid using formwork and to avoid unsightly grout runs thixotropic grout can be used, which is mechanically mixed to ensure consistency and strength and then pumped into place.

The erection of the wall panels and fl oor units will be rapid. As an indication, six to eight rooms can be erected in a day and the lead time will be 12 weeks 19.


39


5.9 Tolerances

Table 5.1Tolerance for precast elements from

BS 8110: 1997.

It is essential to fully consider tolerances at the design stage to improve buildability and quality of the fi nished building. This section introduces the tolerances that should be considered.

In the Eurocode system, recommended production tolerances are provided in the product standards for precast concrete. The production tolerances can be varied in the execution specifi cation and the values here are for guidance only, but note stricter tolerances may incur a cost premium. As a reference the tolerances for length, cross-section and squareness given in BS 8110 are given in Table 5.1, which are currently used by the UK precast concrete industry. Table 5.2 gives the tolerances of lengths, heights, thickness and diagonal dimen-sions for wall elements from BS EN 1499220. Table 5.3 gives the tolerance for length, width and thickness for fl oors from BS EN 1374721.

The European product standard introduces two classes for tolerances, the tighter standard, class A, being generally more onerous than BS 8110, but Class B is less stringent.

Section property Permitted deviationReference dimensions 0–3.0 m 3–4.5 m 4.5–6 m 6–12 m

Length, squareness ± 6 mm ± 9 mm ± 12 mm ± 18 mm

Reference dimensions 0–0.5 m 0.5–0.75 m 0.75–1.0 m 1.0–1.25 m

Cross-section ± 6 mm ± 9 mm ± 12 mm ± 15 mmNote: The tolerance for squareness is the difference between the two diagonal dimensions.

Class Permitted deviationReference dimensions 0–0.5 m 0.5–3 m > 3–6 m > 6–10 m > 10 m

A ± 3 mm ± 5 mm ± 6 mm ± 8 mm ± 10 mm

B ± 8 mm ± 14 mm ± 16 mm ± 18 mm ± 20 mmNote:These tolerances are applicable to lengths, heights, thickness and diagonal dimensions. The tolerance for squareness is the difference between the two diagonal dimensions

Section property Permitted deviation

Length ±20 mm

Width +5 mm–10 mm

Thickness ±10 mma

Key:a For fl oor units less than 100 mm thick refer to BS EN 13747.

5.9.1 Production tolerances

Table 5.2Tolerance for wall elements from

BS EN 14992: 2007.

Table 5.3Tolerance for fl oor elements from

BS EN 13747: 2005.


40

Erection tolerances are the geometrical tolerances relating to location, verticality and other aspects of construction assembly. The erection tolerances are provided in BS EN 1367022, which is due for publication in 2008. The tolerances may be amended in the execution specifi cation; in the UK the National structural concrete specifi cation for building construc-

tion (4th edition)23 will refl ect the requirements of EN 13670, with some amendments to refl ect UK practice.

Construction tolerances are the combination of production and erection tolerance, but are not necessarily a summation of the production and erection tolerances. BS EN 13670 introduces the ‘box’ principle in which all elements must fi t within their prescribed envelope or box. Further advice is given in the guidance notes to the National Structural Concrete

Specifi cation for Building Construction23.

Clause 10.9.5.2 of Eurocode 216 gives detailed guidance on determining the bearing lengths for precast elements and should be referred to.

The construction tolerances for the structural frame should be considered when detailing the interfaces with other building elements. BS 560624 gives good guidance on how to consider these variations in the design.

An example is forming an opening in a precast panel into which a window is to be fi xed. The opening should be specifi ed larger than the window to allow for variations (see Figure 5.13). BS 5606 gives guidance on how to determine how much larger the opening should be.

A further example where consideration should be given to tolerances is the appearance of the façade. Given that panels will vary in size, as will the position and size of the openings within the panels, there should be a clear understanding of how the panels should be set out. A number of options exist including:

setting out the panels to give a constant joint thickness setting out the panels so that windows are aligned setting out the panels so the centre point of the panels are aligned.

Discussion and coordination are the most effective ways of achieving a design that overcomes tolerance issues.

5.9.2 Erection tolerances

5.9.3 Construction tolerances

5.9.5 Coordination with other building elements


5.9.4 Bearings


41

Figure 5.13Designing to allow for dimensional variations.

5.10.1 Building classes


Max. space between components

Min. space

Max. comp.size

Min. comp.size

Min.jointsize

Min.jointsize

Max.jointsize

Max.jointsize

Target comp.size

Design of buildings to resist disproportionate collapse is a requirement of the Building Regulations. This section explains how to meet the requirements for crosswall construction.

Precast concrete structures require careful detailing to ensure that the structure is robust and meets the requirements of Approved Document A25. This classifi es buildings according to type and occupancy for the purpose of designing to resist disproportional collapse. Table 5.4 summarises the requirements for the occupancy of buildings likely to be constructed using crosswall.

Class Building type and occupancy Tying requirements1 House not exceeding four storeys. No additional measures.

2A Five-storey occupancy houses.Hotels, fl ats, apartments and other residential buildings not exceeding four storeys.

Horizontal ties; orEffective anchorage of fl oors to supports.

2B Hotels, fl ats, apartments and other residential buildings exceeding four storeys, but not exceeding 15 storeys.

Horizontal ties and vertical ties; orAllowance for removal of support (refer to Approved Document A).

3 Hotels, fl ats, apartments and other residential buildings exceeding 15 storeys.

Systematic risk assessment (refer to Approved Document A)

The types of tie required for a Class 2B building are shown in Figure 5.14 and can be sum-marised as:

floor ties – connecting floors over an internal wall horizontal perimeter ties – connecting floors to perimeter walls internal ties – running parallel to the internal walls peripheral ties – around the perimeter of the floor vertical ties – connecting vertical walls to provide continuity.

The tying requirements for a Class 2A building are similar, except the vertical ties can be omitted. Further details of the ties are shown in Figure 5.15 for solid fl oor units, and Figure 5.16 for hollowcore fl oor units.

Class 3 buildings are outside the scope of this guide but guidance can be found in BS EN 1991-1-7 Annex B26 and New approach to disproportionate collapse27.

5.10 Robustness

Table 5.4Building classes and corresponding tying

requirements.


42

Verticaltie

Peripheral tie

Vertical tie

Horizontalperimeter tie

Verticaltie

Internal tie

Figure 5.14Ties for crosswall construction.

Eurocode 2, Part 1-128 gives guidance on the design of ties as detailed in the following subsections.

Peripheral tiesAt each fl oor and roof level an effectively continuous tie should be provided within 1.2m of the edge. Structures with internal edges (e.g. atria and courtyards) should also have similar peripheral ties.


5.10.2 Design of ties

Tie bar diameter f

Minimum ( + 2 + 10)f HaggIn-situ concrete

b) Sectiona) Plan

In-situ infill

Projecting barsGable beam

Plug in open cores

At least one (often two) core(s)opened for approximately 300 mm

Perimeter tie

Perimeter tie

Above: Figure 5.15Position of internal tie within longitudinal

joints of hollowcore units.

Right: Figure 5.16Perimeter ties where hollowcore units span

parallel to the edge beam.


43


Internal tiesAt each fl oor and roof level, internal ties should be provided in two directions approximately at right angles. The internal ties, in whole or in part, may be spread evenly in slabs or may be grouped at walls or other positions. If located in walls, the reinforcement should be within 0.5m of the top or bottom of the fl oor slabs.

In each direction the tie needs to be able to resist a force, which should be taken as:

Ftie,int= (1/7.5)(gk + qk)(lr/5)Ft ≥ Ft

where: (gk + qk) = average permanent and variable floor actions (kN/m2) lr = greater of the distances (in metres) between centres of the columns,

frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration

Ft = (20 + 4n0) ≤ 60 kN (where n0 is the number of storeys).

The maximum spacing of internal ties should be limited to 1.5lr.

Ties to wallsWalls at the edge and corner of the structure should be tied to each fl oor and roof. In corner walls, ties should be provided in two directions. The tie should be able to resist a force of:

Ftie, fac = maximum (2Ft; ls Ft/2.5; 0.03NEd)

where: Ftie, fac = in kN/m run of wall Ft = (20 + 4n0) ≤ 60 kN (where n0 is the number of storeys) ls = floor-to-ceiling height (in metres) NEd = total design ultimate vertical load in wall or column at the

level considered.

Tying of external walls is required only if the peripheral tie is not located within the wall.

Vertical tiesEach wall panel carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of resisting the load received by the wall panel from any one storey under accidental design situation. Figures 5.17 and 5.18 show typical details for vertical ties.

The peripheral tie should be able to resist a tensile force of:

Ftie,per = (20 + 4no) ≤ 60 kN

where: no = number of storeys.


44


Wall panel

Vertical tie castinto wall panel

Bolt connectingvertical tie tovertical tie inpanel below

Reinforcement frompanel below

Reinforcement placedafter panel erected

Wall panel

Slab panel

Reinforcement U-bar castinto slab panel

Void cast intowall panel

b)Bespoke vertical ties cast into wall panelsa) Reinforcement placed in-situ

Figure 5.17Typical vertical tie details.

Bespoke tie prior to casting into panel *

Connection detail joining two bespoke vertical ties *

Figure 5.18Vertical ties cast into wall.

Photos : Bison Concrete Products Limited *.

and Bell and Webster Concrete Limited †

Tie using standard reinforcement †


45

Tunnel form construction 6

6.1 Site

6.2.1 Tunnel form system capabilities

6. Tunnel form construction

The site layout, location and boundary conditions may impact on the design and construction of a tunnel form project. The particular issue to consider for a tunnel form project is whether the formwork modules can be lifted clear of the building so they can be moved to their next position. Generally 5 m clearance is required on one side of the building, although shorter tunnel form units can be manufactured (but these will slow productivity). It may be possible to oversail adjacent land but the risk should be assessed on a case-by-case basis (note that Network Rail will not allow railways to be oversailed).

The preliminary sizes given in this section are focused on strength requirements; other requirements such as acoustics (see section 3.2) may also determine the minimum requirements.

The typical fabrication sizes of tunnel forms are given in Figure 6.1. Any combination of two half-tunnel modules is possible, allowing every room width between 2400 mm (2 × 1200 mm modules) and 6600 mm (2 × 3300 mm modules). It is also possible to mix modules, for example a 3000 mm room width can be achieved using 1200 mm and 1800 mm modules. Spans under 2000 mm can be accommodated using CAM-action tunnels, which are full tunnels. Wider tunnels can be formed with the use of a table form between the two half tunnels, but this slows down the process because an extra lift is required. The length of the tunnel form elements are between 2.5 and 12.5 m in 1.25 m increments.

Any fl oor-to-ceiling height between 2.46 and 3.50 m can be accommodated with the tunnel forms, although this should be consistent from fl oor to fl oor. Small variations in wall height can be incorporated by using higher kickers with the same forms. Double-height walls (up to 4.5 m) can be achieved using extension legs. This allows double-height living space in duplex apartments, or an open stairwell/hallway for low-rise construction or lobby/concierge areas.

SlabsThe initial sizing of the fl oor can be carried out using the data in Figure 6.2. Tunnel form can support slabs up to 350 mm thick. The tunnel forms can be strengthened to accomodate deeper slabs.

6.2 Initial sizing

6.2.2 Structural sizes


46

Removable wheel with jack

Triangulation wheel(raised during castingto avoid loading slab)

Lower triangulation

Triangulation brace

Articulated diagonalstrut

Horizontalpanel

Verticalpanel

1550 250

Minimum width

34

2366

Maximum width

Horizontal panel1550 5-600 250

Additionalpanel

Slidingpanel

Maximum width

Horizontal panel

2150 5-900

Additionalpanel

Slidingpanel

250

Minimum width

2150 25034

Horizontalpanel

Verticalpanel

Articulated diagonal strut

Triangulation brace

Lower triangulation



2366

950 250

34

Verticalpanel

Horizontalpanel

Articulated diagonal structure

Triangulation brace

Lower triangulation



Minimum width

Maximum width

Horizontal panel Additionalpanel

Slidingpanel

950 5-400 250

2366

1200-1600 mm

Self-weight 130 kg/m2

1800-2400 mm

2400-3300 mm

a) Small module

c) Medium module

b) Large module

< 2000 mm

d) CAM — Action full tunnel



CAM — action to allow

shrinking of formwork

Figure 6.1Details of the tunnel form modules.

6 Tunnel form construction


47

Figure 6.2Initial sizing of one-way spanning slabs.


Key


350

300

250

200

150

1004.0 5.0 6.0 7.0 8.0 9.0

Multiple span(solid)

Single span(dotted)

Slab

dept

h(m

m)

Span (m)

1.5 kN/m2

(Ψ2= 0.3)

2.5 kN/m2

(Ψ2= 0.3)

5.0 kN/m2

(Ψ2= 0.6)

7.5 kN/m2

(Ψ2= 0.6)

Single span, m 4.0 5.0 6.0 7.0 8.0 9.0Overall slab depth, mm

IL = 1.5 kN/m2 129 159 191 233 281 334

IL = 2.5 kN/m2 136 168 200 238 286 338

IL = 5.0 kN/m3 150 185 223 259 311 374

IL = 7.5 kN/m2 162 198 237 275 338 397

Reinforcement, kg/m2 (kg/m3)

IL = 1.5 kN/m2 6 (46) 8 (52) 11 (57) 13 (54) 15 (54) 19 (56)

IL = 2.5 kN/m2 6 (44) 8 (50) 11 (54) 15 (62) 19 (65) 19 (56)

IL = 5.0 kN/m3 7 (50) 10 (56) 12 (56) 18 (70) 19 (60) 23 (62)

IL = 7.5 kN/m2 8 (51) 11 (55) 15 (62) 18 (66) 23 (68) 24 (60)

Multiple span, m 4.0 5.0 6.0 7.0 8.0 9.0Overall slab depth, mm

IL = 1.5 kN/m2 125 133 158 185 230 271

IL = 2.5 kN/m2 125 141 167 194 235 276

IL = 5.0 kN/m3 128 156 183 215 257 301

IL = 7.5 kN/m2 136 165 197 226 273 327

Reinforcement, kg/m2 (kg/m3)

IL = 1.5 kN/m2 4 (36) 6 (47) 8 (48) 10 (55) 12 (53) 14 (52)

IL = 2.5 kN/m2 6 (48) 8 (53) 8 (49) 13 (65) 12 (52) 14 (52)

IL = 5.0 kN/m3 8 (59) 10 (63) 12 (67) 15 (69) 17 (67) 20 (68)

IL = 7.5 kN/m2 10 (71) 13 (76) 15 (77) 17 (77) 20 (73) 23 (71)

Design assumptionsReinforcement fyk = 500 N/mm2. Main bar diameters and distribution steel as required. To

comply with defl ection criteria, service stress, σs, may have been reduced.

Loads A superimposed dead load (SDL) of 1.50 kN/m2 (for fi nishes, services, etc.) is included. Expressions (6.10a) and (6.10b) have been used.

Concrete C32/40, density 25 kN/m3, 20 mm quartzite (gravel) aggregate.



48

WallsGenerally the walls are sized to be as narrow as possible to increase the net fl oor area and will vary depending on the structural, acoustic and thermal requirements. Often the mini-mum wall thickness will be determined by the acoustic requirements. There may be occasions, such as in tall buildings where there are high compressive loads on the lower walls, or adjacent to long fl oor spans where there are high bending moments due to the notional eccentricity of the slab loading, when a thicker wall is required.

The tunnel form systems can accommodate wall thicknesses of 100 to 600 mm. Optimum construction usually results in 180 mm thick walls; primarily due to acoustic rather than structural requirements.

Any slim concrete wall requires a concrete mix with good workability. This is best achieved through the use of superplasticisers or water-reducing agents, as opposed to increased water content which could reduce strength and durability. A workable mix, combined with a skilled use of vibrators, results in a surface suitable for direct fi nishes. The small increase in mix cost is more than saved in the time and cost of not requiring a follow-on trade to apply a surface fi nish. Trials should be conducted on site to ensure that the mix and proposed placement result in a good surface fi nish.

A key feature of the tunnel form system is the use of heaters during the winter months to ensure suffi cient concrete strength gain to permit early striking times. Space heaters are placed within the area beneath the formwork which is sealed at each end by tarpaulin curtains. The heaters are used to provide a slow, evenly spread heat between 50°C and 70°C. The system manufacturer can provide formulae and data for calculating the heating capacity required. Generally, the curtains are closed and the area is heated for a period of one hour before placing the concrete, until the concrete achieves the required strength for striking. This manner of heating promotes the curing of the concrete.

The covered heated curing method also means hydration occurs at an early stage. This reduces problems with water loss due to evaporation on conventional slabs before the concrete has reached full strength.

The steel forms are fi nished with 4 mm steel plating fabricated to a tolerance of 1 mm.

Given the accuracy of fabrication, the quality of fi nishes achieved is limited by the care and attention of the workforce in cleaning and preparing the shutters and placing the concrete. As with any concrete, greatest attention is required when compacting the kickers, avoiding aggregate segregation at the bottom of the wall, and ensuring compaction in the corners. The joint between the wall and the kickers is usually concealed behind the skirting board. Contractors should make an allowance to rub down fi ns at formwork joints, fi ll occasional blowholes, and fi ll tie bar holes. The effort required to make good will be proportional to the effort in placing the concrete, and hence varies from site to site.

6.3 Concrete placing and curing

6.4 Finishes



49


In many cases, the walls only require a good-quality paint or wallpaper to present an acceptable fi nish. However, due to variability of weather and concrete supply, a good cost plan should include an allowance for a skim to cover over any defects with concrete com-paction or cold joints.

Typically, the slab surface is fi nished with a hand-fl oat rather than power-fl oating due to the starter bars along each wall line and the time taken to power-fl oat. Most end-users will apply the fl oor fi nishes directly onto the slab or use a smoothing compound of latex or synthetic polymer as a base.

Design of tunnel form construction is relatively simple. Design typical slab one way spanning between walls; select mesh size. It is important

to use mesh in the walls and slabs where possible since it speeds up the daily cycle because it requires less time to lift and fix into position.

Design slab above openings in wall (i.e. corridors or doors) as two-way span using tables for small regular panels. Use finite element for large irregular panels. Select mesh size.

Calculate the permanent actions on the walls. For multi-storey construction, the cumulative permanent dead weight will be greater than the tunnel formwork load.

Design lowest wall as a simple plain wall (assuming stability cores). Select mesh size. Check lowest wall using appropriate concrete strength (i.e. less than the 28-day

strength) for weight of formwork and weight of concrete supported by formwork applied as a line load adjacent to the wall.

Large spans with a table form may also have a prop at mid-span. Calculate back-propping requirements if necessary. Typically the slab is propped at 2 m centres.

High multi-storey construction may enable reduced mesh size in the walls at upper levels, although this is not common.

See design example in Appendix C for further details.

Usually every party wall is cast in concrete, but every other party wall may be omitted by using longer spans; this has the benefi t of providing future fl exibility.

Tunnel form structures are monolithic and therefore the benefi ts of continuity can be used in the structural design.

When using tunnel form construction, it is possible to make changes to the position and size of openings late in the design process. However, late changes can lead to aborted work and have an impact on the programme. It is therefore advisable to ensure the co-ordination of services and structure in suffi cient time to avoid late changes, except where absolutely necessary.

Generally the details are similar to any other in-situ reinforced concrete frame. However, there are situations where the tunnel formwork system requires the designer to take account of this in the detailing, for instance at the slab–wall interface as shown in Figure 6.3.

6.5 Design checks required

6.6 Design


50

Figure 6.3Detail of the slab–wall interface.


Meshreinforcement

Meshreinforcement

Meshreinforcement

Loose barLap

a) Mesh only junction b) Loose bars used for junction

The formwork is specially adapted for each project. The repetitive nature of the system and the use of prefabricated standard dimensioned forms and reinforcing mats/cages simplify the whole construction process, producing a smooth and fast operation. The techniques used are already familiar to the industry, but with tunnel form construction there is less reliance on skilled labour. A typical 24-hour construction sequence is shown in Figure 6.4.

On average, a team of nine to twelve site operatives plus a crane driver can strike and fi x 300 m2 of formwork each day, including placing approximately 35 to 60 m3 of ready-mixed concrete using a skip or 80 m3 using a concrete pump. The work can continue in all weather except high winds, and heaters can be used to accelerate the concrete curing process.

The schedule provided by the repetitive 24-hour cycle means each operative knows exactly what to do and when, and works to a precisely detailed plan. The smaller work teams and predictable, measurable daily production rates simplify and enhance overall control of the project. Known completion times make scheduling of material deliveries and follow-on trades more accurate and optimise cash fl ow by facilitating ‘just-in-time’ principles. By quickly providing protection, the system allows the follow-on trades to commence work on completed rooms while work proceeds on upper fl oors.

The lead-in time will generally be approximately 4 weeks 19.

6.7 Construction6.7.1 Construction sequence

6.7.2 Programme


51


Concrete cube test completed byengineer

Striking of tunnel forms can begin

cleaned, oiled and repositioned

Wall reinforcement placed inadvance

Services are placed within thewalls

Final elements are lifted intoposition

Once two half tunnels are in place

then reinforcement can be placed

on deck

Conduits are placed for services

within the slab and then reinforce-

ment completed

Concrete pouring can commence

Walls followed by slabs(2 to 3 hours)

Vibrators are used to ensure a

high-quality finish

Curtains are closed and spaceheaters inserted

Heating of the tunnel forms helps

to accelerate the curing process

The exercise is repeated thefollowing day

07:00

07:30

11:00

14:30

17:00

First half tunnel form is removed

Figure 6.4Construction sequence for tunnel form

construction.Illustrations: Outinord International Ltd


52


Figure 6.5Typical tunnel form.

Illustrations: Outinord International Ltd

The key to designing for any systemised form of construction is to understand how the system works on site. Figures 6.1 and 6.5 illustrates the composition of a typical tunnel form.

The sliding and additional soffi t panels are of standard dimensions (1200, 1800 or 2400 mm). The additional panel is manufactured to the dimension to suit the span, enabling a range of module widths to be easily constructed on site with the same unit. The vertical panel is of a standard 2400 mm dimension. Higher fl oor-to-ceiling heights are accommodated by manufacturing an ‘upper extension’ panel of the required dimensions. This panel easily fi xes to the vertical panel.

The wheels and ‘triangulation’ support are only used when striking and positioning the units. These components are retracted before the fresh concrete is placed, hence this load is transferred through the ‘inclined strut’ to the wall line. This reduces the back-propping requirements and negates the requirement to design for construction loading.

Openings and ducts are blocked out by fi xing stop-ends to the steel formwork using magnets. The ends of walls and slabs are also closed using stop-ends held onto the formwork using magnets. This ensures fast placement and repeated use of high-quality formers for the openings.

Accuracy from one level to the next is maintained by the use of precast concrete cruciforms. These fi t into the space between the tunnel forms to provide an accurate line for the location of the formwork for the walls above (see Figure 6.6). This means that accuracies of ±3 mm are achieved for room sizes.

High dimensional tolerance is achieved with conical tie bars to ensure constant wall thickness, and cruciforms to ensure accurate positioning of the formwork in the next lift. The tunnel form system allows extensive use of pre-cut mesh reinforcement, resulting in quicker placement and fi xing.

6.7.3 Details

Figure 6.6Kicker formwork and concrete cruciforms to

control accuracy.Photos: Outinord International Ltd


53


Like all in-situ concrete frames, it will generally be found that no additional reinforcement is required to ensure a robust structure with tunnel form construction. Reinforcement provided for other purposes may be used as the reinforcement acting as ties in in-situ concrete. Indeed, normal detailing of reinforcement ensures that the ties are adequately anchored. Tunnel form structures are likely to far exceed the robustness requirements.

The tunnel form system incorporates stripping platforms, void platforms and gable-end (end-wall) platforms with integrated edge protection. These platforms also allow for circulation around the tunnel form and the structure.

Tunnel form is a particular formwork system specifi cally designed to maximise the speed of construction for cellular-type structures built with in-situ concrete. More typical formwork systems, such as vertical panel systems, horizontal panel systems and table forms can also be used for this type of structure. Much of the design guidance in this publication can equally well be applied to these formwork systems.

6.8 Robustness

6.9 Health and safety

6.10 Alternatives to tunnel form


54

7. Twinwall

Hybrid concrete wall panels comprise two skins of precast concrete, connected by steel trusses, which hold the precast skins apart at a constant spacing, which act as permenant formwork to the in-situ concrete (see Figure 7.1).

The precast skins areconnected and spacedby steel lattice

Main horizontal andvertical reinforcementfor the wall is fittedwithin the precast skins

Figure 7.1Typical twinwall panel.

The precast skins contain the main horizontal and vertical reinforcement for the wall, in the form of a cross-sectional area of mesh or bars which can be specifi ed by the designer. How-ever, starter bars and continuity reinforcement must be provided within the in-situ portion.

The site layout, location and boundary conditions may impact on the design and construction of a twinwall project. The particular design considerations to consider for a twinwall project are arrangements for unloading the units. It is far more effi cient to use a ‘just-in-time’ delivery system, where the units are lifted from the lorry into their fi nal position. In this case an unloading area that can be used throughout the working day is required.

The location and size of the crane are also important considerations, especially as precast units tend to require a crane with a higher lifting capacity. In particular the need to oversail beyond the site, especially public highways or railways (note that Network Rail will not allow oversailing) may well infl uence the crane location or perhaps even the structural solution.

The preliminary sizes given in this section are focused on strength requirements; other requirements such as acoustics (see section 3.2) may also determine the minimum requirements. Manufacture, transportation and placing of the units impose limits on the maximum sizes (see section 5.8).

7.1 Site

7.2 Initial sizing

7 Twinwall


55

Twinwall 7

SlabsTypically a lattice girder slab is used in twinwall construction. The sizing data for lattice girder slabs are given in Figure 7.2. Units are usually 1200 or 2400 mm wide and can be used for spans of up to 10 m.

Hollowcore slabs may also be used and the sizing chart in section 5.1. is applicable. How-ever, it should be noted that the precast skin of the twinwall panel will be too narrow to be considered a permanent bearing.

Key


300

250

200

150

1003 4 5 6 7

Span (m)Sl

abde

pth

(mm

)

8

1.5 kN/m2

(Ψ2= 0.3)

2.5 kN/m2

(Ψ2= 0.3)

5.0 kN/m2

(Ψ2= 0.6)

7.5 kN/m2

(Ψ2= 0.6)

Single span, m 3.0 4.0 5.0 6.0 7.0 8.0 9.0Overall thickness, mm

IL = 1.5 kN/m2 115 117 148 187 226 261 288

IL = 2.5 kN/m2 115 125 158 196 233 264 291

IL = 5.0 kN/m3 115 147 186 220 250 276

IL = 7.5 kN/m2 131 170 208 238 265 291

Design assumptionsReinforcement fyk = 500 N/mm2.

Cover cnom = 20 mm; Δc, dev= 0 mm.

Loads A superimposed dead load (SDL) of 1.50 kN/m2 (for fi nishes, services, etc.) is included. BS EN 199018 , Expressions (6.10a) and (6.10b) have been used.



Figure 7.2Initial sizing of lattice girder slabs.


56

7 Twinwall

Overall wall thicknesses below 200 mm are diffi cult to achieve because the precast skin thickness is typically 50 to 70 mm each side (plus tolerance), and the thickness of the in-situ concrete in between must accommodate starter and continuity reinforcement with suffi cient space for the concrete to fl ow around the bars. The fi nal wall thickness can range typically from 200 to 350 mm in total width, although thicker walls are possible. It is worth noting that, due to the manufacturing process, tolerances on the inside faces of the precast skin are not well controlled and can reduce the space available for in-situ concrete or starter bars by 10 to 15 mm each side.

Generally the walls are sized to be as narrow as possible to increase the net fl oor area. There may be occasions, such as in tall buildings where there are high compressive loads, or adjacent to long fl oor spans where there are high bending moments due to the notional eccentricity of the wall, when a thicker wall is required.

To make effi cient use of the precast moulds, it is important to strike the precast elements in the factory as quickly as possible. For this reason precast concrete manufacturers prefer to use higher-strength concrete than is generally used for in-situ concrete. The typical class of concrete used for twinwall panels is C35/45. Self-compacting concrete is also increasingly used in the precast factory to reduce noise due to use of vibrators to compact the concrete.

The in-situ concrete to be placed between the walls should be able to fl ow between the two precast faces, and around the starter bars and continuity reinforcement. Using a vibrator poker may be diffi cult or impossible. The use of self-compacting concrete should be con-sidered. A smaller aggregate size, for example 10 mm, may also be appropriate.

The use of high-quality formwork moulds gives the external faces of the panels a smooth fi nish. The fi nish quality is suitable to receive a plaster fi nish or wallpaper. It should be noted that the colour will not be consistent and it is therefore not advised that exposed concrete fi nishes are used.

Typical details for connecting the twinwall and lattice girder slabs are shown in Figures 7.3 and 7.4.

Where the design engineer is relying on others to design particular elements responsibility for the design of the elements and the continuity reinforcement should be clearly set out.

In BS EN 1499220 twinwall is classifi ed as composite walls to be designed like a solid wall. This approach can easily be applied to the general wall condition. However, care is required at joints to ensure the full section width can be assumed. This is important where local bearing is the critical load case, for example on a transfer beam or above a column.

7.2.1 Walls

7.3 Concrete

7.4 Finishes

7.5 Design details


57

Twinwall 7

Figure 7.3Internal twinwall connection with a lattice

girder slab. Precast concrete


In-situ concrete


Slab reinforcement

In-situ concrete


Tie reinforcement

Lap length

In-situ concrete

In-situconcrete

Precast concrete



Tie reinforcement


Lap length

Lap length

Tie reinforcement

Figure 7.4External twinwall connection with a lattice

girder slab.


58

7 Twinwall

Manufacture, transportation and placing of the units is a key consideration in the design of a twinwall project. These considerations are similar to those for crosswall construction and are explained in section 5.8.

An effi cient construction sequence is the key to maximising the economy of twinwall construction. Ideally the twinwall panel will be lifted directly from the lorry into its fi nal position. The panels are typically propped on one side only, until the fl oor above has been cast.

The lattice girder slabs can then be placed, and again these must be propped, until the concrete in the fl oor has suffi cient strength to carry its own weight. Continuity wall and slab reinforcement is then fi xed in position before the wall and slab concrete can be poured.

The panels are typically erected with the base of the wall around 30 mm above the fl oor slab. A timber batten is usually placed on one side of the wall to act as a setting-out guide when lifting the twinwall panels into position (see Figure 7.5). A second timber batten is fi xed after the panel is placed. These timbers are removed after casting the in-situ concrete and this is the principal means of checking that the in-situ concrete has reached the base of the pour.

7.6 Construction

Ensuring the maximum section width can be used requires care on site to ensure the following:

Tight lateral tolerance of the wall on the bearing surface to ensure the wall above is aligned with the wall below.

Tight vertical tolerance, as an out-of-plumb wall could result in a reduced bearing surface. Good working practice on site to ensure compaction of the concrete in bearing between

the base of the wall and the slab. The infill should be a minimum thickness of 30 mm, with no air bubbles or honeycombing. Where possible, good compaction is achieved with self-compacting concrete and 10 mm aggregate.

Minor damage to the precast element does not result in a reduced bearing area. Appropriate reduced effective depth due to a single line of central starter bars at the

joints.

It is not possible in the space available to cover all the design requirements for a twinwall building, but detailed guidance is given in The Concrete Centre publication Design of

Hybrid Concrete Buildings29.


59

Twinwall 7

The wall and fl oor panels can be erected as fast as for crosswall construction; however, the in-situ element needs to be cast as each fl oor is completed, which slows the speed of the frame construction. In mitigation there is no need to install a separate screed. To maximise the benefi ts there should be repetition of panels. The panels should be detailed to improve buildability.

Figure 7.5Position and levelling of twinwall panel.

7.6.1 Speed of construction

7.7 Tolerances Section 5.9 on tolerances in the crosswall section should be referred to. In addition the following should be considered. The inside faces of the precast skins are an unfi nished surface and can vary by 10 to 15 mm, with implications for the space available for in-situ concrete and starter and continuity reinforcement when the panels arrive on site.

Mesh reinforcement is cast into each precast skin. A 50 mm thick precast skin could accommodate, for example:

20 mm cover to external face (or as appropriate to meet durability requirements) 10 mm vertical bar 8 mm horizontal bar 10 mm cover to internal face (while not required for durability in the permanent

condition, some cover here is advisable).

Clearly, walls which require larger bar sizes to achieve required levels of reinforcement, or walls in exposed conditions, will in turn need thicker precast skins to achieve required covers.


60

7 Twinwall

An advantage of the twinwall system is that it is simple to tie the structure together with reinforcement in the in-situ concrete, to meet the requirements of the Building Regulations to avoid disproportional collapse.

7.8 Robustness


61

Appendix A. Volumetric precast concrete prison cells

As an alternative to the systems described in this publication, volumetric precast concrete modules may be used for prisons. The system consists of modules, each containing four cells.

As with the other systems described in this publication, this system benefi ts from the fi re protection, acoustic properties and robustness of concrete. The latter is of particular benefi t in this application. Off-site construction has benefi ts for construction adjacent to existing prisons. It enables the on-site workforce to be reduced, which in turn minimises the number of security clearances required for personnel. Off-site construction also reduces the on-site construction period, which again improves security.

All the walls between cells and the roof are cast in one concrete pour using special moulds, which have been designed to simplify the demoulding process (see Figure A.1).

The window grills are cast into the concrete to increase security (see Figure A.2).

The modules are fi tted out with (see Figure A.3): sanitary ware windows furniture


Figure A.1Mould for volumetric prison cells.

Photo: Precast Cellular Structures Limited


62

Figure A.3Sanitary ware and services

are installed off-site.Photo: Precast Cellular Structures Limited

The complete 40-tonne module can then be transported to site, where it is placed on a pre-prepared ground-fl oor slab. Further units can be stacked on top of the ground-fl oor units, with the roof of the lower unit forming the fl oor of the upper unit.


Figure A.2Window grills are cast in for increased

security. Photo: Precast Cellular Structures Limited


63

Project details Calculated by OB

Job no.CCIP - 032

Checked byPG

Sheet no.CW1

ClientTCC

DateJul 08

Appendix B. Crosswall worked example

Crosswall worked exampleCrosswall worked exampleSlab design

Bathroompod

1.5 m 2.0 m

3.5 m

Variable actions

Screed

Self-weight

ActionskN/m/m width of slab

PermanentSelf-weight = 0.15 x 25 = 3.7560 mm screed = 0.06 x 22 = 1.30gk over full span = 5.05

Pod = 3.00Deduct because there is no screed under pod = 1.30gk over part span = 1.70

VariableResidential = 1.50Services and finishes = 0.50qk = 2.00

CoverNominal cover, cnom

cmin,dur = minimum cover due to environmental conditionsAssuming XC1 and using C35/45 concrete,cmin,dur = 15 mm

BS EN 1992-1-1, Table 4.1, BS 8500-1, Table A.4

Δcdev = allowance in design for deviation Assuming no measurement of cover Δcdev = 10 mm

BS EN 1992-1-1,4.4.1.2(3)

∴ cnom = cmin + Δcdev = 15 + 10 = 25 mmFire

Check adequacy of section for 1-hour fire resistance (i.e. REI 60) Thickness, hs,min = 80 mm cf. 175 mm proposed ∴ OK BS EN 1992-1-24.1(1),

5.1(1), Table 5.8 Axis distance, amin = 20 mm cf. 25 + 8/2 = 29 i.e. not critical ∴ OK

cnom = 25 mm

Load combination (and arrangement)By inspection, BS EN 1990 Exp. (6.10b) governs ∴ γGk = 1.25 and γQk = 1.5

BS EN 1990 Exp. (6.10b)

Design of one-way spanning 150 mm-thick continuous slab.


64


Job no.CCIP - 032

Checked byPG

Sheet no.CW2

ClientTCC

DateJul 08

AnalysisTotal ultimate limit state load across full width of slab = 1.25 x 5.05 + 1.5 x 2 = 9.31 kN/m/m widthAdditional ultimate limit state load under bathroom pod = 1.25 x 1.7= 2.13 kN/m/m width

Reactions (ULS):

RLHS = 1 ( 9.31 x 3.52 + 2.13 x 1.5 x 2.75) = 18.8 kN/m width 3.5 2

RRHS = 1 ( 9.31 x 3.52 + 2.13 x 1.52) = 17.0 kN/m width 3.5 2 2

Determine point of zero shear (i.e. location of maximum moment)Distance from right-hand support = 17.0/9.31 = 1.83 kN/m

Maximum sagging moment:MEd = 17.0 x 1.83 – (9.31 x 1.832) = 15.5 kNm/m width 2

Shear force (ULS), (maximum occurs at left-hand support), VEd = 16.3 + 2.5 = 18.8 kN/m

Flexural designEffective depth:

d = 150 x 25 x 8/2 = 121 mm

Flexure in span:

K = MEd/bd2fck = 15.5 x 106/(1000 x 1212 x 35) = 0.03

z = d/2 [1 + √(1 – 3.53K)] ≤ 0.95d

z = 121/2 [1 + √(1 – 3.53 x 0.03)] ≤ 0.95 x 121

z = 118 ≤ 114

z = 114 mm

As = MEd/fydz = 15.5 x 106/(500/1.15 x 114) = 313 mm2/m

Minimum area of reinforcement:

As,min = 0.17% for fck = 35 MPaAs,prov = 385/(1000 x 121) = 0.32% OK

BS EN 1992-1-1, Exp (9.1N)

Either use A393 mesh or B385 mesh (note the latter uses significantly less steel).

Deflection By inspection; deflection not critical.

Shear VEd = 18.8 kN/m width

vEd = 18.8 x 103/1000 x 121 = 0.16 MPa

vRd,c = 0.54 MPa BS EN 1992-1-1, 6.2.2(1)

∴ No shear reinforcement required


Crosswall worked exampleSlab design


65


Job no.CCIP - 032

Checked byPG

Sheet no.CW3

ClientTCC

DateJul 08


Wk = 3.5 x 3.0 x 1.0 = 10.5 kN Taking wind pressure as 1.0 kN/m2

(Assume this load is shared equally between the two 6 m long walls)Gk,fl = 5.05 x 3.5 = 17.7 kN/m (Floor load on wall)

Gk,w = 25 x 0.18 x 3.0 = 13.5 kN/m (Self-weight of wall)

Gk,tot = 17.7 + 13.5 = 31.2 kN/m (Total vertical permanent load)

Ultimate limit state loads:Axial load, N = 0.9 x 31.2 x 6 = 168.5 kNMoment, M = 1.5 x 5.3 x 3 = 23.9 kNm

Check for tension is the wall:

σt = N – M = 168.5 x 103 – 23.9 x 106

Lt (t l2/6) 6000 x 180 (180 x 60002/6)

= 0.156 – 0.022 = 0.134 MPa (no tension in wall)It may be necessary to check for tension at every floor level.

Crosswall worked exampleWall design

Wall design: Wall is 180 mm thick, check top storey of wall for lateral loads, vertical loads OK by inspection.


66


Job no.CCIP - 032

Checked byPG

Sheet no.TF1

ClientTCC

DateJul 08

Slab design: Try 175 mm-thick continuous slab.

Tunnel form worked exampleTunnel form worked exampleSlab design

Permanent actions Variable actions

6 m 6 m 6 m

Appendix C. Tunnel form worked example

ActionskN/m/m run

Permanent

Self-weight = 0.175 x 25 = 4.4gk = 4.4

VariableResidential = 1.5Services and finishes = 0.5qk = 2.0

CoverNominal cover, cnom

cmin,dur = minimum cover due to environmental conditionsAssuming XC1 and using C35/45 concrete,cmin,dur = 15 mm

BS EN 1992-1-1, Table 4.1, BS 8500-1, Table A.4

Δcdev = allowance in design for deviation Assuming no measurement of cover Δcdev = 10 mm

BS EN 1992-1-1, 4.4.1.2(3)

∴ cnom = cmin + Δcdev = 15 + 10 = 25 mm

FireCheck adequacy of section for 1-hour fire resistance (i.e. REI 60) Thickness, hs,min = 80 mm cf. 175 mm proposed ∴ OK BS EN 1992-1-2, 4.1(1),

5.1(1) and Table 5.8 Axis distance, amin = 20 mm cf. 25 + 8/2 = 29, i.e. not critical ∴ OK

cnom = 25 mm

Load combination (and arrangement)By inspection, BS EN 1990, Exp. (6.10b) governs ∴ γGk = 1.25 and γQk = 1.5

BS EN 1990, Exp. (6.10b)

Analysis Using bending moment coeffi cients for a continuous slab, a coeffi cient of 0.086Fl applies to the end span and fi rst interior support. Check end span for fl exure and shear capacity as well as defl ection.


67


Job no.CCIP - 032

Checked byPG

Sheet no.TF2

ClientTCC

DateJul 08


Design moment:F = total design ultimate load = (1.25 x 4.4 + 1.5 x 2.0) x 6 = 51.0 kN/m widthM = 0.086Fl = 0.086 x 51.0 x 6 = 26.3 kNm/m widthShear force:

V = 0.6F = 0.6 x 51.0= 30.6 kN

Flexural designEffective depth:

d = 175 – 25 – 8/2 = 146 mmFlexure in span:

K = MEd/bd2fck = 26.3 x 106/(1000 x 1462 x 35) = 0.035

z = d/2 [1 + √(1 – 3.53K)] ≤ 0.95

z = 146/2 [1 + √(1 – 3.53 x 0.035)] ≤ 0.95 x 145

z = 141 ≤ 138

z = 138 mm

As = MEd/fydz = 26.3 x 106/(500/1.15 x 138) = 438 mm2/m

Minimum area of reinforcement:As,min = 0.17% for fck = 35 MPaAs,prov = 438/(1000 x 146) x 100 = 0.30% OK

BS EN 1992-1-1, Exp (9.1N)

Use B503 mesh

DeflectionCheck span-to-effective-depth ratio ρ = 0.30% Basic span-to-effective-depth ratio = 60.6Actual span-to-effective-depth ratio = 6000/145 = 41.4 Deflection OK

BS EN 1992-1-1, Exp (7.16a)<Table 7.4N & NA><Exp. (7.17)>

ShearVEd = 30.6 kN/m

vEd = 30.6 x 103/1000 x 146 = 0.21 MPa

vRd,c = 0.54 MPa BS EN 1992-1-1, 6.2.2(1)

∴ No shear reinforcement required

Construction situation.Assume tunnel forms will be struck at 15 MPa cube strength and lifted onto new slab. Check slab for self-weight and load from formwork. (Note once tunnel form has been positioned there is no load on the interior of the slab.)

Leg loads from tunnel forms = (110/100 × 3)/2 = 1.7 kN per leg at 2 m from wall

Tunnel form worked exampleSlab design


68


Job no.CCIP - 032

Checked byPG

Sheet no.TF3

ClientTCC

DateJul 08


Self-weight of slab = 4.4 kN/m

Moment, MEd ≈ 0.086 × 4.4 × 62 + 1.7 × 2.0 = 13.6 + 3.4 ≈ 17.0 kNm

Flexural designFlexure in span:

K = MEd/bd2fck = 17.0 x 106/(1000 x 1462 x 12) = 0.068

z = d/2 [1 + √(1 – 3.53K)] ≤ 0.95d

z = 145/2 [1 + √(1 – 3.53 x 0.068)] ≤ 0.95 x 145

z = 136 ≤ 138

z = 136 mm

As = MEd/fydz = 17.0 x 106/(500/1.15 x 136) = 288 mm2/m

Less than the requirements for permanent situation OK

Tunnel form worked exampleSlab design


69

References

References

1 BRITISH STANDARDS INSTITUTION. BS 8204. Screeds, bases and in-situ fl oorings. Part 1: Concrete

bases and cement sand levelling screeds to receive fl oorings – code of practice. BSI, London, 2003.

2 BRITISH STANDARDS INSTITUTION, BS 8204-2. Screeds, bases and in-situ fl oorings. Part 2:

Concrete wearing surfaces – code of practice. BSI, London, 2003.

3 BRITISH STANDARDS INSTITUTION. BS 8204. Screeds, bases and in-situ fl oorings. Part 7:

Pumpable self-smoothing screeds – code of practice. BSI, London, 2003.

4 BRITISH STANDARDS INSTITUTION. BS 8203. Code of practice for installation of resilient fl oor

coverings. BSI, London, 2001.

5 GATFIELD, M.J. Report 184: Screeds, fl oorings and fi nishes – selection, construction and

maintenance. CIRIA, London, 1998.

6 THE CONCRETE CENTRE. Precast concrete in buildings. TCC, Camberley, 2007, Ref. TCC/03/31.

7 ELLIOTT, K.S. Multi-storey precast concrete framed buildings. Blackwell Science, Oxford, 1996.

8 DEPARTMENT FOR COMMUNITIES AND LOCAL GOVERNMENT. Building Regulations (England

and Wales) Approved document L. DCLG, London, 2006.

9 THE CONCRETE SOCIETY. Technical report TR67 Movement, restraint and cracking in concrete

structures. The Concrete Society, Camberley, 2008.

10 BRITISH STANDARDS INSTITUTION. BS EN 1992-1-2. Eurocode 2: Design of concrete structures.

Part 1-2: Structural fi re design. BSI, London, 2004.


and Wales) Approved document E (2004). DCLG, London, revised 2006.

12 ROBUST DETAILS LTD. Robust Details Part E Resistance to the passage of sound (Edition 2). RDL, Milton Keynes, 2005.

13 THE CONCRETE CENTRE. Thermal mass. TCC, Camberley, 2005, Ref. TCC/05/05.

14 THE CONCRETE CENTRE. Thermal mass for housing. TCC, Camberley, 2006, Ref. TCC/04/05.

15 DE SAULLES, T. Utilisation of thermal mass in non-residential buildings. The Concrete Centre, Camberley, 2006, Ref. CCIP-020.

16 BRITISH STANDARDS INSTITUTION. BS EN 1992-1-1. Eurocode 2: Design of concrete structures –

Part 1-1 General rules for buildings. BSI, London, 2004.

17 BRITISH STANDARDS INSTITUTION. BS 8110. The structural use of concrete. Part 1: Code of

practice for design and construction. BSI, London, 1997.

18 BRITISH STANDARDS INSTITUTION. BS EN 1990. Eurocode: Basis of structural design. BSI, London, 2002.

19 THE CONCRETE CENTRE. Concrete framed buildings. TCC, Camberley, 2006, Ref. TCC/03/024.

20 BRITISH STANDARDS INSTITUTION. BS EN 14992. Precast concrete products. Wall elements. BSI, London, 2007.

21 BRITISH STANDARDS INSTITUTION. BS EN 13747. Precast concrete products. Floor plates for fl oor

systems. BSI, London, 2005.

22 BRITISH STANDARDS INSTITUTION. EN 13670. Execution of concrete structures. Part 1: Common. BSI, London, due 2008.

23 CONSTRUCT. National structural concrete specifi cation for building construction. The Concrete Society, Camberley, due 2008.

24 BRITISH STANDARDS INSTITUTION. BS 5606. Guide to accuracy in building. BSI, London, 1990.


and Wales) Approved document E (2004). DCLG, London, revised 2006.


70

26 BRITISH STANDARDS INSTITUTION. BS EN 1991-1-7. Eurocode 1: Actions on structures. Part 1-7:

General actions – accidental actions. BSI, London, 2006,

27 ALEXANDER, S. New approach to disproportionate collapse. The Structural Engineer, London, 7 Dec 2004, p14-18.

28 BRITISH STANDARDS INSTITUTION. BS EN 1992-1-1. Eurocode 2: Design of concrete structures.

Part 1-1: Design of concrete structures. General rules and rules for buildings. BSI, London, 2004.

29 TAYLOR, H.P.J. and WHITTLE, R. Design of Hybrid Concrete Buildings. The Concrete Centre, Camberley, 2008, Ref. CCIP-030.

References


CC

IP-032R

esidential Cellular C

oncrete Buildings

O.B

rooker BEng CEng MICE M

IStructE R.H

ennessy BEng(Hons) CEng M

ICE MIStructE

Residential Cellular Concrete Buildings

This design guide is intended to provide the structural engineer with essential guidance for designing cellular-type structures. It is written for the structural engineer who has knowledge of building structures in general but who has limited or no experience of cellular structures. This guide highlights areas that require close coordination between the structural and services engineers and the architect.

Guidance is provided on selecting an appropriate solution, sizing the structure and carrying out detailed design. Detailing issues are covered, some of which should be considered at the early stages of a project to achieve an effi cient building confi guration.

CCIP-032 Published September 2008 ISBN 978-1-904482-46-8Price Group P



CI/Sfb

UDC69.056.5

Owen Brooker is senior structural engineer for The Concrete Centre where he promotes effi cient concrete design through guidance documents, presentations and the national helpline. A consultant by background, he is also author of a number of guides on the application of Eurocode 2.

Richard Hennessy is structures knowledge manager working in the structures discipline development group of Buro Happold. Richard is a structural engineer and was able to bring his fi rst-hand project experience and also Buro Happold’s collective experience of the tunnel form technique to this publication.

Residential CellularConcrete BuildingsA guide for the design and specifi cation of concrete buildings using tunnel form, crosswall or twinwall systems


O.Brooker BEng CEng MICE MIStructE

R.Hennessy BEng(Hons) CEng MICE MIStructE


CC

IP-010C

ost Model Study – C

omm

ercial BuildingsA

report comm

issioned by The Concrete C

entre

Cost Model Study –Commercial BuildingsA comparative cost assessment of the construction of multi-storey offi ce buildings


A report commissioned by The Concrete Centre

Cost Model Study – Commercial Buildings

This comprehensive and independent cost study was undertaken to evaluate a number of structural frame options for a three-storey offi ce building in an out-of-town location and a six-storey offi ce building in a city centre location. A total of 14 fl oor design options were evaluated, budget costings were assigned to all elements of construction and adjustments were made to refl ect time-related costs attributable to differences in the construction programme.

The publication outlines the analysis, the detailed costings and programmes for each structural alternative, and provides a useful resource for architects, engineers and contractors involved with evaluating the cost competitiveness of structural options for multi-storey offi ce construction.

CCIP-010 Published October 2007 ISBN 1-904482-36-8Price Group P



CI/Sfb

UDC624.94.04.033

Francis Ryder, Head of Cost at The Concrete Centre, has project managed this cost model study for commercial buildings.

For more information visit www.concretecentre.com/publications

CMS-commercial cover.indd 1CMS-commercial cover.indd 1 01/10/2007 11:26:5801/10/2007 11:26:58



CCIP-010 Published October 2007 ISBN 1-904482-36-8 Price Group P© The Concrete Centre


CCIP publications are available from the Concrete Bookshop atwww.concretebookshop.com Tel: +44 (0)7004 607777

All advice or information from The Concrete Centre is intended for use in the UK only by those who will evaluate the signifi cance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by The Concrete Centre or its subcontractors, suppliers or advisors. Readers should note that the publications from The Concrete Centre are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Cover photo: Cardinal Place © Anthony Weller/VIEW. Printed by Alden press, Witney, UK.

AcknowledgementsThe Concrete Centre, as the organisation who commissioned this independent study, would like to acknowledge the contributions of the following companies on this project:

Allies and Morrison – Architectural DesignEstablished in 1984, Allies and Morrison’s expertise includes master planning, architecture, landscape, design, interior design and conservation. Allies and Morrison routinely work on a number of master plans and played a key role in preparing master plan proposals for the London 2012 Olympics and the regeneration of the Lower Lea Valley. Past award winning commissions include One Piccadilly Gardens, Manchester; the BBC Media Village at White City; Girton College Library and Archive and the British Council in Lagos, Nigeria.www.alliesandmorrison.co.uk

Arup - Structural Design. Arup is an international fi rm of consulting engineers, with over 55 years of international experience in providing consultancy in engineering, design, planning and project management services in every fi eld related to building, civil, and industrial projects. Arup aims to provide a consistently excellent multi-disciplinary service by adding value through technical excellence, effi cient organisation, personal service and a strong commitment to sustainable design. www.arup.com

Davis Langdon LLP - Quantity SurveyingDavis Langdon LLP provides a range of integrated project and cost management services designed to maximise value for clients investing in infrastructure, construction and property, with extensive experience in projects and programmes across a broad range of sectors and building types. Davis Langdon has a culture of achieving excellence and delivers success through limiting risk, forecasting and controlling cost, managing time and resources, and maximising value for money according to the specifi c needs of the client and brief. www.davislangdon.com

Mace - Programming Mace is one of the world’s most diverse management and construction companies and is a renowned global business providing management and construction services to the public and private sectors, with a reputation for fi nding the best solutions to complex projects. Mace has been responsible for the successful delivery of a number of award-winning projects, including the More London development incorporating City Hall, Heathrow T5 and the City of London’s fourth tallest tower, 51 Lime Street. www.mace.co.uk

The following proprietary products are referenced in this publication. Slimdek® is a registered trademarks of Corus UK Ltd. Ribdeck® is a registered trademark of Richard Lees Steel Decking Ltd.



Contents

1. Summary 3

2. Introduction 5

3. Method of study 6

4. Building A – 3-Storey business park location 11

5. Building B – 6-Storey central city location 22

6. Programmes 35

7. Summary of costs 45

8. Study fi ndings 49

9. Commentary from The Concrete Centre 62

A1. Appendix A – Detailed programmes 68

Commercial Buildings - Cost Mode1 1Commercial Buildings - Cost Mode1 1 02/10/2007 11:18:4902/10/2007 11:18:49


3

Summary

1. Summary

This cost model study compares the costs of constructing three- and six-storey commercial buildings using a variety of short-span and long-span options in two different locations, taking into account construction and Category A fi t-out, and the effect of programme times on cost.

Designs were commissioned for a three-storey offi ce building in an out-of-town business park location in the south east and a six-storey offi ce building located in central London. The buildings were based upon appropriate structural grids commonly in current use, with designs and specifi cations suited to current market conditions. Architectural design was undertaken by Allies and Morrison, all structural designs were carried out by Arup, and costings were undertaken by Davis Langdon.

The designs were taken to normal outline design stage, the only differences being directly attributable to the structural frame material. Budget costings were assigned to all elements of construction, from substructure, superstructure and external envelope through to pre-liminaries, with the exception of external works, which were considered to be too highly site-specifi c to permit accurate costing. Adjustments were made to the costings to refl ect time-related costs attributable to differences in construction programmes.

Whilst identifying the variation in the costs of frames, the study also considers the effects that the choice of framing material and method of construction have on other elements of the building, as well as the other benefi ts that the choice of frame can generate.

The study demonstrates the need to consider all elements of the building cost, rather than simply the cost of the structure, and highlights the extent to which elements other than the structure are affected by the choice of frame solution.

In terms of overall construction cost for the three-storey building, the most economic solution was found to be the RC Flat Slab option, closely followed by the steel Composite option (+0.5%), with the Post-Tensioned Flat Slab and In-situ + Hollowcore options in equal third place (+1.2%). The Steel + Hollowcore option was in fi fth place (+2.4%), with the Slimdek option being the least economic (+5.1%).

In terms of overall construction cost for the six-storey building, the most economic solution was also found to be the RC Flat Slab option, closely followed by the Post-Tensioned Flat Slab option (+0.1%), with the steel Composite option in third place (+0.9%) and the In-situ + Hollowcore option in fourth place (+1.0%). The Steel + Hollowcore option was in fi fth place (+3.5%), with the Slimdek option again being the least economic (+5.0%). Of the two long-span options on this building, the Post-Tensioned Band Beam option and the Long-Span Composite option are respectively 2.2% and 2.3% more costly than the Flat Slab option.


4

Thus in consideration of the construction cost, an average of 1.0% separates the four most economic short-span options, rising to 5.1% when all six options are considered. For the two long-span solutions considered, the difference in total construction cost is negligible at 0.1%.

The main conclusions are that, for modern commercial buildings, the variation in total construction cost is relatively small across the range of structural options considered and that they are all relatively competitive. Clearly, therefore, it is the effect on other construction related factors in the project which need to be considered in the selection of the most appropriate structural choice. Factors such as cash fl ow, overall project time, fi re protection, use of fl at soffi ts and lower fl oor to fl oor height are discussed in detail in the study.

Summary


5

Introduction

2. Introduction

This Cost Model Study – Commercial Buildings was undertaken to provide both a com-parison and an understanding of the construction costs associated with commercial buildings using a variety of different structural solutions.

Cost is usually the major criterion in assessing design and construction alternatives and construction professionals require current studies in order to provide weight to their decisions. The Reinforced Concrete Council (RCC) published a cost model study on commercial buildings in 1993 (GOODCHILD, C.H. Cost Model Study, British Cement Association 97.333, 1993). The Concrete Centre identifi ed that this study needed to be updated because building types in the contemporary market are signifi cantly different from those that formed the basis of the 1993 study.

The value of the RCC study was found to be not so much in the cost results but in the detailed and rigorous assessment of how structural frame choice can affect the cost of other items, such as cladding, internal planning, fi re protection, services, fi t-out, etc. It is the independent assessment of current building types reported in this document that will be of most enduring value to quantity surveyors, architects, engineers and other construction professionals.

Thus, The Concrete Centre commissioned a study, undertaken in 2005 and 2006 by the following consultants:

Allies and Morrison Architectural Design Arup Structural Design Davis Langdon LLP Quantity Surveying Mace Programming

The objective of the study was to provide a cost comparison between various structural options for buildings of three-and six-storeys, on clear sites, in out-of-town and city centre locations respectively. Identical specifi cations were required, with the only permissible variations being directly attributable to the materials used in the structural frame.

It is emphasised that the study was undertaken on an independent basis. The structural design for all options was carried out by Arup and costs were prepared by Davis Langdon, based on pricing data obtained from their national cost database of recent projects and therefore refl ecting the current marketplace.

Procurement and construction planning/programming studies also formed part of the commissions, in order that the effects of programme on costs could be included. These were carried out by Mace.

The cost models were developed using current best practice and are reported upon in this publication. The process of designing and costing alternative methods of constructing otherwise identical buildings raises many interesting issues for those com-missioning, designing and constructing buildings. As will be shown, there are many useful conclusions to be drawn, over and above those relating simply to cost.


6

Method of study

3. Method of study

The brief given to the design team asked for the outline designs of multi-storey buildings on open clear sites, one case being an out-of-town business park in the south east and the other case being in central London. The designs were to refl ect contemporary commercial practice and the design team’s best judgement. They would be used for preparing budget costs and for making comparisons of the effects of the choice of different structural frames.

The choice, size and location of the buildings to be investigated were based on the design team’s judgement of current commercial practice and demand, and to avoid unduly favouring one structural solution over another.

Designs were commissioned for a three-storey offi ce building in an out-of-town business park location in the south east (Building A) and a six-storey offi ce building located in central London (Building B). The buildings were based upon appropriate structural grids commonly in current use, using pad or piled foundations. Specifi cations were suited to current market conditions, which suggested that Building A be an air-conditioned, L-shaped building with curtain walling and some natural ventilation and that Building B be a rec-tangular, air-conditioned building with curtain walling.

Building A was chosen to refl ect a framed building of average size (4,650m2) in a com-mercial/business park setting. It is representative of a typical low-rise building in the centres of current development activity.

Building B, containing retail space at ground fl oor level, was chosen to refl ect a high-quality framed building of average size (14,200m2 of offi ces and 2,300m2 of retail space) in Central London. It is acknowledged that a building of this type in London would normally have a basement. However, it was considered that inclusion of this element could unduly favour some of the structural options over others above ground. Accordingly, the basement construction has been excluded from the study.

The shape and form of the buildings were determined to suit typical market requirements in terms of performance and cost.

Indicative sketches for the two buildings, showing the building form, follow on page 7.

Brief

Concepts and initial studies


7

The form of Building A is an L-shape with a full-height atrium, a central service core and secondary stairs and service access located towards the ends of the building, with a limited amount of undercroft parking. Air conditioning is provided by a fan-coil system providing full climate control when active.

The internal environment is designed to maximise daylighting and allow some mid-season free cooling from natural ventilation, which saves energy and lowers CO2 emissions. This is achieved with fl oor plates 23.5m wide, confi gured around a grid of three bays of 7.5m, allowing a degree of cross-ventilation from the perimeter windows.

The building envelope comprises grid stick curtain wall cladding, incorporating fl oor to ceiling double glazing units and aluminium clad insulated spandrels, permitting good daylighting to most of the working areas.

Method of study

Building A: Three-storey. Building B: Six-storey.

Typical fl oor plans and cross-sections for both building projects used in this study


8

Method of study

The form of Building B is rectangular, arranged around a central atrium and incorporating a fan-coil unit air-conditioning system, with service cores located towards the ends of the building. The form of the building is designed with a low envelope to volume ratio, which, in addition to maximising investment return, helps minimise heat loss during the winter. The building is fully sealed, requiring full climate control year round. The building envelope comprises unitised curtain walling, incorporating fl oor to ceiling double-glazing units and stone clad insulated spandrels.

The fl oor plate depths are 9.5m to the core walls on the E-W axis and 15.5m to either the core walls or the atrium on the N-S axis. The building can be operated with single or split tenancies, with splitting by vertical division and requiring a glazed wall to the atrium.

Layouts involving circular columns and cantilevers were not pursued (other than the inclusion of two feature columns to the edge of the atrium on Building A) as they may have unduly favoured some structural solutions over others. Also, utilisation of exposed concrete inside the building to reduce capital and running costs of the air conditioning by using the thermal mass of the structure has not been considered in the base case compari-son, as this may also have unduly favoured some structural solutions over others. This is a potential benefi t which is discussed further in Chapter 9 - Commentary from The Concrete

Centre.

Investigations to determine the optimum structural grid for the proposed buildings were carried out. Grids of 7.5 × 7.5m, 9.0 × 6.0m and 9.0 × 9.0m were considered.

For Building A, a 7.5 × 7.5m grid was established as optimum and was adopted for all frame options in the study, long spans not being considered appropriate. For Building B, a 7.5 × 9.0m grid is more representative of the current market for a city centre site. It also permitted exploration of a long-span option in the study, by creating a 15.0 × 9.0m grid.

The resulting gross fl oor areas were to be approximately 1,500m2 per fl oor based on a 7.5 × 7.5m structural grid for Building A and approximately 2,750m2 per fl oor based on a 9.0 × 7.5m structural grid for Building B.

For Building A, six options were developed. For Building B, six options were developed for the short-span situations (7.5m) and two options for a long-span situation (15.0m), giving eight options in total.

The structural options were chosen as being representative of current best practice and most likely to be proposed by the design team for a commercially viable project. Indicative diagrams and descriptions for each of the options are shown in the fi gures which follow.


9

Method of study

The two buildings were taken up to normal outline design stage. The buildings were all to commercial developers’ standards with associated outline specifi cations. The only diffe-rences were directly attributable to the choice of structural solution.

The architectural schemes, layouts and specifi cations were based on contemporary commercial practice and current regulations. The new Part L of the Building Regulations had not come into effect at the time the designs were undertaken and is not therefore taken into account in the study.

Offi ce fl oors were designed to be for an open-plan confi guration on a 1.5m planning module, to allow for possible subdivision of the fl oors into two tenancies. Cellular offi ce layouts were allowed for. Potential partitions may be aligned with external wall mullions or piers at 1.5m centres. Initial fl oor plans and core layouts were adjusted and modifi ed following liaison and discussion between the design team members. In particular, core areas were modifi ed as necessary to accommodate structural and engineering services’ requirements and to suit the peculiarities that result from the choice of structural solution.

No design was undertaken for external works and landscaping, these aspects being so highly site-specifi c as to preclude meaningful consideration. The extent, layout and complexity of external works are to a large extent dictated by the size, confi guration and orientation of the site for each particular project, together with constraints imposed by location and external factors such as planning. The extent to which external works are likely to be infl uenced to any signifi cant degree by the choice of structural solution is considered to be minimal, and consequently, consideration of external works is beyond the scope of this study.

Short-span options - Building A and B Long-span options - Building B only

Option 1 - Flat Slab Option 2 - Composite Option 3 - PT Flat Slab Option 7 - PT Band Beams

Reinforced In-situ concrete fl at slab and columns

Steel beams and metal decking, both acting compositely with In-situ concrete fl oor slabs. Steel columns

Post-tensioned In-situ concrete fl at slab and reinforced In-situ concrete columns

Post-tensioned In-situ concrete fl at slab and band beams with reinforced In-situ concrete columns

Short-span options - Building A and B

Option 4 - Steel + Hollowcore Option 5 - In-situ + Hollowcore Option 6 - Slimdek Option 8 - Long-Span Composite

Steel beams acting compositely with precast concrete hollowcore fl oor slabs. Steel columns

Reinforced In-situ concrete beams and columns with precast concrete hollowcore fl oor slabs

Slimdek system comprising asymmetric beams and metal decking, both acting compositely with In-situ concrete fl oor slabs. Steel columns

Long-span cellular steel beams and metal decking, both acting compositely with In-situ concrete fl oor slab. Steel columns

Scheme designs


10

Specifi cations and drawings

The fi nal structural zones represent those considered, by the design team’s experience and judgement, to be optimum depths for the structures.

Design criteria and outline specifi cations were fi nalised and scheme drawings were prepared for each building for all structural options. The design information is presented in this document as follows:

Design criteria Architectural, structural and services

Outline specifi cations Architectural, structural and services

Architectural drawings Typical fl oor plans

Structural drawings Partial fl oor plans and fl oor zone for each of the following options:

Flat Slab

In-situ + Hollowcore

PT Flat Slab

Composite

Steel + Hollowcore

Slimdek

PT Band Beams

Long-Span Composite

Costings were based on drawings and specifi cations prepared for all options, for both buildings.

Structural schemes were prepared for each frame option to allow for an order of cost to be assessed and thus a comparison made (and not for an absolute cost to be determined). The level of information provided on each scheme was equivalent to that which would be prepared in a normal scheme design. Quantities and estimates of cost and areas were pre-pared from the scheme design information. Budget costings were assigned to all elements of construction, from substructure, superstructure and external walls through to preliminaries, using rates appropriate to the specifi cations and locations and a base date of June 2006.

The costings were presented in the form of summaries and are contained within Chapter 7 Summary of costs, where information on key rates is also presented.

Detailed construction programmes were prepared on the basis of the drawings, specifi ca-tions and quantities outlined in this report; these are presented in the form of bar charts and are contained within Chapter 6 Programmes. Procurement programmes and contractor lead times were also considered.

A more detailed explanation of the planning and programming, including notes on the assumptions made and the logic used, is given within Chapter 6 Programmes, and examples of the detailed programmes are contained within Appendix A.

Basis of costing and quantities

Planning and programming

Method of study


11

Building A - Design criteria

4. Building A

The following design criteria, representative of current good practice and commercial standards, form the basis of the study.

Plan dimensionsPlanning grid 1500 × 1500mm

Partition grid 1500 × 1500mm

Structural grid 7500 × 7500mm

Vertical dimensions 3950–4160mm (see drawings)

Floor to ceiling height 2700mm

Raised fl oor 250mm

Occupancy

Density One person per 10m2 of nett internal fl oor area.

Design target populations Three-storey 407 total, 319 on upper fl oors.

Ancillary accommodation

Core areas Include male and female toilets and cleaners’ cubicle on each fl oor, disabled toilets and PABX equipment on the ground fl oor.

Codes of practice and standards

Concrete BS 8110 Part 1: 1997 (amendments 1 & 2) - Structural use of concrete

Structural steelwork BS 5950 Part 1: 2000 - Structural use of steelwork in buildings

Loads

Imposed load Offi ces: 4.0 + 1.0kN/m2 for partitionsRoof: 0.75kN/m2

Dead load Self-weight plus superimposed dead load of 0.9kN/m2

Line loads External cladding 8kN/mAtrium glazing 8kN/mInternal blockwork walls 10kN/m

Defl ections

General Defl ections will be limited in accordance with the guidance in the appropriate Code of Practice.

Fire rating 1hr

Vibration Natural frequency limited to 4Hz.

Ground conditions

Bearing pressure It has been assumed that the site provides a bearing capacity suitable for pad foundations and a ground-bearing ground fl oor slab with an N value of approximately 30 in a Standard Penetration Test. It has been assumed that the water table is below founding level.

Lateral stability Frame action

Propping Propping is required for the Slimdek system during construction. No propping is required for the other steel frames.

General All normal services to be provided to typical contemporary commercial standards, including: heating, lighting, ventilation, lifts, hot and cold water supply, drainage, fi re services, small power, provision for communications, lightning protection, etc.

Ventilation

General Air-conditioned using fan-coil system, with partial natural ventilation.

Design criteria

Architectural

Structural

Services


12

Building A - Outline specifi cations

External envelope

External wall Proprietary grid stick curtain walling system, 170mm thick overall.

Internal atrium walls(if required for tenancy split)

Grid stick curtain wall incorporating fl oor to ceiling single glazing units and aluminium clad spandrels.

Rain screen to stair cores Aluminium panel rain screen with ventilated insulated cavity supported on slab-bearing blockwork walls or grid frame.

Plant rooms Proprietary curtain walling panel system.

Plant screen Coated aluminium louvres connected to steel panels bolted to slab upstands.

Flat roofs Inverted roof build-up with monolithic hot applied bitumen polymer membrane, insulation and ballast.

Offi ce areas

Floors Proprietary medium duty raised fl oor system, 250mm overall, and carpet.

Ceilings 500 × 500mm pre-fi nished fully demountable perforated tile with concealed suspension system.

External walls Grid stick curtain wall incorporating fl oor to ceiling double glazing units and aluminium clad insulated spandrels. Solar control by soft coat glass and fritting to south/west elevations and soft coat glass to north and east façades. Mullions to be top hung from roof level.Façades incorporate high-level openable vents allowing cross ventilation for daytime cooling during temperate weather and/or night time purging.Operation of the façade vents by remote control to prevent users opening at the wrong times.

Columns Emulsion-painted plastered concrete or painted dry-lined encased steel columns.

Skirtings Recessed fl ush painted softwood.

Entrance halls/ground fl oor lift lobbies

Floor Carpet.

Walls Emulsion painted dry-lining or plaster.

Ceilings Pre-fi nished 500 × 500mm metal tile with concealed grid.


Furniture Reception desk.

Doors Stainless steel revolving doors.

Lift lobbies (upper fl oors)

Floors Carpet.



Toilets

Floor Unglazed ceramic tiles.

Walls Glazed ceramic tiles. Full-height cubicle partitions and doors.


Skirtings Ceramic tile.

Lighting Downlighters.

WCs Suspended WC pans with concealed cisterns.

Wash basins Fully or semi-recessed vanity mounted hand basins with concealed UPVC pipework, polished granite top.

Urinals White vitreous china.

Mirrors Full height and width.

Vanity shelf Polished granite.

Fittings Polished stainless steel fi ttings and shaving point.

Hand drying Recessed paper towel dispensers.

Outline specifi cations

Architectural


13


Staircases

Floors Precast terrazzo (primary stair) or granolithic (secondary stair) treads and risers with non-slip nosings, with mild steel painted stringer.

Walls Emulsion painted suspended plasterboard dry-lining.

Ceilings Emulsion painted suspended plasterboard dry-lining.

Handrail Polished stainless steel top and secondary rails.

Balustrade Polished stainless steel posts.

Internal doors

Doors Hardwood veneered plywood solid core doors with overpanels and painted hardwood frames.

Ironmongery Polished stainless steel.

Plant spaces

Enclosed Floors: screed laid to falls. Walls/ceilings: unfi nished structure/blockwork.

Open/external Precast paving slabs and gravel ballast.

Intake room Unfi nished structure.

Substructures

Foundations Mass/reinforced concrete pads, cast on 75mm blinding on compacted formation.

Slab Ground-bearing slab with edge thickening and mesh reinforcement to top face. Joints provided with debonded bars on all gridlines to control cracking. Allowance made for lift pits and manholes.

Superstructures

Structural frames Specifi cation as given on partial fl oor plans.Plant room enclosures: steel frame (25kg/m2) supporting lightweight cladding.

Fire One hour fi re protection to all structural members apart from roof structure (no fi re protection required). The building is not sprinkler-protected.

Air conditioning

Design data 22°C dry bulb +/– 1°C.50% RH +10%/–15% RH.0.25 air changes per hour for offi ce areas.

Internal thermal loads:Occupants 8W/m2

Offi ce lighting 10W/m2

Offi ce small power 18W/m2

Occupancy One person per 10m2.

Fresh air allowance 12 litres per second per person.

Supply All offi ces air-conditioned by means of four-pipe fan-coil system.

Air handling Roof-mounted air handling units serving all areas of the building. Chilled water generated by a central refrigeration plant.

Heating

General: Low temperature hot water system. Gas fi red boiler plant in roof plant room.

Architectural continued..

Structural

Services


14


Ventilation

Supply Vents incorporated within suspended ceiling. All toilet areas provided with mechanical supply and extract system.

Plumbing services

Cold water Rising main to cold water storage tank feeding central core and LTHW system. Separate drinking water system.

Hot water Hot water from central roof mounted storage feeding core areas.

Roof drainage Rainwater outlets connected to vertical stacks.

Foul drainage All foul waste to discharge into Local Authority foul water drainage system.

Fire services Hose reels.

Control systems

Control All mechanical services plant and equipment controlled by central BMS.

Electrical services

Load densities Offi ces:Lighting 12W/m2

Small power 15W/m2

Air conditioning 60W/m2

Miscellaneous 10W/m2

Lighting Generally to L2 offi ce standard; control by switches with key switches for the emergency fi ttings. Emergency fi ttings to be self-contained.

Small power Distribution within raised fl ooring via fl oor boxes. Cleaners’ sockets to walls and circulation spaces.

Communications Provision within fl oor boxes for tenants’ installations.

Lightning Protection system complying with BS 6651:1999.

Lift installation

Design criteria Designed to serve an overall, building population of one person per 14 m2. 15% of the design target population to be handled in a fi ve minute period.

Services


15

Building A - Architectural drawings

Architectural drawings

In the structural drawings which follow, one page is dedicated to each structural option. On each page is part of a typical fl oor which represents the area highlighted in blue. In addition a cross-section through the fl oor zone accompanies each plan.

Typical fl oor plan

A B C D

7500 7500 7500

7500 7500 7500 7500 7500

E F G H I

6

5

4

3

2

1

7500

7500

7500

7500

7500

Full height

atrium


16

Building A - Structural drawings

Structural drawings

All Columns450 x 450 unlessotherwise noted

Full HeightAtrium

Lifts Void

Void

Stairs

ToiletZone

7500 7500 7500 7500

7500

7500

7500

7500

200mm R C Shear walls

800 ØR C Column

800 ØR C Column

300

600

150

300 mm RC slab

Services zone

Ceiling and lighting zone

Section through floor zone

1050

Flat Slab1. 300mm concrete flat slab to upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Assumed design imposed loads: Roof: 0.75kN/m2

Plant room: 7.5kN/m2

Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 300mm Services zone: 600mm Floor zone = 1050mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 4000mm

]


17

Composite1. 130mm lightweight concrete slab on 1.2mm Ribdeck AL on

steel frame to upper floors and roof.2. Lightweight concrete class C 32/40.3. High-yield reinforcement.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 130mm Services zone(1): 807mm Floor zone = 1087mm Ceiling/lighting: 150mm ≈ 1090mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 4040mm (1) including downstand beams

350 (min)

130

457 x 152 UB52

Services zone



150

A193 mesh

1.2mm ribdeck AL1087

800

(min)

Lift

7500

406 x 140 UB46 406 x 140 UB46 406 x 140 UB46 406 x 140 UB46

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

7500 7500 7500

7500

6000

7500

7500

7500

Void406 x 140 UB46

457 x 152 UB52

457 x 152UB52

254

x102

UB25

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

356

x127

UB33

ToiletZone

457 x 152 UB52

457 x 152 UB52

457 x 191UB82

457 x 152 UB52

457 x 191 UB67

356

x127

UB33

406

x140

UB39

406

x140

UB39

356

x127

UB33

Full HeightAtrium

356

x127

UB33

508ØCHS153

457 x 152 UB52

356

x127

UB33

356

x127

UB33

406 x 140 UB46

356

x127

UB33

356

x127

UB33

356

x127

UB33

Void

203 x 203 UC46

508ØCHS153

Stairs

457

x191

UB82

457

x191

UB82

203 x 203 UC46 C3C3

C3 C3

C1 C1

C2C2

C2 C2 C3 C3 C1

C3 C1 C1

C1C2C2C2C2

203 x 203UC46

C3 = 305 x 305 UC97

C2 = 254 x 254 UC89

C1 = 254 x 254 UC73

406 x 140UB39 4

06

x140

UB39

]



18


Void

Void

Full HeightAtrium

7500

7500

7500

7500

7500750075007500

600 x 600 R C Beam (typical)All Columns400 x 400 unlessotherwise noted

ToiletZone

Lifts

600 x 250 R C Beam (edge)

200mm R CShear walls

Stairs

800 ØR C Column

800 ØR C Column

600 x 250 R C Beam (trimmer)

200

800(min)

600

Services zone



A142 mesh in 50mm

(min) structural topping

250

150mm

hollowcore

450 450

425

600

1150

600

350(min)

150

50

Typical edge

beam

600

In-situ + Hollowcore1. 150mm precast concrete hollowcore units with 50mm (min)

mesh reinforced structural topping to upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab and topping: 200mm Services zone(1): 800mm Floor zone = 1150mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 4100mm (1) including downstand beams

]


19


Full HeightAtrium

Lifts Void

Stairs

ToiletZone

7500 7500 7500 7500

7500

7500

7500

7500

200mmR C Shear walls

800 ØR C Column

800 ØR C Column


Void

250

600

1000

150

Services zone



Reinforcement

250mm Post-tensioned slab

PT duct

PT Flat Slab1. 250mm post-tensioned concrete flat slab to upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Post-tensioning: Each post-tensioning tendon has five No. 12.7 mm

diameter strands.5. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

6. Vertical dimensions: Slab: 250mm Services zone: 600mm Floor zone = 1000mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 3950mm

]


20

7500 7500

139.7 x 5.0 CHSФ 139.7 x 5.0 CHSФ 139.7 x 5.0 CHSФ 139.7 x 5.0 CHSФ

139.7 x 5.0 CHSФ139.7 x 5.0 CHSФ

139.7 x 5.0 CHSФ

139.7 x 5.0 CHSФ

457 x 191 UB74

457

x191

UB67

457

x191

UB67

457

x191

UB67

7500 7500

7500

6000

7500

7500

7500

Void533 x 210 UB82 406 x 140 UB39

457 x 191 UB67

406 x 140 UB39406 x 140 UB39

406

x178

UB54

406

x178

UB54

457

x191

UB67

457

x191

UB67

457

x191

UB67

Lifts

ToiletZone

457

x191

UB67

533 x 210 UB82

457

x191

UB67

406

x178

UB54

152 x89 UB16

508Ø CHS153

406

x178

UB54

406

x178

UB54

457

x191

UB67

Void

203 x 203 UC60

406 x 178 UB54

305

x305

UC97

203 x 203UC60

406

x178

UB54

508Ø CHS153

457

x191

UB67

Full HeightAtrium

Stairs

305

x305

UC97203 x 203 UC60

C2C2C2

C2

C2

C2

C2C2

C2

C2

C2

C1C3C3

C4 C4C4

C3

C1

C3 C3

C3 = 305 x 305 UC118

C4 = 305 x 305 UC137

C2 = 305 x 305 UC97

C1 = 254 x 254 UC89


350 (min)

250

457 x 191 UB67

Services zone



150

A142 mesh in 50mm structural topping

200 mm Hollowcore

1207

800

(min)

Steel + Hollowcore1. 200mm precast concrete hollowcore units with 50mm (min)

mesh reinforced structural topping on steel frame to upper floors and roof.

2. Concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 250mm Services zone(1): 807mm Floor zone = 1207mm Ceiling/lighting: 150mm ≈ 1210mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 4160mm (1) including downstand beams

]


21


Void

300

ASB

155

300

ASB

155

203 x 102 UB23

203 x 102 UB23 203 x 102 UB23 203 x 102 UB23 203 x 102 UB23

75007500 7500 7500

Lifts Void

Stairs

305

x305

UC97

203 x 203 UC60

203 x 203 UC60

200 ASB124

203 x 203UC60

305

x305

UC97

508Ø CHS153

508Ø CHS153

280ASB74

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

300

ASB

155

203 x 102 UB23

203 x 102 UB23

300 ASB196

300 ASB196

203 x 102 UB23

300 ASB196 300 ASB196 280 ASB74

7500

7500

7500

7500

6000

Full HeightAtrium

ToiletZone

C1 = 254 x 254 UC89

C2 = 254 x 254 UC107

C3 = 305 x 305 UC97

C4 = 305 x 305 UC118

C5 = 305 x 305 UC137

C3C3

C3C3

C3C1

C2 C4 C5 C1

C1C2

C2

C5

C2 C2

C5 C5

C2 C1

C1

342

600

150

SD225 Deep decking

Services zone



1092

In situ concrete slab

300 ASB 155

A193 mesh

Slimdek1. 342mm (overall) concrete slab on SD225 deep decking on

asymmetric steel beams to upper floors and roof.2. Concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 342mm Services zone: 600mm Floor zone = 1092mm Ceiling/lighting: 150mm ≈ 1095mm Floor to ceiling: 2700mm Raised floor: 250mm Total: 4045mm

]


22

Building B - Design criteria

5. Building B

The following design criteria, representative of current good practice and commercial standards, form the basis of the study.

Plan dimensionsPlanning grid 1500 × 1500mm

Partition grid 1500 × 1500mm

Structural grid 7500 × 9000mm

Vertical dimensions 3950–4235mm (see drawings)

Floor to ceiling height 3200mm GF–1st Floor; 2700mm 1st–5th Floor

Raised fl oor 250mm

Occupancy

Density One person per 10m2 of nett internal fl oor area.

Design target populations 1,215 total.Ancillary accommodation

Core areas Include male and female toilets and cleaners’ cubicle on each fl oor, disabled toilets and PABX equipment on the ground fl oor.

Codes of practice and standards

Concrete BS 8110 Part 1: 1997 (amendments 1 & 2) - Structural use of concrete

Structural steelwork BS 5950 Part 1: 2000 - Structural use of steelwork in buildings

Loads

Imposed load Offi ces: 4.0 + 1.0kN/m2 for partitionsRoof: 0.75kN/m2

Dead load Self-weight plus superimposed dead load of 0.9kN/m2

Line loads External cladding 8kN/mAtrium glazing 8kN/mInternal blockwork walls 10kN/m

Defl ections

General Defl ections will be limited in accordance with the guidance in the appropriate Code of Practice.

Fire rating 1½hrs

Vibration Natural frequency limited to 4Hz.

Ground conditions

Bearing pressure The ground is assumed as typical made ground to GFL –5m and clay from GFL –5m to depth. Piles are 750mm diameter open bored piles using C30/37 concrete.

A pile capacity working load capacity of 1MN @ 14m penetration into the clay, varying linearly to 2MN @ 23m penetration. Maximum pile length 28m.

It is assumed that the ground does not have suffi cient capacity to carry a ground-bearing slab and that all options would have a rein-forced In-situ concrete suspended slab. In long-span options, inter-mediate piles are provided to reduce the span of this ground slab.

Lateral stability Braced frame using shear walls.

Propping Propping is required for the Slimdek system during construction. No propping is required for the other steel frames.

Design criteria

Architectural

Structural


23

Services

Building B - Outline specifi cations

General All normal services to be provided to typical contemporary commercial standards, including: heating, lighting, ventilation, lifts, hot and cold water supply, drainage, fi re services, small power, provision for communications, lightning protection, etc.

Ventilation

General Air-conditioned using four-pipe fan coil system.

External envelope

External wall Proprietary curtain walling system, 170mm thick overall.

Shop fronts atground fl oor

Floor-to-ceiling ground-supported single glazing in aluminium grid frame, with aluminium louvre spandrel system over for ventilation of shops.

Internal atrium walls(if required for tenancy split)

Unitised curtain wall incorporating fl oor-to-ceiling single glazing units and aluminium clad spandrels.Individual units to be 1500 or 3000mm wide × storey height, top hung from slab edge.

Plant rooms Proprietary curtain walling panel system.

Plant screen Coated aluminium louvres connected to steel panels bolted to slab upstands.

Flat roofs Inverted roof build-up with monolithic hot-applied bitumen polymer membrane, insulation and ballast.

Atrium roof Fritted double glazed units in an aluminium grid frame.

Offi ce areas

Floors Proprietary medium duty raised fl oor system, 250mm overall, and carpet.

Ceilings 500 × 500mm pre-fi nished fully demountable perforated tile with concealed suspension system.

External walls Unitised curtain wall incorporating fl oor to ceiling double glazing units and stone clad insulated spandrels. Individual units to be 1500 or 3000mm wide × storey height top hung from slab edge. Solar shading to south and west façades in the form of external horizontal/vertical brises-soleils cantilevered off the face of the building to allow façade cleaning access.

Columns Emulsion painted plastered concrete or painted dry-lined encased steel columns.


Retail space

Generally left as shell fi nish for fi t-out by tenants

External walls Floor to ceiling ground supported single glazing in aluminium grid frame, with aluminium louvre spandrel system over for ventilation.Stone panel rain screen with ventilated insulated cavity supported on ground-bearing blockwork walls or grid frame.

Entrance halls/ground fl oor lift lobbies

Floor Carpet.




Furniture Reception desk.

Doors Stainless steel revolving doors.

Lift lobbies (upper fl oors)

Floors Carpet.



Outline specifi cations

Architectural


24


Toilets

Floor Unglazed ceramic tiles.

Walls Glazed ceramic tiles. Full height cubicle partitions and doors.


Skirtings Ceramic tile.

Lighting Downlighters.

WCs Suspended WC pans with concealed cisterns.

Wash basins Fully or semi-recessed vanity mounted hand basins with concealed UPVC pipework, polished granite top.

Urinals White vitreous china.

Mirrors Full height and width.

Vanity shelf Polished granite.

Fittings Polished stainless steel fi ttings, and shaving point.

Hand drying Recessed paper towel dispensers.

Staircases

Floors Precast terrazzo (primary stair) or granolithic (secondary stair) treads and risers with non-slip nosings, with mild steel painted stringer.

Walls Emulsion painted suspended plasterboard dry-lining.

Ceilings Emulsion painted suspended plasterboard dry-lining.

Handrail Polished stainless steel top and secondary rails.

Balustrade Polished stainless steel posts.

Internal doors

Doors Hardwood veneered plywood solid core doors with overpanels and painted hardwood frames.

Ironmongery Polished stainless steel.

Plant spaces

Enclosed Floors: screed laid to falls. Walls/ceilings: unfi nished structure/blockwork.

Open/external Precast paving slabs and gravel ballast.

Intake room Unfi nished structure.

Substructures

Foundations Piled foundations (750mm diameter open-bored piles, maximum length 28m); intermediate piles provided for long-span options.

Slab In-situ reinforced concrete suspended slab. Allowance made for lift pits and manholes.

Superstructures

Concrete Specifi cation as given on partial fl oor plans.Plant room enclosures: steel frame (25kg/m2) supporting lightweight cladding.

Steel Steel frames as shown on partial fl oor plans.

Fire One-and-a-half-hour fi re protection to all structural members apart from roof structure (no fi re protection required). The building is not sprinkler-protected.

Architectural continued..

Structural


25


Air conditioning

Design data 22°C dry bulb +/– 1°C.50% RH +10%/–15% RH.0.25 air changes per hour for offi ce areas.

Internal thermal loads:Occupants 8W/m2

Offi ce lighting 10W/m2

Offi ce small power 18W/m2

Occupancy One person per 10m2.

Fresh air allowance 12 litres per second per person.

Supply All offi ces air-conditioned by means of four-pipe fan coil system

Air handling Roof-mounted air handling units serving all areas of the building. Chilled water generated by a central refrigeration plant.

Heating

General Low temperature hot water radiant panels. Gas fi red boiler plant in the roof plant room.

Ventilation

Supply Vents incorporated within suspended ceiling. All toilet areas provided with mechanical supply and extract system.

Plumbing services

Cold water Rising main to cold water storage tank feeding central core and LTHW system. Separate drinking water system.

Hot water Hot water from central roof mounted storage feeding core areas.

Roof drainage Rainwater outlets connected to vertical stacks.

Foul drainage All foul waste to discharge into Local Authority foul water drainage system.

Fire services Hose reels.

Control systems

Control All mechanical services plant and equipment controlled by central BMS.

Electrical services

Load densities Offi ces:Lighting 12W/m2

Small power 15W/m2

Air conditioning 60W/m2

Miscellaneous 10W/m2

Lighting Generally to L2 offi ce standard; control by switches with key switches for the emergency fi ttings. Emergency fi ttings to be self-contained.

Small power Distribution within raised fl ooring via fl oor boxes. Cleaners’ sockets to walls and circulation spaces.

Communications Provision within fl oor boxes for tenants’ installations.

Lightning Protection system complying with BS 6651:1999.

Lift installation

Design criteria Designed to serve an overall, building population of one person per 14 m2. 15% of the design target population to be handled in a fi ve minute period.

Services


26

Building B - Architectural drawings

Architectural drawings

Typical fl oor plan

In the structural drawings which follow, one page is dedicated to each structural option. On each page is part of a typical fl oor which represents the area highlighted in blue. In addition a cross-section through the fl oor zone accompanies each plan.

9000 9000 9000 9000 9000 9000 9000 9000

7500

7500

9000

7500

7500

A B C D E F G H I

1

2

3

4

5

6

Full height

atrium


27

550 x 550 (G-2)450 x 450 (2-R)


550 x 550 (G-2)450 x 450 (2-R)

550 x 550 (G-2)450 x 450 (2-R)

550 x 550 (G-2)450 x 450 (2-R)

550 x 550 (G-2)450 x 450 (2-R)

550 x 550 (G-2)450 x 450 (2-R)

9000 90009000 9000

7500

7500

9000 Atrium

LiftsStairs

ToiletZone

Void Lobby200mm R CShear walls

Building B - Structural drawings

Flat Slab1. 325mm concrete flat slab to upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 325mm Services zone: 600mm(1) Floor zone = 1075mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm (2)

Raised floor: 250mm Total: 4025mm (1) increase to 750mm for GF - 1st (2) increase to 3200mm for GF - 1st

325

600

150

325 mm RC slab

Services zone



1075

Structural drawings

]


28


PT Flat Slab1. 250mm post-tensioned concrete flat slab to upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Post-tensioning: Each post-tensioning tendon has five No. 12.7 mm

diameter strands.5. Assumed design imposed loads: Roof: 0.75 kN/m2


Offices: 5.0kN/m2

6. Vertical dimensions: Slab: 250mm Services zone: 600mm(1) Floor zone = 1000mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm (2)


9000 90009000 9000

7500

7500

9000

All Columns500 x 500 (G-1)450 x 450 (1-R)

LiftsStairs

ToiletZone

Void

Atrium

Lobby200mm R CShear walls

250

600

1000

150

Services zone



Reinforcement

250mm Post-tensioned slab

PT duct

]


29

Void

7500

9000

7500

9000 9000

254 x146UB31

406 x 140 UB39

457

x191

UB67

457

x191

UB67

457

x191

UB67

457

x191

UB67

406 x 140 UB46

406 x 140 UB39

406 x 140 UB46

406 x 140 UB46

406 x 140 UB46

406 x 140 UB39

406 x 140 UB46

406 x 140 UB46

406 x 140 UB46

9000 9000

LiftsStairs

ToiletZone

Atrium


406

x140

UB46

406 x 140 UB46

457

x191

UB67

406 x 140 UB46 406 x 140 UB39

406 x 140 UB39 254 x 146 UB31

406 x 140 UB39

406 x 140 UB46 406 x 140 UB46

254 x 146 UB31

457

x191

UB67

406

x140

UB39

254

x146

UB31

254 x 146 UB31

406

x140

UB46

406 x 140 UB46

406 x 140 UB39

406 x 140 UB46

406 x 140 UB46

406

x140

UB46

457

x191

UB67

457

x191

UB67

457

x191

UB67

457

x191

UB67

406 x 140 UB46

406 x 140 UB39

406 x 140 UB46

406 x 140 UB46

406 x 140 UB46

C1 = 254 x 254 UC73

C2 = 305 x 305 UC97

C3 = 356 x 368 UC129

C2C2

C2

C3

C2C2

C3

C2

C3

C2C2

C3C2

C1

C2

C2 C3

C3


350 (min)

130

457 x 191 UB67

Services zone



A193 mesh

1.2mm ribdeck AL1087

150

800

(min)

Composite1. 130mm lightweight concrete slab on 1.2mm Ribdeck AL on

steel frame to upper floors and roof.2. Lightweight concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 130mm Services zone(1): 807mm(2) Floor zone = 1087mm

Ceiling/lighting: 150mm ≈ 1090mm Floor to ceiling: 2700mm (3)

Raised floor: 250mm Total: 4040mm (1) including downstand beams (2) increase to 950mm for GF - 1st (3) increase to 3200mm for GF - 1st

]


30

In-situ + Hollowcore1. 150mm precast concrete hollowcore units with 50mm (min)

mesh reinforced structural topping to upper floors and roof. In-situ reinforced concrete beams and columns.

2. Concrete class C 32/40.3. High-yield reinforcement.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions : Slab and topping: 200mm Services zone(1): 800mm(2) Floor zone = 1150mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm (3)


A

A

9000 9000

All Columns450 x 450

ToiletZone

Stairs

Void

A A

A A

A

B

B

B

B

A

A

A

A

A

B

BB

B

9000 9000

LiftsAtrium


A A

A

A A

C

B B

A A

B B

Beam Schedule

A 600 x 600 R C Beam

B 600 x 250 R C Beam

C 600 x 425 R C Beam

7500

9000

7500

200

800(min)

600

Services zone



A142 mesh in 50mm

(min) structural topping

250

150mm

hollowcore

450 450

425

600

1150

600

350(min)

150

50

Typical edge

beam

600


]


31

PT Band Beams1. 225mm post-tensioned concrete flat slab with band beams to

upper floors and roof.2. Concrete class C 32/40.3. High-yield reinforcement.4. Post-tensioning: Each post-tensioning tendon has five No. 12.7

mm diameter strands.5. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

6. Vertical dimensions: Slab: 225mm Services zone(1): 800mm(2) Floor zone = 1175mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm (3)


225

325

800

(min)

Services zone



2500 (typical)

150

PT Duct

1175

350

(min)

All Columns800 x 800

550 x 2500 P TBeam (typical)

LiftsStairs

ToiletZone

Void

Atrium


550 x 1750 P Tedge beam

550 x 2750 P Tbeam

7500

7500

9000

9000 9000 9000 9000


]


32

Long-Span Composite1. 130mm lightweight concrete slab on 1.2mm Ribdeck AL on steel

frame to upper floors and roof. Steel columns and cellular beams.2. Lightweight concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab and decking: 130mm Services zone(1): 800mm(2) Floor zone = 1080mm Ceiling/lighting: 150mm Floor to ceiling: 2700mm (3)


130

Cellular beam

457 x 191 UB67 +

533 x 210 UB92

Services zone



800

(min)

A142 mesh1.2 mm ribdeck AL

1080710

(Typical)

150

406

x140

UB39

406 x 140 UB39

356

x127

UB33

533 x 210 UB82

610 x 229 UB101

610 x 229 UB101610 x 229 UB101

610 x 229 UB101

356

x127

UB33

406

x140

UB39

406

x140

UB39

406

x140

UB39

406

x140

UB39

406

x140

UB39

406

x140

UB39

Lifts

Stairs

Void

Lobby


ToiletZone

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

457

x191

UB67

+533

x210

UB92

406

x140

UB39

533 x 210 UB82 533 x 210 UB82 533 x 210 UB82 533 x 210 UB82

610 x 229 UB101

533 x 210 UB82

7500

7500

9000

9000 90009000 9000

Atrium

C3

C3

C1

C2 C3 C3 C3 C3

C3C3C4

C4 C3 C3

C1 = 254 x 254 UC73

C2 = 305 x 305 UC97

C3 = 356 x 368 UC129

C4 = 356 x 368 UC177

610 x 229 UB101


]


33

Steel + Hollowcore1. 200mm precast concrete hollowcore units with 50mm (min)

mesh reinforced structural topping on steel frame to upper floors and roof.

2. Concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 250mm Services zone(1): 883mm(2) Floor zone = 1283mm Ceiling/lighting: 150mm ≈ 1285mm Floor to ceiling: 2700mm (3)


350 (min)

250

533 x 210 UB82

Services zone



A142 mesh in 50 mm (min) structural topping

200 mm Hollowcore

1283

150

800

(min)

9000 90009000 9000

533 x 210 UB82

406

x140

UB46

406

x140

UB46

457 x 191 UB82

457 x 191 UB67 457 x 191 UB67 457 x 191 UB67 457 x 191 UB67

406

x140

UB39

406

x140

UB39

7500

7500

254 x146

UB31

203

x133

UB25

203

x133

UB25

203

x133

UB25

203

x133

UB25

203

x133

UB25

203

x133

UB25

203

x133

UB25

203

x133

UB25

533 x 210 UB82 533 x 210 UB82 533 x 210 UB82

9000

Lifts

Stairs

ToiletZone

Void

Atrium


406 x178

UB54

406

x178

UB60

457 x 191 UB82

533

x210

UB101

406 x 178UB54

457 x 191 UB67

533

x210

UB101

406 x 178 UB54

305 x 165UB40

457

x191

UB67

457 x 191 UB67

457 x 191 UB67

406 x 178 UB54

533

x210

UB82

457 x 191 UB67 457 x 191 UB67

406

x178

UB60

C1 C3

C3

C3

C3 C4

C4

C3

C2 C2

C3 C3

C3C3

C2

C3 C3

C2

C1 = 305 x 305 UC97

C2 = 305 x 305 UC158

C3 = 356 x 368 UC129

C4 = 356 x 368 UC153


]


34


Slimdek1. 342mm (overall) concrete slab on SD225 deep decking on

asymmetric steel beams to upper floors and roof.2. Concrete class C 32/40.3. Steel grade S355.4. Assumed design imposed loads: Roof: 0.75kN/m2


Offices: 5.0kN/m2

5. Vertical dimensions: Slab: 342mm Services zone: 600mm(1) Floor zone = 1092mm Ceiling/lighting: 150mm ≈ 1095mm Floor to ceiling: 2700mm (2)


342

600

150

SD225 Deep decking

Services zone



1092

In situ concrete slab

300 ASB 249

A193 mesh

203

x102

UB23

203

x102

UB23

Atrium

300 ASB196

300 ASB196

300 ASB155

300

ASB249

203

x102

UB23

300 ASB249

300 ASB249

280

ASB74

280

ASB74

203

x102

UB23

9000

300 ASB249 300 ASB249 300 ASB249

9000 90009000

300 ASB249300 ASB249300 ASB249

203

x102

UB23

203

x102

UB23

300

ASB249

300

ASB155

203

x102

UB23

Void

300 ASB249280 ASB124

280ASB74

300 ASB249

203

x102

UB23

280 ASB74

300 ASB153

280 ASB74

300

ASB155

280

ASB74

Stairs

ToiletZone

Lobby

300 ASB153


280 ASB124

Lift

C1

C1 C1

C1C1

C2 C2 C2

C2

C2

C2 C2

C2

C2

C2 C2 C2

C2

7500

7500

9000

C1 = 305 x 305 UC97

C2 = 356 x 368 UC129

]


35

Programmes

6. Programmes

A comparison of the overall programme durations, showing each of the periods from procurement to completion, is given in tabular and graphical form in this Chapter. Detailed programmes for the Flat Slab and Composite options for both Building A and Building B are also presented in Appendix A – Detailed programmes.

Structuraloption

Procurementtime (weeks)

Lead time (weeks)

Overall construction time (weeks)

Frame construction time (weeks)

Overallproject

time (weeks)

Building A

Flat Slab 10 4 50 10 64

PT Flat Slab 10 4 51 11 65


10 4 52 13 66

Composite 10 12 48 8 70

Steel + Hollowcore

10 12 48 7 70

Slimdek 10 12 48 7 70

Building B

PT Flat Slab 10 6 66 17 82

Flat Slab 10 6 67 18 83

PT Band Beams 10 7 66 17 83


10 6 70 22 86

Steel + Hollowcore

10 16 65 21 91

Slimdek 10 16 65 21 91

Composite 10 16 67 23 93

Long-Span Composite

10 18 67 23 95

The procurement element is identical for each option at ten weeks, comprising two weeks for collation of information, four weeks for bidding, three weeks for bid evaluation and one week for award of contract, assuming a traditional approach to works package sub-contracting.

Building AThe lead time for the Flat Slab, In-situ + Hollowcore and PT Flat Slab options is four weeks, comprising one week for working drawings, one week for drawing approval, one week for material procurement and one week for mobilisation.

Procurement programme

Lead times

NoteFrame construction time for

Composite, Steel + Hollowcore, Slimdek and

Long-Span Composite options includes construction of concrete jump-form core.


36

Programmes

The lead time for the structural frame for the Composite, Steel + Hollowcore and Slimdek options is 12 weeks, comprising four weeks for working drawings, one week for drawing approval, one week for material procurement, fi ve weeks for manufacture and one week for mobilisation.

Building BThe lead time for the structural frame for the Flat Slab, In-situ + Hollowcore and PT Flat Slab options for the short-span options is six weeks, comprising one week for working drawings, one week for drawing approval, two weeks for material procurement and two weeks for mobilisation. For the long-span PT Band Beam option, an extra week is required for procurement, increasing the lead time to seven weeks.

The lead time for the structural frame for the Composite, Steel + Hollowcore and Slimdek options for the short-span options is 16 weeks, comprising four weeks for working drawings, one week for drawing approval, two weeks for material procurement, eight weeks for manufacture and one week for mobilisation. For the Long-Span Composite option, an extra two weeks are required for manufacture, increasing the lead time to 18 weeks.

Other elementsWith regard to lead times, the most critical element is cladding, which is required rela-tively early in the construction and for which the lead time can be as much as 45 weeks for complex, high-quality curtain walling systems. Clearly, it would be unlikely that incurring such a long lead time after contract award would be a viable option on most projects. Accordingly, the procurement process for cladding would generally need to be set in motion before contract award and several solutions are available to overcome this problem.

It is possible for a client to enter into a framework agreement with one or more cladding manufacturers, under which production space can be reserved to suit an anticipated project schedule. This route is most likely to be adopted by an experienced client with an ongoing stream of developments. Alternatively, a client may pre-order the cladding prior to awarding a contract, in order to guarantee delivery to suit an eventual construction programme. In either case, the client bears the fi nancial risk of such a commitment to the cladding manufacturer.

The early appointment of a contractor under a two-stage tender approach can prove effec-tive in overcoming the problem and may also prove benefi cial by involving the contractor’s expertise in buildability and programming in the cladding procurement. Alternatively, a long-term partnering or alliancing approach can alleviate the diffi culty; however, the risk apportionment on such a basis needs to be appropriate to the project and must be fully understood and carefully considered by all parties.

Lifts and some M&E plant also tend to have long lead times, especially non-standard equipment, but as these are generally required later in the project, greater scope exists for managing the risks associated with pre-ordering.


37

Programmes

Assumptions and logicA fi ve-day week was assumed. Holidays have not been shown on the programme and no allowance has been made for inclement weather. A simplifi ed view has been taken of such factors as logistics, site access, boundary constraints, cranage, etc., where it has been assumed that there would be no access or supply problems. These aspects are highly site-specifi c and could result in shorter or longer construction periods. On activities not related to the structure, similar resources and sequences have been assumed for all the options.

It was assumed that the ground fl oor slab would be fully or substantially complete before a steel frame is erected. Whilst it is possible for steelwork to be erected from the pile caps before the ground fl oor slab is constructed, thus saving time on the critical path, many steelwork contractors prefer the ground fl oor slab to be installed, as it is safer for the steelwork erectors to work from mobile elevated working platforms positioned on a fl at surface, as well as providing a clear lay-down area for the steelwork.

It is also possible for a concrete frame to be built before the ground fl oor slab is con-structed, the columns being cast from pile caps and the ground fl oor slab being installed subsequently. The time savings are similar for both materials.

It was assumed that the frame for Building A would be erected using a mobile crane and that one tower crane is used for the erection of Building B. It was also assumed that long-lead items such as cladding, lifts and some plant would be pre-ordered.

With the Flat Slab, PT Flat Slab and Slimdek options, although the availability of a clear unimpeded soffi t would permit greater use of prefabrication in the M&E services distribu-tion, with consequent programme savings, no allowance has been made for any reduction in the construction programme as a result of this potential benefi t.

Building AThe construction programmes range from 50 to 52 weeks for the buildings constructed using the Flat Slab, In-situ + Hollowcore and PT Flat Slab options, compared with a 48-week period for each of the buildings constructed using the Composite, Steel + Hollowcore and Slimdek options.

Building BOf the short-span buildings, construction programmes range from 65 weeks for the Steel + Hollowcore and Slimdek options, the PT Flat Slab option at 66 weeks, closely followed by the Flat Slab and Composite options at 67 weeks, with the In-situ + Hollowcore option at 70 weeks.

The construction programmes for the long-span options are almost identical, with the PT Band Beam option being marginally shorter at 66 weeks, compared to 67 weeks for the Long-Span Composite option.

Construction programmes


38

Building A - Programmes

Construction programmes Building A

Testing & commissioning

2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor

Fit-out second floor

Fit-out ground floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

Composite — 48 weeks

Number of weeksEstablish site

8

8

16

8

5

9

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12


2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor



Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

Steel + Hollowcore — 48 weeks


7

8

16

8

5

9

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12


39


2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor



Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

Flat Slab — 50 weeks


10

8

16

8

5

8

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12



2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor



Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

Slimdek — 48 weeks


7

8

16

8

5

9

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12


40



2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor



Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

PT Flat Slab


11

8

16

8

5

8

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12

— 51 weeks


2Substructures

Superstructure

Roof finishes

Roof installations

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor



Final fix

External works

Activity

Week number

0 10 20 30 40 50 60

In-Situ + Hollowcore — 52 weeks


13

5

16

8

5

8

8

5

9

17

12

0 10 20 30 40 50 60

8

11

13

13

12


41

Construction programmes Building B

Building B - Programmes


Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

Steel + Hollowcore — 65 weeks


21

9

20

14

10

10

Fit-out fourth floor

Fit-out fifth floor

17

12

15

15

18

22

19

15

2

10

10

10

10

12


Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

Slimdek — 65 weeks


21

9

20

14

10

10


Fit-out fifth floor

18

12

15

15

18

22

19

15

2

10

10

10

10

12


42



Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

PT Band Beams — 66 weeks


17

9

20

14

10

10


Fit-out fifth floor

19

12

16

15

20

22

19

19

2

10

10

10

10

12


Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

PT Flat Slab — 66 weeks


17

9

20

14

10

10


Fit-out fifth floor

19

12

16

15

20

22

19

19

2

10

10

10

10

12


43


Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

Composite — 67 weeks


23

9

20

14

10

10


Fit-out fifth floor

17

12

15

15

19

22

19

18

2

10

10

10

10

12



Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

Flat Slab — 67 weeks


18

9

20

14

10

10


Fit-out fifth floor

19

16

16

15

25

22

19

17

2

10

10

10

10

12


44



Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

Long-Span Composite


23

9

20

14

10

10


Fit-out fifth floor

17

12

15

15

19

22

19

17

2

10

10

10

10

12

— 67 weeks


Substructures

Superstructure

Roof finishes

Roof installations

Atrium glazing

External envelope

Cores and risers

Toilet fit-out

M & E first fix

M & E second fix

Lifts

Fit-out first floor


Fit-out third floor

Final fix

External works

Activity

Week number

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

In-Situ + Hollowcore — 70 weeks


22

9

20

14

10

10


Fit-out fifth floor

18

12

16

15

22

22

19

19

2

10

10

10

10

13


45

Summary of costs

7. Summary of costs

Basis of pricingPrices used in this study have been prepared by Davis Langdon, based on pricing data obtained in June 2006 from their national cost database of recently tendered projects. Rates for Building A are based on construction in south east England and rates for Building B are based on construction in central London.

PreliminariesThe cost of the main contractor’s preliminaries for each option was based on two separate elements. A lump sum was included to allow both for non-work-related aspects such as contractual requirements for insurances, employer’s facilities, etc. and for fi xed one-off costs such as site establishment, access roads, crane bases, services connection charges, etc.

Separate allowances were made for time-related costs, such as management and staff, site accommodation, services and facilities, cranage, etc. Such costs vary according to pro-gramme duration and the sequencing of operations within the programme. Adjustment of these costs has been made to refl ect the different construction durations identifi ed in the programmes produced by Mace.

For Building A, preliminaries on average equate to an on-cost of 13.6% of the basic construction cost and for Building B, preliminaries on average equate to an on-cost of 15.5% of the basic construction cost.

Finance and rental costsThe study did not include assessment of the costs of fi nancing the project, nor consideration of return on rentals.

Summary tablesThe itemised costs for Building A and Building B are presented in the following tables, followed by the key rates used in the study.

Costs


46

Summary of costs - Building A

Building A4642 m2 GIFA

Element Short-span optionsFlat Slab Composite In-situ + Hollowcore PT Flat Slab Steel + Hollowcore Slimdek

Element total (£) Element total (£) Element total (£) Element total (£) Element total (£) Element total (£)

Substructure 199,480 189,765 202,641 200,512 195,452 192,107

Frame/upper fl oors 564,827 568,078 591,645 642,599 643,704 872,208

Roof fi nishes 241,208 241,208 241,208 241,208 241,208 241,208

Stairs 63,000 63,000 63,000 63,000 63,000 63,000

External cladding 1,166,600 1,174,480 1,187,720 1,154,800 1,199,980 1,175,460

Internal planning 141,230 154,110 145,255 139,740 156,630 153,900

Wall fi nishes 51,010 50,040 49,684 48,820 52,240 50,240

Floor fi nishes 274,432 274,432 274,432 274,432 274,432 274,432

Ceiling fi nishes 125,308 125,308 125,308 125,308 125,308 125,308

Fittings 60,000 60,000 60,000 60,000 60,000 60,000

Sanitary 208,890 208,890 208,890 208,890 208,890 208,890

Mechanical 1,285,834 1,311,551 1,285,834 1,285,834 1,311,551 1,285,834

Electrical 637,811 650,567 637,811 637,811 650,567 637,811

Lifts 70,000 70,000 70,000 70,000 70,000 70,000

BWIC 172,470 172,470 172,470 172,470 172,470 172,470

Contingency 394,658 398,542 398,692 399,407 406,907 418,715

Preliminaries 735,000 715,000 755,000 745,000 715,000 715,000

Overheads and profi t 383,505 385,646 388,175 388,190 392,840 402,995

TOTAL £6,775,263 £6,813,088 £6,857,765 £6,858,021 £6,940,180 £7,119,578

Element Short-span optionsFlat Slab Composite In-situ + Hollowcore PT Flat Slab Steel + Hollowcore Slimdek

£/m2 % £/m2 % £/m2 % £/m2 % £/m2 % £/m2 %Substructure 43 2.9 41 2.8 44 3.0 43 2.9 42 2.8 41 2.7

Frame/upper fl oors 122 8.3 122 8.3 127 8.6 138 9.4 139 9.3 188 12.3

Roof fi nishes 52 3.6 52 3.5 52 3.5 52 3.5 52 3.5 52 3.4

Stairs 14 0.9 14 0.9 14 0.9 14 0.9 14 0.9 14 0.9

External cladding 252 17.2 253 17.2 256 17.3 249 16.8 258 17.3 253 16.5

Internal planning 30 2.1 33 2.3 31 2.1 30 2.0 34 2.3 33 2.2

Wall fi nishes 11 0.8 11 0.7 11 0.7 11 0.7 11 0.8 11 0.7

Floor fi nishes 59 4.1 59 4.0 59 4.0 59 4.0 59 4.0 59 3.9

Ceiling fi nishes 27 1.8 27 1.8 27 1.8 27 1.8 27 1.8 27 1.8

Fittings 13 0.9 13 0.9 13 0.9 13 0.9 13 0.9 13 0.8

Sanitary 45 3.1 45 3.1 45 3.0 45 3.0 45 3.0 45 2.9

Mechanical 277 19.0 283 19.3 277 18.8 277 18.7 283 18.9 277 18.1

Electrical 137 9.4 140 9.5 137 9.3 137 9.3 140 9.4 137 9.0

Lifts 15 1.0 15 1.0 15 1.0 15 1.0 15 1.0 15 1.0

BWIC 37 2.5 37 2.5 37 2.5 37 2.5 37 2.5 37 2.4

Contingency 85 5.8 86 5.8 86 5.8 86 5.8 88 5.9 90 5.9

Preliminaries 158 10.8 154 10.5 162 11.0 160 10.9 154 10.3 154 10.0

Overheads and profi t 83 5.7 83 5.7 84 5.7 84 5.7 84 5.7 88 5.7

TOTAL £1,460 £1,468 £1,477 £1,477 £1,495 £1,534


47

Element Short-span options Long-span options Short-span options

Flat Slab PT Flat Slab Composite In-situ + Hollowcore

PT Band Beams Long-Span Composite

Steel + Hollowcore

Slimdek

Element total (£)

Element total (£)

Element total (£)

Element total (£)

Element total (£)

Element total (£)

Element total (£)

Element total (£)

Substructure 891,672 865,937 815,468 885,169 907,622 848,868 860,967 852,231

Superstructure 1,811,939 2,016,344 1,878,457 1,846,453 2,227,681 2,201,664 2,275,704 3,011,992

Roof fi nishes 545,080 545,080 545,080 545,080 545,080 545,080 545,080 545,080

Stairs 132,000 132,000 132,000 132,000 132,000 132,000 132,000 132,000

External cladding 5,951,060 5,849,590 5,957,935 6,053,840 6,086,885 5,957,935 6,208,265 5,974,270

Internal planning 297,080 293,790 355,728 300,225 301,360 355,638 366,552 356,352

Wall fi nishes 234,455 229,931 256,770 233,226 227,825 241,566 264,162 263,112

Floor fi nishes 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221

Ceiling fi nishes 702,366 702,366 702,366 702,366 702,366 702,366 702,366 702,366

Fittings 132,500 132,500 132,500 132,500 132,500 132,500 132,500 132,500

Sanitary 824,000 824,000 824,000 824,000 824,000 824,000 824,000 824,000

Mechanical 4,544,360 4,544,360 4,635,247 4,544,360 4,544,360 4,635,247 4,635,247 4,544,360

Electrical 2,690,688 2,690,688 2,739,502 2,690,688 2,690,688 2,739,502 2,739,502 2,690,688

Lifts 600,000 600,000 600,000 600,000 600,000 600,000 600,000 600,000

BWIC 601,800 601,800 601,800 601,800 601,800 601,800 601,800 601,800

Contingency 1,584,467 1,589,670 1,600,806 1,594,420 1,626,854 1,626,404 1,654,152 1,679,848

Preliminaries 3,350,000 3,310,000 3,350,000 3,470,000 3,310,000 3,350,000 3,270,000 3,270,000

Overheads and profi t 1,563,641 1,565,717 1,577,693 1,579,401 1,597,694 1,599,707 1,618,771 1,640,869

TOTAL £27,624,328 £27,660,993 £27,872,572 £27,902,748 £28,225,936 £28,261,499 £28,598,289 £28,998,690

Building B16,480 m2 GIFA

Element Short-span options Long-span options Short-span options

Flat Slab PT Flat Slab Composite In-situ + Hollowcore

PT Band Beams Long-Span Composite

Steel + Hollowcore

Slimdek

£/m2 % £/m2 % £/m2 % £/m2 % £/m2 % £/m2 % £/m2 % £/m2 %Substructure 54 3.2 53 3.1 49 2.9 54 3.2 55 3.2 52 3.0 52 3.0 52 2.9

Superstructure 110 6.6 122 7.2 114 6.7 112 6.6 135 7.9 134 7.7 138 7.9 183 10.3

Roof fi nishes 33 2.0 33 2.0 33 1.9 33 1.9 33 1.9 33 1.9 33 1.9 33 1.9

Stairs 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5

External cladding 361 21.5 355 21.0 362 21.3 367 21.6 369 21.5 362 21.0 377 21.6 363 20.5

Internal planning 18 1.1 18 1.1 22 1.3 18 1.1 18 1.1 22 1.3 22 1.3 22 1.2

Wall fi nishes 14 0.8 14 0.8 16 0.9 14 0.8 14 0.8 15 0.9 16 0.9 16 0.9

Floor fi nishes 71 4.2 71 4.2 71 4.2 71 4.2 71 4.1 71 4.1 71 4.1 71 4.0

Ceiling fi nishes 43 2.5 43 2.5 43 2.5 43 2.5 43 2.5 43 2.5 43 2.4 43 2.4

Fittings 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5

Sanitary 50 3.0 50 3.0 50 2.9 50 2.9 50 2.9 50 2.9 50 2.9 50 2.8

Mechanical 276 16.5 276 16.3 281 16.5 276 16.2 276 16.0 281 16.3 281 16.1 276 15.6

Electrical 163 9.7 163 9.7 166 9.8 163 9.6 163 9.5 166 9.6 166 9.5 163 9.2

Lifts 36 2.2 36 2.2 36 2.1 36 2.1 36 2.1 36 2.1 36 2.1 36 2.1

BWIC 37 2.2 37 2.2 37 2.1 37 2.1 37 2.1 37 2.1 37 2.1 37 2.1

Contingency 96 5.7 96 5.7 97 5.7 97 5.7 99 5.7 97 5.7 100 5.8 101 5.8

Preliminaries 203 12.1 201 11.9 203 12.0 211 12.4 201 11.7 203 11.8 199 11.4 198 11.2

Overheads and profi t 95 5.7 94 5.7 95 5.7 95 5.7 97 5.7 97 5.7 98 5.7 99 5.7

TOTAL £1,676 £1,678 £1,691 £1,693 £1,713 £1,715 £1,735 £1,759

Summary of costs - Building B


48

Summary of costs

Key ratesKey rates used in the structural elements of the study are tabulated below:

Building element Unit Building A Building B

Concrete in walls m3 £125 £125

Concrete in suspended slabs m3 £115 £120

Lightweight concrete in suspended slabs m3 £145 £145

Concrete in beams m3 £115 £120

Concrete in columns m3 £115 £120

A142 mesh reinforcement m2 £3 £3

A193 mesh reinforcement m2 £4 £4

Reinforcement in suspended slabs tonne £820 £840

Reinforcement in beams tonne £820 £840

Reinforcement in walls tonne £820 £840

Reinforcement in columns tonne £820 £840

Post-tensioning to fl oor slabs m2 £27 £27

Intumescent coating - 60 minute (site applied) m2 £13 –

Intumescent coating - 90 minute (site applied) m2 – £20

Formwork to walls m2 £32 £31

Formwork to soffi ts of suspended slabs m2 £31 £32

Formwork to beams m2 £42 £42

Formwork to columns m2 £42 £42

Formwork to columns - curved m2 £63 –

150mm hollowcore planks m2 £46 £47

200mm hollowcore planks m2 £48 £54

Solid grade S355 steel beams tonne £1,390 £1,405

Solid grade S355 steel columns tonne £1,390 £1,405

Solid grade S355 steel columns hollow sections tonne £1,730 –

ASB grade S355 steel beams tonne £1,590 £1,600

Cellular grade S355 steel beams tonne – £1,545

Core walls SHS steel bracing tonne £1,770 –

Ribdeck AL 1.2mm steel decking m2 £21 £21

SD225 steel decking (propped) m2 £36 £39

Shear studs -19mm × 100mm No £1 £1

Shear studs -19mm × 120mm No £1 £1

Key rates used in other elements of the study are tabulated below:

Other element Unit Building A Building B

External cladding

Curtain walling m2 £360 £830

Rain-screen m2 £260 £935

Brise-soleil m2 £310 £310

Atrium walling m2 £360 £480

Shop fronts m2 – £470

Internal planning

Non-structural dry-lined metal stud partitions m2 £55 £65

Blockwork walls to retail units m2 – £80


49

Study fi ndings

8. Study fi ndings

Building A – 3 storeyIn terms of overall construction cost for Building A, the most economic option, the Flat Slab, was found to be between 0.6% and 4.8% less expensive than the alternative structural solutions.

Building B – 6 storeyIn terms of overall construction cost for Building B, for the short-span situation, the most economic option, the Flat Slab, was found to be between 0.1% and 4.7% less expensive than the alternative structural solutions.

OverallThe most signifi cant differential for both buildings occurred using the Slimdek option, for which the overall construction costs were found to be between 5.0% and 5.1% more expensive than the most economic option, after adjusting time-related preliminaries for construction programme difference. When only the costs of the structural frame and upper fl oors are considered, the Slimdek option was found to be between 54.1% and 66.4% more expensive than the most economic option.

Building AWith regard to speed of construction, for Building A the construction programmes for the Composite, Steel + Hollowcore and Slimdek options are all identical at 48 weeks, with 50 weeks required for the Flat Slab option, 51 weeks for the PT Flat Slab option and 52 weeks for the In-situ + Hollowcore option.

Building BWith regard to speed of construction, for the short-span options in Building B, the con-struction programmes for both the Steel + Hollowcore and Slimdek options are identical at 65 weeks, with 66 weeks required for the PT Flat Slab option; the Flat Slab option and Composite options identical at 67 weeks and 70 weeks for the In-situ + Hollowcore option.

For the long-span options in Building B, the PT Band Beam option was found to have a programme of 66 weeks, compared to a programme of 67 weeks for the Long-Span Composite option.

When considering a ten week procurement time and a lead time of 4-7 weeks for the Flat Slab, In-situ + Hollowcore, PT Flat Slab and PT Band Beam options; and, 12–18 weeks for the Composite, Steel + Hollowcore, Slimdek and Long-Span Composite options; the overall programmes are as summarised below:

Costs

Programme


50

Study fi ndings

Building A – short-span Building B – short-span Building B – long-spanFlat Slab 64 weeks PT Flat Slab 82 weeks PT Band Beams 83 weeks

PT Flat Slab 65 weeks Flat Slab 83 weeks Long-Span Composite 95 weeks

In-situ + Hollowcore 66 weeks In-situ + Hollowcore 86 weeks

Composite 70 weeks Steel + Hollowcore 91 weeks

Steel + Hollowcore 70 weeks Slimdek 91 weeks

Slimdek 70 weeks Composite 93 weeks

The study fi ndings are presented in the following manner:

The costings are divided into the following eight primary components which together make up the overall cost of each scheme design:

Substructures Frames and upper floors Cladding Internal planning Roof finishes and internal finishes Mechanical and electrical services Preliminaries Contingency and overheads and profit.

For each component, the costs per m2 of gross internal fl oor area for each of the eight options are compared graphically and in tabular form, with the most economical option for that component being used as the base for comparison.

The costs of each primary component are also broken down where appropriate; for example ‘frames and upper fl oor costs’ are sub-divided into concrete frame, formwork and reinforce-ment, steel frame, decking & slabs and fi re protection.

(Minor differences between the fi gures used in the Study fi ndings and the Summary of costs are

due to rounding.)

Building A

Internal Planning

2%

Substructure 3%

Frame and Upper Floors10%

Preliminaries11%

Contingency & O/h&P

12%

External Cladding17%

M&E, Lifts & BWIC34%

Roof Finishes & Internal Finishes

11%

Average Elemental Breakdown

Building B

Internal Planning

1%

Substructure 3%

Frame and Upper Floors

8%Preliminaries

12%

Contingency & O/h&P

11%

External Cladding21%

M&E, Lifts & BWIC34%

Roof Finishes & Internal Finishes

10%

Average Elemental Breakdown


51

Study fi ndings - Building A

Overall costs

£/m2 % difference

Flat Slab £1,460 -

Composite £1,468 +0.5%

In-situ + Hollowcore £1,477 +1.2%

PT Flat Slab £1,477 +1.2%

Steel + Hollowcore £1,495 +2.4%

Slimdek £1,534 +5.1%

Overall costsBased on the building footprints and outline specifi cations compiled by Allies and Morrison, together with the structural design information and calculation provided by Arup, all six structural options are within 5.1% of each other, after adjusting time-related preliminaries for construction programme differences.

Of particular note is the signifi cance of M&E services costs in the overall comparison, representing an average of 34% of total costs, and of the external cladding, representing an average of 17% of total costs.

As illustrated in the fi gure and table, the Slimdek option was found to be 5.1% more expensive than the Flat Slab option, with both options providing clear, unimpeded soffi ts.

These fi gures are based on cost per m2 of gross internal area. The differences in cost would be even greater if net internal areas had been considered, due to the larger area taken up by a steel core. However, as this level of detail would not normally be apparent at outline design stage, it has not been examined further in this study.

Study fi ndings – Building A

In Table 1 and Table 2 which follow, showing the construction costs for each element of the building, the % comparison is related to the cost for the most economic option for the element in question.

PT Fl

at S

lab

1550

1525

1500

1475

1450

1425

1400

Flat

Sla

b

In-s

itu +

Hol

low

core

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

Ove

rall

cons

truc

tion

cost

s


52

Element Substructures Frame and upper fl oors

Percentage of total cost 3% 10%

Findings Foundations for the three-storey building are simple pads. Costs for the ground-fl oor slab and associated earthworks are identical for all solutions.

On an overall basis, costs for the complete substructure were found to be lowest for the Composite and Slimdek options, with costs for the alternative options ranging from +2.4% to +7.3%.

Costs for the earthworks and foundations only (excluding the GF slab) were found to be lowest for the Composite option, with costs for the alternative options ranging from +4.5% to +13.6%, which is the consequence of smaller pads being utilised for the lighter buildings.

When the costs of the frame and upper fl oors only are compared on a like-for-like basis, the most economic option is the Flat Slab, with costs for the alternative structural options ranging from +4.1% to +54.1%. A signifi cant feature is the premium required to achieve a clear, fl at soffi t with the Slimdek system as opposed to with alternative fl at soffi t solutions, the Flat Slab and PT Flat Slab options. This is shown graphically and in tabular form below.

It should be appreciated that, in cost plans, the infi ll to the steel core bracing in a steel-framed building is often allocated to the Internal Planning element. In this study, this would have produced an imbalance of approximately 7.5% in the comparisons, which has been adjusted in the table below.

This highlights the need for designers to be aware that the structure of a cost plan may not readily reveal the full effects of the choice of a particular structural frame. Examination of the cost plan at a more detailed level than elemental totals may therefore prove benefi cial in informing the structural choice.

Relative costs

Percentage comparison with Flat Slab option


Note for frame and

upper fl oors

*Stairs have been

excluded from the

comparison, being of

equal cost for all

solutions.

45 40 35 30 25 20 15 10 5 0

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

GF slabFoundationsEarthworks

Table 1Elemental cost

comparison.

180

160

140

120

100

80

60

40

20

0

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

Fire protectionDecking & slabsSteel frame

FormworkReinforcementConcrete frame

Substructure costs

£/m2 % difference

Composite £41 -

Slimdek £41 -

Steel + Hollowcore £42 +2.4%

Flat Slab £43 +4.9%

PT Flat Slab £43 +4.9%

In-situ + Hollowcore £44 +7.3%

Frame and upper fl oors costs

£/m2 % difference

Flat Slab £122 -

Composite £122 -




Slimdek £188 +54.1%

*


53

External cladding Internal planning

17% 2%

For the costs of external cladding (curtain walling to main elevations and atrium, together with rain screens, brise-soleil, external doors and cladding to roof plant areas), the most economic option is the PT Flat Slab, with cost for the alternative solutions ranging from +0.9% for the Flat Slab option to +4.2% for the Steel + Hollowcore option. However, whilst the percentage variation between options may appear small, it should be borne in mind that, the actual cost variation can be signifi cant for this element.

The variation in cost is related to the area of cladding resulting from the necessary storey heights, which vary from 3950mm on the PT Flat Slab option to 4160mm on the Steel + Hollowcore option, to accommodate the different structural zones.

With the wall-to-fl oor ratio on this building form, a 5.3% increase in fl oor-to-fl oor height produces a 6.0% increase in cladding cost over three storeys.

For the Internal planning (internal partitions, internal glazing to atrium, WC cubicles and internal doors) the most economic solutions are the Flat Slab and PT Flat Slab options in equal place. Costs for the alternative solutions range from +3.3% for the In-situ + Hollowcore option to +13.3% for the Steel + Hollowcore option.

This cost range refl ects the adjustment of the imbalance relating to the infi ll to steel braced cores, referred to in the Frame and upper fl oors element.

Account has been taken in the costing of the added complexity of fi re and acoustic sealing of partition heads against the irregular soffi ts of steel decking and around irregularly shaped intersecting steel frame members.


External cladding costs

£/m2 % difference

PT Flat Slab £215 -


Slimdek £219 +1.9%

Composite £219 +1.9%



Internal planning costs

£/m2 % difference

PT Flat Slab £30 -

Flat Slab £30 -


Slimdek £33 +10.0%



225

220

215

210

205

200

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

35

30

25

20

15

10

5

0

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

DoorsAtrium glazingInternal planning

Note for external

cladding

** Undercroft treatment

has been excluded from

the comparison, being of

equal cost for all

solutions.

**


54

Element Roof fi nishes and internal fi nishes, fi xtures and fi ttings

Mechanical and electrical services


Findings The costs of the roof fi nishes were the same across all the structural options, as is also the case for the fi xtures and fi ttings.

Slight differences in internal fi nishes costs are entirely contained within the wall fi nishes and refl ect the dissimilar storey heights, which differ by 5.3% between the lowest (the Flat Slab option) and the highest (the Steel + Hollowcore option). At this outline stage of design, these differentials are so small as to be lost in the rounding of the fi gures.

In respect of the direct costs of lifts, mechanical services, electrical services, sanitary installations and builder’s work in connection, there was no noticeable difference between all of the structural solutions.

However, with regard to the relative ease of installation of the mechanical and electrical services, a premium is incurred for the additional complexity where the services distribution has to be installed around downstand beams of varying depth, cross-section and number, as are found with the Composite and Steel + Hollowcore options.

Relative costs



Table 1 cont’dElemental cost

comparison.

100

80

60

40

20

0

£/m²

Ceiling finishes Floor finishesWall finishes

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

Finishes costs

£/m2 % difference

PT Flat Slab £97 -

In-situ + Hollowcore £97 -

Composite £97 -

Slimdek £97 -

Flat Slab £97 -

Steel + Hollowcore £97 -

Mechanical and electrical costs

£/m2 % difference

Flat Slab £512 -



Slimdek £512 -



550 500 450 400 350 300 250 200 150 100 50

0

Flat

Slab

In-s

itu +

Hol

low

core

PT Fl

at Sl

ab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

£/m²

BWCLifts

MechanicalSanitary

Electical

Note for fi nishes *Roof fi nishes and

fi xtures and fi ttings

have been excluded

from the comparison,

being of equal cost for

all solutions.

*


55

Preliminaries Contingency, overheads and profi t

11% 12%

The budget for preliminaries for each option was based on two separate elements. A lump sum to allow for both non-work-related aspects, such as contractual requirements for insurances, employer’s facilities, etc., together with fi xed one-off costs such as site establishment, access roads, crane bases, services connection charges, etc.

The second element relates to time-related costs, such as management and staff, site accommodation, services and facilities, cranage, etc. Such costs therefore vary according to programme duration and the sequencing of operations within the programme. Adjustment of these costs has been made to refl ect the different construction durations between 48 and 52 weeks identifi ed in the programmes. (see Chapter 6 Programmes).

Detailed consideration of items within the Preliminaries, e.g. size of particular cranes, was beyond the scope of this study.

A design contingency of 7.5% has been included within the budget costs, to refl ect the outline nature of the design information developed at this stage of a project. The budget costs also contain an allowance of 6% in respect of overheads and profi t. It should be borne in mind that, at this stage of the design, the allowance for contingency is the equivalent of 70% of the cost of the frame and upper fl oors on the most economic solutions.


Preliminaries costs

£/m2 % difference

Composite £154 -


Slimdek £154 -




Contingency, overheads and profi t costs

£/m2 % difference

Flat Slab £168 -





Slimdek £177 +5.4%

180

160

140

120

100

80

60

40

20

0

Overheads & profitContingency

£/m²

Flat

Slab

In-s

itu +

Hol

low

core

PT Fl

at Sl

ab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

180

160

140

120

100

80

60

40

20

0

Time-related preliminariesFixed preliminaries

£/m²

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek


56

Overall costs

Based on the building footprints and outline specifi cations compiled by Allies and Morrison, together with the structural design information and calculation provided by Arup, all eight structural options are within 5.5% of each other, after adjusting time-related preliminaries for construction programme differences.

Of particular note is the signifi cance of M&E services costs in the overall comparison, representing an average of 33% of total costs, and of the external cladding, representing an average of 21% of total costs.

As illustrated in the fi gure and table, the Slimdek option was found to be 5.5% more expensive than the most economic option, the Flat Slab, with both options providing clear, unimpeded soffi ts.

These fi gures are based on cost per m2 of gross internal area, with all options having concrete cores.

In terms of overall construction costs, for short-span options, the most economic solution was found to be the Flat Slab option, with alternative solutions being between 0.7% and 5.5% more expensive.

For long-span options, the PT Band Beam solution was found to be more economic than the Long-Span Composite solution. The Long span options are shown on the right of the charts as shown below.

Study fi ndings - Building B

1750

1725

1700

1675

1650

1625

1600

PT B

and

Beam

s

Long

span

Com

posit

eLong spanShort span

£/m²

£/m²

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

Study fi ndings – Building B

Overall costs£/m2 % difference

Flat Slab £1,676 -

PT Flat Slab £1,678 +0.1%

Composite £1,691 +0.9%

In-situ + Hollowcore £1,693 +1.0%

Steel + Hollowcore £1,735 +3.5%

Slimdek £1,759 +5.0%

PT Band Beams £1,713 +2.2%

Long-Span Composite £1,715 +2.3%

Ove

rall

cons

truc

tion

cost

s


57


Element Substructures Frame and upper fl oors


Findings Foundations for Building B are piled, with varying pile depths, pile cap sizes and confi gurations for each option. Costs for the ground fl oor slab and associated earthworks are identi-cal for all solutions.

On an overall basis, costs for the complete substructure were found to be lowest for the Composite option, with costs for the alternative options ranging from +6.1% to +12.2%.

Costs for the earthworks and foundations only (excluding the GF slab) were found to be lowest for the Composite option, with costs for the alternative options ranging from +6.6% to +18.2%, which is the consequence of fewer piles, shorter pile lengths and smaller pile caps needed for the lighter buildings.

When the costs of the frame and upper fl oors only are com-pared, the most economic option is the Flat Slab, with costs for the alternative structural options ranging from +1.8% to +66.4%. A signifi cant feature is the premium required to achieve a clear, fl at soffi t with the Slimdek system as opposed to with alternative fl at soffi t solutions, the Flat Slab and PT Flat Slab options. This is shown graphically and in tabular form below.

For the long-span options, the frame and upper fl oors costs were almost identical, the PT Band Beams option being 0.8% more expensive than the Long-Span Composite option. Both long-span solutions were an average of 22.3% higher than the most economic short-span solution, the Flat Slab.

Relative costs

Percentage comparison with Flat Slab option Substructure costs

£/m2 % difference

Composite £49 -

Slimdek £52 +6.1%





Long-Span Composite £52 +6.1%

PT Band Beams £55 +12.2%

Table 2Elemental cost

comparison.

605550454035302520151050

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Spa

nCo

mpo

site

£/m²

GF slabFoundationsEarthworks

Frame and upper fl oor costs£/m2 % difference

Flat Slab £110 -





Slimdek £183 +66.4%



Note for frame and

upper fl oors

*Stairs have been

excluded from the

comparison for clarity,

being of equal cost for

all solutions.

200

180

160

140

120

100

80

60

40

20

0

£/m²

Fire protectionDecking & slabsSteel frame

FormworkReinforcementConcrete frame

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Spa

nCo

mpo

site

*


58

Element External cladding Internal planning


Findings For the costs of external cladding (curtain walling to main elevations and atrium, together with rainscreens, brise-soleil, external doors and cladding to roof plant areas), the most economic option is the PT Flat Slab, with costs for the alternative solutions ranging from +1.7% for the Flat Slab option to +6.2% for the Steel + Hollowcore option. However, whilst the percentage variation between options may appear small, it should be borne in mind that, the variation in terms of actual cost can be signifi cant for this element.

The variation in cost is related to the area of cladding resulting from the necessary storey heights, which vary from 3950mm on the PT Flat Slab option to 4235mm on the Steel + Hollowcore option, to accommodate the different structural zones.

With the wall-to-fl oor ratio on this building form, a 7.2% increase in fl oor-to-fl oor height produces a 6.1% increase in cladding cost over the six storeys.

For the Internal planning (internal partitions, WC cubicles and internal doors) the most economic solutions are the Flat Slab, PT Flat Slab and In-situ + Hollowcore options in equal place. Costs for the alternative solutions are 22.2% higher, with minor differences between the Composite, In-situ + Hollowcore, Slimdek and Long-Span Composite options. As with Building A, the costing takes account of the added complexity of fi re and acoustic sealing of partition heads against the irregular soffi ts of steel decking and around irregularly shaped intersecting steel frame members. The effect of this factor on Building B is more signifi cant due to the quantity of blockwork walls within the ground fl oor retail space.

Such a large cost range refl ects the effects of both the differences in storey height to accommodate the different structural zones and the cost premium incurred as a result of this added complexity.

Relative costs



External cladding costs£/m2 % difference




Slimdek £363 +2.3%






comparison.

375

365

355

345

335

325

£/m²

Flat

Sla

b

In-S

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Spa

nCo

mpo

site

Internal planning costs£/m2 % difference

PT Flat Slab £18 -

Flat Slab £18 -



Slimdek £22 +22.2%


PT Band Beams £18 -


25

20

15

10

5

0

Flat

Sla

b

In-S

itu +

Hol

low

core

PT Fl

at S

lab

Com

posit

e

ISte

el +

Hol

low

core

Slim

dek

PT B

and

Beam

s

Long

Spa

nCo

mpo

site

£/m²

DoorsInternal planning


59

Roof fi nishes and internal fi nishes, fi xtures and fi ttings


10% 33%

The costs of the roof fi nishes were the same across all the structural options and they are therefore not included in the comparison of the internal fi nishes shown graphically and in tabular form below, which is also the case for the fi xtures and fi ttings.

For the internal fi nishes (fl oor wall and ceiling fi nishes) the most economic solutions are the PT Flat Slab and PT Band Beam option in equal place, with costs for the alternative solutions ranging from +0.8% for the Flat Slab, In-situ + Hollowcore and Long-Span Composite options, to +1.6% for the Composite, Steel + Hollowcore and Slimdek options.

These differences in internal fi nishes costs are entirely contained within the wall fi nishes and refl ect the dissimilar storey heights, which differ by 5.3% between the lowest (the Flat Slab option) and the highest (the Steel + Hollowcore option).

In respect of the direct costs of lifts, mechanical services, electrical services, sanitary installations and builder’s work in connection, there was no noticeable difference between all of the structural solutions.

However, with regard to the relative ease of installation of the mechanical and electrical services, a premium is incurred for the additional complexity where the services distribution has to be installed around downstand beams of varying depth, cross-section and number, as are found with the Composite, Steel + Hollowcore and Long-Span Composite options.


60055050045040035030025020015010050

0

Flat

Slab

In-s

itu +

Hol

low

core

PT Fl

at Sl

ab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Span

Co

mpo

site

£/m²

BWCLifts

MechanicalSanitary

Electical

140

120

100

80

60

40

20

0

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Spa

nCo

mpo

site

£/m²

Ceiling finishes Floor finishesWall finishes

Flat

Sla

b

In-s

itu +

Hol

low

core

PT Fl

at S

lab

Finishes costs£/m2 % difference






Slimdek £129 +1.6%



Mechanical and electrical costs£/m2 % difference

Flat Slab £562 -



Slimdek £562 -





Note for fi nishes *Roof fi nishes and

fi xtures and fi ttings

have been excluded

from the comparison

for clarity, being of

equal cost for all

solutions.

*


60


Element Preliminaries Contingency, overheads and profi t


Findings The budget for preliminaries for each option was based on two separate elements. A lump sum to allow for both non-work-related aspects, such as contractual requirements for insurances, employer’s facilities, etc., together with fi xed one-off costs such as site establishment, access roads, crane bases, services connection charges, etc.

Separate allowances were made for time-related costs, such as management and staff, site accommodation, services and facilities, cranage, etc. Such costs therefore vary according to programme duration and the sequencing of operations within the programme. Adjustment of these costs has been made to refl ect the different construction durations between 65 and 70 weeks identifi ed in the programmes (see Chapter 6 Programmes).

Detailed consideration of items within the Preliminaries, e.g. size of particular cranes, was beyond the scope of this study.

A design contingency of 7.5% has been included within the budget costs, to refl ect the outline nature of the design information developed at this stage of a project. The budget costs also contain an allowance of 6% in respect of overheads and profi t. It should be borne in mind that, at this stage of the design, the allowance for contingency is the equivalent of 87% of the superstructure cost on the most economic solution.

Relative costs

Percentage comparison with Flat Slab option Preliminaries costs

£/m2 % difference

Slimdek £199 -









comparison.

200

180

160

140

120

100

80

60

40

20

0

Flat

Slab

In-s

itu +

H

ollo

wco

re

PT Fl

at Sl

ab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Span

Com

posit

e

£/m²

Overheads & profitContingency

200

180

160

140

120

100

80

60

40

20

0

Flat

Sla

b

In-s

itu +

H

ollo

wco

re

PT Fl

at S

lab

Com

posit

e

Stee

l +H

ollo

wco

re

Slim

dek

PT B

and

Beam

s

Long

Spa

n Co

mpo

site

Time-related preliminariesFixed preliminaries

220

£/m²

Contingency, overheads and profi t costs£/m2 % difference

Flat Slab £191 -





Slimdek £202 +5.8%




61

Study fi ndings

It is evident from the study fi ndings presented that the effects of the choice of a parti-cular structural solution do not arise solely within the Frame and upper fl oors element of the cost plan.

The charts below summarise those elements where costs are directly affected by the choice of frame and show the percentage variation in cost for each frame option, when compared with the most economic option, the Flat Slab, as the base case. The explanation of the reasons for the variations is given in the study fi ndings above.

Summary comparison charts

Building B Flat Slab PT Flat Slab Composite In-situ + Hollowcore

PT Band Beams

Long-Span Composite

Steel + Hollowcore

Slimdek

Substructure

Base

cas

e fo

r com

paris

on

–2.9% –8.5% –0.7% +1.8% –4.8% –3.4% –4.4%

Frame and upper fl oors +10.9% +3.6% +1.8% +22.7% +21.8% +25.5% +66.4%

External cladding –1.7% +0.1% +1.7% +2.3% +0.1% +4.3% +0.4%

Internal planning –1.1% +19.7% +1.1% +1.4% +19.7% +23.4% +20.0%

Wall fi nishes –1.9% +9.5% 12.7% –2.8% +3.0% +12.7% +12.2%

M and E, lifts and BWIC

0% +1.4% 0% 0% +1.4% +1.4% 0%

Contingency +0.3% +1.0% +0.6% +2.7% +2.6% +4.4% +6.0%

Time-related preliminaries

–1.5% 0% +4.6% –1.5% 0% –3.1% –3.1%

Overheads and profi t

+0.1% +0.9% +1.0% +2.2% +2.3% +3.5% +4.9%

Building A Flat Slab Composite In-situ + Hollowcore

PT Flat Slab Steel + Hollowcore

Slimdek

Substructure

Base

cas

e fo

r com

paris

on

–4.9% +1.6% +0.5% –2.0% –3.7%

Frame and upper fl oors +0.6% +4.1% +13.1% +13.9% +54.1%

External cladding +0.8% +2.1% –1.1% +3.3% +0.9%

Internal planning +10.0% +3.3% 0% +13.3% +10.0%

Wall fi nishes –1.9% –2.6% –4.3% +2.4% –1.5%

M and E, lifts and BWIC

+1.6% 0% 0% +1.6% 0%

Contingency +1.0% +1.0% +1.2% +3.1% +6.1%

Time-related preliminaries

–3.9% +3.9% +2.0% –3.9% –3.9%

Overheads and profi t

+0.6% +1.2% +1.2% +3.0% +5.4%


62

Commentary from The Concrete Centre

9. Commentary from The Concrete Centre

The main conclusion to be drawn from the study is that, of the range of structural options commonly used in the construction of modern commercial offi ces, for both the three-storey out-of-town building and for the six-storey city centre building, the most economic structural solution was found to be the RC Flat Slab option. This produced savings of between 1% and 6% in overall construction costs in comparison with alternative solutions.

The main source of savings lies in the superstructure, when the frame, cladding and internal planning are all taken into account. There are minimal differences in the fi nishes, other than those caused by variations in storey heights depending on the structural solution adopted.

Foundations for the heavier options cost more, but account for a relatively small propor-tion of the overall cost, the difference between the foundations for lighter and heavier buildings equating to less than 0.3% of the overall costs.

Preliminaries are very similar, other than time-related aspects, although individual projects may have logistical diffi culties, site constraints, access, adjacent buildings, etc. that are particular to that project and will affect the preliminaries. Such aspects are intrinsically project specifi c and are therefore beyond the scope of the study.

There are no differences in the design or specifi cation of the mechanical and electrical services as a result of the structural designs selected; however, those designs involving downstand beams of varying depths, cross-section and number incur a cost premium as a result of the added complexity of installing the services around such projections.

A cost premium is incurred in the case of the buildings with the heavier structural frame. To some extent this cost premium can be offset by adopting post-tensioned slabs, which are typically some 15% lighter. In the case of Building B, the foundations to the post-tensioned options are between 3% and 4% less expensive than those for the Flat Slab option.

With appropriate adjustment for the location of costs of core walls and bracing infi ll within the elemental summaries in order to achieve a like-for-like comparison, the frames and upper fl oors for the RC Flat Slab option have been shown to be less expensive than the alternative structural solutions, which were between 1% and 54% more expensive for Building A and between 2% and 66% more expensive for Building B.

Foundations

Main conclusion

Differences in cost

Frame and upper fl oors


63


It should be appreciated that in most cost plans, the infi ll to the bracing of a steel-braced core, which is an integral component of the choice of structure, is generally not included within the costs of the structure, but is allocated to the Internal planning element. Conse-quently any comparison of the costs of the frame and upper fl oors only could be distorted by a signifi cant amount.

The thinner the overall structural and services zone, the lower the cladding cost. Given that the cladding on the buildings in the study represents between 17% and 22% of the construction cost, minimising the cladding area represents considerable value to the client. The minimum fl oor-to-fl oor height is almost always achieved with a fl at soffi t and separate services zone, offering the potential for additional storeys in high-rise buildings and thus improved rental or sales return. Smaller fl oor-to-fl oor heights have reduced cladding areas and hence lowered costs, and of increasing importance is the potential benefi t that a reduced cladding area has on the building’s energy use.

It should be noted that a premium is incurred in sealing and fi re stopping at partition heads against profi led soffi ts of metal decking and around non-rectangular-shaped intersecting frame members. Failure to consider this aspect can result in expensive and time-consuming remedial work later in the construction programme.

Mechanical and electrical services represent a large proportion of the overall construction costs of the buildings, averaging 34% for Building A and 33% for Building B. The design team was briefed not to design the services in detail, nor to take into account any benefi ts associated with the potential for fabric energy storage. Nonetheless, it should be noted that the removal of suspended ceilings in order to benefi t from the thermal mass of the concrete within the buildings would reduce the overall capital project costs for all options by approximately 2% for Building A and by approximately 3% for Building B.

Types of ventilationBoth buildings have been assumed as fully air-conditioned and, whilst natural ventilation and thermal mass can be used to eliminate air conditioning, these were not considered in this study.

FlexibilityA fl at soffi t provides a clear zone for services distribution, free of any downstand beams. This reduces co-ordination effort for the design team and therefore the risk of errors, permits fl exibility in design and allows co-ordination effort to be focused elsewhere. Services installation is simplest below a fl at soffi t, permitting maximum off-site fabrication of services, higher quality of work and quicker installation.

Internal planning

External cladding



64


These advantages can typically produce cost savings on initial services installation costs but, more importantly, because they facilitate the use of pre-fabricated services equipment packages, they can offer reduced installation programmes, together with cost-in-use benefi ts in the form of reduced maintenance downtime due to ease of equipment change-out, greater fl exibility and less disruption to an occupier’s business operations.

Flat soffi ts also allow greater future adaptability for building refurbishment, new layouts and cellular arrangements; in addition, different service requirements are straightforward and more easily accommodated.

These benefi ts are some of the main reasons for the development of Slimdek; however, this study shows the signifi cant cost premium incurred with this option and shows how the RC Flat Slab or PT Flat Slab options are the most economic ways of getting a clear, fl at soffi t.

Differences in nett lettable area resulting from the different structural options adopted have not been considered in the study. However, it should be noted that there are two main areas in which such differences are found: stairs and core areas.

Typically, stairs are re-sized as a result of the reduced storey height module, producing slightly increased net lettable areas.

The area occupied by a concrete core tends to be slightly smaller than that needed for a steel core, due to the allowance for steel bracing zones and the structural concrete walls serving a dual function as partitions.

The RCC study - referred to in the Introduction - found that, on an overall basis, the difference can be as much as 1.5% extra nett lettable fl oor area, and this fi nding is still valid.

General conclusionsThe lead times for the Flat Slab, In-situ + Hollowcore, PT Flat Slab and PT Band Beam options are signifi cantly shorter than those for the Composite, Steel + Hollowcore, Slimdek and Long-Span Composite options.

For Building A, during the eight-week saving in lead time, nearly 90% of the frame for the Flat Slab option could be constructed, whilst the 50 weeks overall construction programme for the Flat Slab option is only marginally longer than the 48 weeks for the Composite solution.

For Building B, the ten-week saving in lead time equates to the period required to construct the frame for the Flat Slab up to fourth-fl oor level and commence the walls and columns from the fourth to fi fth fl oor, i.e. approximately 60% of the complete frame. The overall construction programmes for the Flat Slab and the Composite options are identical at 67 weeks.

Nett lettable area

Programmes


65


Construction programmesThe programmes refl ect a pragmatic contractor’s approach to the construction process. Inevitably, different planners would produce slightly different programmes based on a considerable number of variable factors. Overall project programmes are highly infl uenced by the procurement route and type of contract adopted, and alternative procurement approaches such as construction management or design and build would no doubt produce different results. For example, construction management and design and build approaches lend themselves to concrete construction, where the ability to accommodate late information and variations are particularly benefi cial, as the work can be let before the design of following packages has been fi nalised.

The programmes prepared for this study refl ect one procurement approach but, in practice, contractors are more likely to programme to a pre-set completion date in the knowledge of the type of contract, their projected costs, the risk profi le of the project, their knowledge of and relationship with the client and design team, their supply chain and their exposure to both liquidated damages and to market forces in play at the time of the project.

A practical view had to be taken of such factors as logistics, site access, boundary constraints, cranage, etc., which are essentially site-specifi c. It could be argued that the steelwork could have started on-site sooner, with earlier sub-contract award or longer periods for design, package tendering, mobilisation or foundations making the steelwork lead time less critical or even non-critical. Conversely, the use of a purely domestic sub-contract, without the ability to pre-order, would push the programme back.

Whereas fi re protection used to be a critical activity, modern details such as site-applied intumescent coating have removed fi reproofi ng from the critical path altogether. However, although not on the critical path, the fi reproofi ng activity requires a greater level of detailing and causes disruption that can adversely affect other trades, e.g. diffi culties caused by fi xings penetrating through fi re-proofi ng and damage needing rectifi cation.

Off-site intumescent coatings have been introduced in an effort to reduce the construction time, but these can suffer from signifi cant damage in transit, requiring site remedial work which can eliminate the original saving.

The durations of fi rst fi x, second fi x and M&E installations are essentially the same, with slight differences in quantities appearing to make little difference to the programmes. However, it is becoming increasingly common to use prefabrication for the M&E services distribution, which can offer programme advantages when used in conjunction with the open fl at soffi ts provided by the Flat Slab, PT Flat Slab and Slimdek options. Prefabrication of sections of the M&E installations also offers advantages in subsequent maintenance and refurbishment of the building. No account is taken within the programmes of any construction time savings resulting from such prefabrication.


66


Although the reported costings excluded the effects of fi nance costs, if fi nance costs were to be considered, they should not be limited to the construction period alone as, in most cases, fi nance costs also affect the procurement and lead times.

It is not possible to examine the entire project from inception to completion, as the dura-tion prior to the commencement of procurement cannot be defi ned on a generic basis. However, consideration of the periods that have been identifi ed in the programmes for pro-curement, lead time and construction would produce the following comparison, assuming a rate of 7.75% p.a. (base rate + 2%) and comparing the programme extension or saving against the most economic short-span solution, the Flat Slab option. The PT Band Beam option has been compared with the Long-Span Composite option.

Finance costs

Building B Flat Slab PT Flat Slab

Composite In-situ + Hollowcore

PT Band Beams

Long-Span Composite

Steel + Hollowcore

Slimdek

Construction cost in £/m2 £1,676 £1,678 £1,691 £1,693 £1,713 £1,715 £1,735 £1,759

Overall programme in weeks 83 82 93 86 83 95 91 91

Savings in fi nance costs @ 7.75% p.a. +£0 -£1 +£7 +£2 +£0 +£8 +£6 +£6

£1,676 £1,677 £1,698 £1,695 £1,713 £1,723 £1,741 £1,765

Building A Flat Slab Composite In-situ + Hollowcore

PT Flat Slab Steel + Hollowcore

Slimdek

Construction cost in £/m2 £1,460 £1,468 £1,477 £1,477 £1,495 £1,534

Overall programme in weeks 64 70 66 65 70 70

Savings in fi nance costs @ 7.75% p.a. +£0 +£5 +£2 +£1 +£5 +£5

£1,460 £1,473 £1,479 £1,478 £1,500 £1,539

This comparison takes no account of differences in cumulative fi nance costs arising from the different cash fl ow profi les experienced with the differing forms of construction. For example, the Composite, Steel + Hollowcore, Slimdek and Long-Span Composite options require greater expenditure early on than the Flat Slab, In-situ + Hollowcore, PT Flat Slab and PT Band Beam options, where the ‘pay as you pour’ principle works in the client’s favour. A more comprehensive analysis of the construction cash fl ow profi les would be required in order to present a detailed comparison of these effects on fi nance costs.

Initial capital cost is not, of course, the sole driver for clients, whose main objective is optimum value from an overall solution. The wider value aspects of structural solutions in relation to framed buildings are therefore briefl y considered in more detail below.

Overall value vs frame costFrame cost alone should not dictate the choice of structural solution. Rather it should be just one of a number of value issues that should be borne in mind when making the choice of frame material. Only then can one be confi dent that the optimum structural solution has been selected.

Other value considerations


67


Fire protectionFor Flat Slab, In-situ + Hollowcore, PT Flat Slab and PT Band Beam structures, fi re protection is generally not needed, as the material has inherent fi re resistance of up to four hours. This removes the time, cost and separate trades required for fi re protection. Added value bene-fi ts include such factors as enhanced property safety, the potential for lower insurance premiums, re-usability of the structure and considerably reduced down-time for an occupier after a fi re.

Exposed soffi tPotential value to a client exists in those structures with a high thermal mass. By exposing the soffi ts, this can be utilised through fabric energy storage (FES) to reduce initial plant costs, by minimising or eliminating the need for air conditioning and substantially reducing the lifetime operational costs of the asset. Utilisation of FES permits the designer to create naturally ventilated buildings, giving occupants the chance to control their environment, with consequent improvements in employee productivity. Furthermore, suspended ceilings can be reduced or eliminated, giving valuable initial cost and programme benefi ts and reduced lifetime maintenance costs.


68

Appendix A - Detailed programmes

10. Appendix A – Detailed programmes

Project

Dated Drawn by Programme No

Title

Revision comment

Notes

ProgrammeTitle

Client

COMMERCIAL BUILDINGS - COST MODEL STUDYBuilding A : Scheme 1 - Flat Slab

The Concrete Centre 11/07/2006 rev

Line Name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 511

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

Site Set Up

SubstructureRemove Topsoil & RL DigPad FoundationsUnderslab DrainageGround Floor Slab

SuperstructureWalls / Columns Grd to 1st1st Floor SlabWalls / Columns 1st to 2nd2nd Floor SlabWalls / Cols 2nd to 3rd3rd Floor SlabRoof Upstands & Bases

Roof InstallationsRoof FinishesDeliver Main Roof PlantRoof MEP InstallationsInstall Plant Screen Louvres

Curtain Wallling/External CladdingSurvey/Set Out BracketsSecondry Steelwork/FramingGlazing & Spandrel PanelsCapping/Flashing & Roof Upstand Level

Building Watertight

Cores & RisersMEP RisersToilet Fit OutLift Installations

Office Fit Out to Cat ALevel 2H/L MEP 1st Installations

Suspended Ceiling Grid & Service Tiles

H/L MEP 2nd Fix

Raised Flooring

Joinery 1st Fix

Level 1Level G

Close Out

Testing & Commissioning

External Works

Completion

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

+ +16

17

18

19

20

21

22

23

24

25

26

+ +27

+ +28

+ +29

30

31

32

33

34

35

36

+ +37

+ +38

+ +39

+ +40

41

42

Line Name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51


69


Project


Title

Revision comment

Notes

ProgrammeTitle

Client

COMMERCIAL BUILDINGS - COST MODEL STUDYBuilding A : Scheme 4 - Composite


Line Name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 491

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

Site Set Up

SubstructureRemove Topsoil & RL DigPad FoundationsUnderslab DrainageGround Floor Slab

SuperstructureSteelworkMetal DeckingRC ToppingRoof Upstands & Bases

Roof InstallationsRoof FinishesDeliver Main Roof PlantRoof MEP InstallationsInstall Plant Screen Louvres

Curtain Wallling/External CladdingSurvey/Set Out BracketsSecondry Steelwork/FramingGlazing & Spandrel PanelsCapping/Flashing & Roof Upstand Level

Building Watertight

Cores & RisersMEP RisersToilet Fit OutLift Installations



H/L MEP 2nd Fix

Raised Flooring

Joinery 1st Fix

Level 1Level G

Close Out


External Works

Completion

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

1

2

3

4

5

6

7

8

9

10

11

12

+ +13

14

15

16

17

18

19

20

21

22

23

+ +24

+ +25

+ +26

27

28

29

30

31

32

33

+ +34

+ +35

+ +36

+ +37

38

39

Line Name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

Project


Title

Revision comment

Notes

ProgrammeTitle

Client

COMMERCIAL BUILDINGS - COST MODEL STUDYBuilding B : Scheme 1 - Flat Slab


Line Name 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 681

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

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Site Set Up

SubstructurePile ProbingForm Pile MattPiled FoundationsExcavate & Form Pile CapsUnderslab DrainageGround Floor Slab

SuperstructureWalls / Columns Grd to 1st1st Floor SlabWalls / Columns 1st to 2nd2nd Floor SlabWalls / Cols 2nd to 3rd3rd Floor SlabWalls / Columns 3rd to 4th4th Floor SlabWalls / Cols 4th to 5th5th Floor SlabCols / Walls 5th to 6th Roof SlabRoof Upstands & Plant Bases

EnvelopeRoof FinishesRoof InstallationsAtrium GlazingCurtain Wallling/External Cladding

Building Watertight

Cores & RisersM&E RisersToilet Fit OutLift Installations


Perimeter Ceiling Plasterboard Margin


H/L MEP 2nd Fix

Raised Flooring

Joinery 1st Fix

Level 2Level 3Level 4Level 5

Close OutTesting & CommissioningExternal WorksCompletion

1

2

3

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1

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3

4

5

6

7

8

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13

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15

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18

19

20

21

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23

+ +24

+ +25

+ +26

+ +27

28

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+ +30

+ +31

+ +32

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+ +41

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2d

Line Name 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68


70


Project


Title

Revision comment

Notes

ProgrammeTitle

Client

COMMERCIAL BUILDINGS - COST MODEL STUDYBuilding B : Scheme 4 - Composite


Line Name 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 681

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Site Set Up

SubstructurePile ProbingForm Pile MattPiled Foundations (128 No)Excavate & Form Pile CapsUnderslab DrainageGround Floor Slab

SuperstructureConcrete Core (jump form)Structural Steelwork Metal DeckingRC ToppingUpstands/Bases at Roof Level

EnvelopeRoof FinishesRoof InstallationsAtrium GlazingCurtain Wallling/External Cladding

Building Watertight

Cores & RisersM&E RisersToilet Fit OutLift Installations


Perimeter Ceiling Plasterboard Margin


H/L MEP 2nd Fix

Raised Flooring

Joinery 1st Fix

Level 2Level 3Level 4Level 5

Close Out


External Works

Completion

1

2

3

4

5

6

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1

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+ +16

+ +17

+ +18

+ +19

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+ +22

+ +23

+ +24

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45d

Line Name 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68



CC

IP-010C

ost Model Study – C

omm

ercial BuildingsA

report comm

issioned by The Concrete C

entre

Cost Model Study –Commercial BuildingsA comparative cost assessment of the construction of multi-storey offi ce buildings


A report commissioned by The Concrete Centre


This comprehensive and independent cost study was undertaken to evaluate a number of structural frame options for a three-storey offi ce building in an out-of-town location and a six-storey offi ce building in a city centre location. A total of 14 fl oor design options were evaluated, budget costings were assigned to all elements of construction and adjustments were made to refl ect time-related costs attributable to differences in the construction programme.

The publication outlines the analysis, the detailed costings and programmes for each structural alternative, and provides a useful resource for architects, engineers and contractors involved with evaluating the cost competitiveness of structural options for multi-storey offi ce construction.

CCIP-010 Published October 2007 ISBN 1-904482-36-8Price Group P



CI/Sfb

UDC624.94.04.033

Francis Ryder, Head of Cost at The Concrete Centre, has project managed this cost model study for commercial buildings.

For more information visit www.concretecentre.com/publications


Date post:	18-Feb-2016
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