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P R O J E C T

P R O F I L E

P R O J E C T

P R O F I L E

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All advice or information from the British Cement Association is intended for those who will evaluate the significance and limitation of its contents and take responsibilityfor its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all BCA

publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

97.313

First published 1990

ISBN 0 7210 1388 0

Price Group C

British Cement Association 1990

Published by the British Cement Association on behalf of

the industry sponsors of the Reinforced Concrete Campaign.

British Cement Association

Wexham Springs, Slough 5L3 6PL

Telephone Fulmer (0753) 662727

Fax (0753)660399 Telex 848352

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This publication was commissioned by the

Reinforced Concrete Campaign Group.

The Group was set up in 1988 to run a

promotional campaign aimed at providing

better knowledge and understanding of

concrete design and building technology.Its members are Sheerness Steel plc,

Allied Steel and Wire Limited and the

British Reinforcement Manufacturers

Association, representing the major

suppliers of reinforcing steel in the UK,

and the British Cement Association,

representing the major manufacturers of

Portland cement in the UK.

David Bennett is a Senior Engineer in

the Marketing Division of the British

Cement Association.

Bob Gordon was Chief Structural

Engineer for Bovis-Schal on all phases of

the Broadgate development.

FOREWORD

CONTENTS

D.F.H. BennettBSc, MSc CEng, MICE

and

R.W. GordonBSc (Eng), ACGI, DIC, CEng, MICE

THE PROJECT 2

BUILDING Space provisions 3

SPECIFICATION Structure 3

Cladding 4Services 4

Building management system 4

DESIGN Architecture 4

Frame 5

Services 5

CONSTRUCTION Substructure 6

Frame 7

Precast cladding 9

SPEED WITH

BUILT-IN QUALITY 11 APPENDIX Project details 12

1

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BUILDING SPECIFICATION

P R O J E C T P R O F I L E

2

Broadgate, a site of some 29 acres, is located at

Liverpool Street, one of the busiest transport inter-

changes in the City of London, and within walking

distance of all the main City institutions.The entire development - comprising 14 separate

buildings plus extensive retail and leisure facilities -

will provide up to 3.5 million sq ft of net office space

when completed in 1992.

Since 1985, over 2 million sq ft has been

constructed at this site, ranking Broadgate as the

fastest commercial construction project in the UK -

and probably the world.

One of the 14 buildings within Broadgate is

Broadwalk House, situated on the extreme northern

corner of the site, bounded by Appold Street to the

west, Worship Street to the north, and Primrose

Street to the south.

Broadwalk House is not a large building by

comparison with the rest of the development: its

eight storeys, two basements and 33 500 m2 floor

area represents about seven per cent of the total

development. But whatever it may lack in scale is

more than made up for by the sheer speed, quality

and economy of its construction. From start on site,

it took just 20 months to hand over the building to

the client. The frame itself was topped out in 25

weeks, reinforcing the claim that Broadwalk Houseis the most successful construction project within the

Broadgate development.

Designed in reinforced concrete by a team of

professionals who have fully exploited concretes

versatility in order to obtain aesthetic appeal

combined with buildability, Broadwalk House can

be said to have set standards for the rest of the

industry to follow.

To appreciate the significance of the Broadgate

development and the context of Broadwalk House, it

is necessary to reflect on what the office building

market was like before the explosion of information

technology in the early 1980s. The City was

characterized by small buildings, on tight sites which

were, almost without exception, designed from the

outside in, rather than from the needs of the

occupants. The supply of office space was so strictly

regulated by planners and developers that tenants

had come to accept any space, however

inconvenient, and at any price, provided that they

could get their hands on it. The discrepancy in

THE PROJECT

Plan of the

Broadgate

development

Broadwalk House under

construction - from Primrose Street

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quality between office space in London and that

enjoyed in New York, Chicago or Los Angeles, was

enormous.

The Broadgate buildings are the end-product of a

studied and deliberate long-term consultation

between the developers and building end-users, tocreate buildings that work.

Broadwalk House, like other buildings in the

Broadgate development, offers the tenant a choice of

shell and core or base building. Shell and core

consists of the building envelope with lifts, staircases,

main services, toilets, reception lobbies, etc. The

tenant, before occupation, fits out the building

with ductwork, IT cabling, ceilings and raised floors.

With the base building alternative, the building is

already equipped with primary and secondary

ductwork, false ceilings and raised floors.

With its large structural bay layouts and design

innovation, Broadwalk House provides end-users

with a building that is adaptable to their various

requirements.

The principal features of the building are listed

below.

Ground-floor and mezzanine accommodationproviding both office and retail space.

Two levels of trading floors, plus four levels ofoffice space.

A two-level basement car park.

Provision of six passenger lifts from ground toroof, with two escalators from ground to secondfloor to serve the trading floor areas.

Full air-conditioning with zoning options to servediscrete sections of a floor.

Provision for computer, telephone and powerrequirements to meet the increasing demands of

information technology.

Two central atria to admit more natural lightwhich creates a better working environment.

Space provisions

The clear internal dimensions from finished floor

level to underside of ceiling are 2.74 m for typical

office areas and 3.05 m for trading floors. Generally,

the structural span between columns is 9 x 9 m and

9 x 12 m.

The gross floor area of the building is 33 500 m2

(typically 3700 m2 per floor), providing a usable area

of 27 000 m2.

Structure

The foundations consist of large-diameter bored

piles of in situ concrete, with underreaming.

Columns, perimeter beams and small areas of

P R O J E C T P R O F I L E

Typical office

floor section

showing services

Typical

sixth-floor office

interior

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P R O J E C T P R O F I L E

4

non-typical bays are of in situ construction. The

floors are of the composite reinforced concrete

ribbed type, with precast beams spanning up to 12 m

onto in situ spine beams. They have been designed

to carry an imposed load of 3.85 kN/m2, plus

1.85 kN/m2

for services.Cladding

The architectural character of the building highlights

the use of traditional materials in a clear, modern,

functional way. The external cladding is a

combination of high-quality architecturally finished

precast concrete and a curtain walling system. All

windows are double-glazed and framed with

aluminium mullions.

Services

A variable air volume air-conditioning system issupplied with air-handling plant on each floor.

Sprinkler and fire alarms are operated by the

advanced building management system.

There are six high-speed, 21-person passenger

lifts, three executive/fire lifts, and a separate goods

lift to all floors. A twin escalator serves the ground,

mezzanine and the two trading floors.

Additional generator capacity, service facility,

loading bay and storage areas have been provided.

Building management system

The building management and control system ismicro-processor based, capable of operating,

monitoring and controlling equipment from a

remote location. The system has the capacity and

flexibility to service the building for a normal 5-day

or 24 hour, 7-day week, or any other variation.

Interior of

sixth-floor turret

room

Bay window

detail on Appold

Street

Architecture

The architectural master plan of Broadgate was

developed in two distinct phases. The early phase onFinsbury Avenue comprised four buildings centred

around Broadgate Circle. The design of the facades -

using reticulated granite panels - provides a visual

symmetry to the buildings, emphasizing harmony

and uniformity.

In direct contrast to this, the ten buildings in the

later Bishopsgate phase have been designed to give

each one a distinctive architectural style, in order to

enhance corporate image and tenant identity. These

buildings are faced mainly in granite cladding panels,

with the exception of Broadwalk House.

The location of Broadwalk House, on an island

site and separated from the rest of the Bishopgate

DESIGN

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P R O J E C T P R O F I L E

5

buildings by Primrose Street, presented an

opportunity to create something different from the

granite facades of the other buildings.

The choice of an exterior cladding, using

pigmented precast concrete panels to simulate

terracotta, links the building well with the essentialcharacter of the area. The warm colour of the

cladding, the light and shade of the panel relief, the

bay windows profiles, the sweep of the street level

arcade with its row of elegant globes and turreted

corner detail, create a focal point on Appold Street.

All public areas within the building are finished

to the highest quality: the entrance lobby walls

trimmed in cherry wood with hardwood veneer

panelling, the floors paved with polished stone, and

the thoroughfares lit by beautifully crafted wall-

mounted light fixtures, all maintain the hallmark of

quality throughout.

Two atria, clad in buff-coloured drywall panels

with painted timber battens, bring natural light into

the heart of the building, creating a pleasant working

environment and providing visual coherence.

Frame

The design of the frame of Broadwalk House was

finalized only after careful appraisal of all the options

- reinforced concrete, structural steel and pre-

stressed concrete.

Comparative costings and construction timeswere analysed for each option, using data from

previous projects on Broadgate and budget advice

from specialist contractors. These comparisons

showed conclusively that reinforced concrete was the

best buy. It was cheaper than steel by as much as

200/o, and faster to construct.

A ribbed-slab design was chosen for the floors,

with a wide-rib section, in order to maintain an

economical construction depth of 610 mm. The

ribs, spaced at 3.0 m intervals, span three bays - two

of 12.0 m and one of 9.0 m - onto the central spine

beams and perimeter beams, each of which span 9.0 m.The lateral stability of the frame was checked by

analysing the deflections and sway of a three-

dimensional computer model of the structure. The

inherent stiffness of the frame eliminated the need

for shear walls, so enabling fast and economic

construction.

Services

Whilst secondary service ductwork could have been

positioned in the voided areas between ribs, it would

have imposed a limitation on flexibility for end-user

fitting out. Consequently all ductwork was zoned

below soffit level. Entrance lobby

Bringing natural

light into the

building

The central

atrium canopy

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CONSTRUCTION

P R O J E C T P R O F I L E

6

A number of construction features were incor-

porated into the design of the structure to take

account of site constraints, speed of erection, labour

availability, and competitive prices.For the frame and substructure the fast-build

method was used - this incorporated the following

features.

Design of one column/one pile to eliminateconstruction of large pile caps.

Construction of the ground floor slab ahead of thebasement to accelerate the frame.

Elimination of shear walls in order to speed floorcycle time.

Prefabrication of floors was facilitated by

standardization of the building grid and the column

sizes. Specification clauses were drafted to permit

large-area pours and the early removal of soffit

falsework to aid rapid construction techniques.

To ensure that quality was built into the project,

each trade contractor bidding for work was required

to submit a quality plan. This ensured that the

procedures for achieving quality were set down

correctly and that the resources were provided for

the work to be carried out effectively.

Substructure

The bulk excavation for the substructure followed

the installation of the contiguous bored-pile,

earth-retaining system. To simplify the basement

waterproofing and to speed retaining wall

construction, the position of the pile wall was kept

about one metre away from the basement retaining

wall line.

Using two rigs, 320 contiguous bored piles were

constructed at a peak rate of 14 piles a day. The piles

were 750 mm in diameter and generally 11.5 m deep

with temporary casing taken through fill and the 4 m

gravel layer overlying the London clay.Main piling commenced as soon as there was

sufficient space within the excavation for a rig to be

installed. An access ramp was maintained at all times

from ground level to excavation level, for concreting

trucks and spoil lorries. The piling layout followed

the column grid line, i.e. 12.0 x 9.0 m, except for

the core areas which had traditional pile groups and

pile caps. A total of 120 main piles were constructed

using standard boring techniques, and founded

approximately 13.5 m below basement level.

Depending on the pile size, from two to five piles per

day were completed with each rig. The pile loadings

ranged from 12 500 kN to 13 350 kN, and the piles

Constructing the

main piles

Basement,

showing the

intermediate

columns and the

ground-floor

soffit

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varied in size from 750 mm diameter plain shafted,

to 750, 1050 and 1200 mm underreamed piles, with

the largest being 1500 mm in diameter with a

4500 mm underream

The concrete used in all piles had a specified

characteristic strength of 35 N/mm2 at 28 days, anda slump of 125 mm. Following pile construction,

small pile caps were formed to provide a transition

zone between the pile and the column. The individ-

ual pile caps ensured that column starter bars could

be located accurately to grid, thus overcoming the

positional variation associated with pile construc-

tion.

The basement construction comprised two floors

- a basement and intermediate level - and a

perimeter retaining wall. To reduce overall exca-

vation depth, the intermediate basement floor was

designed as an in situ flat slab. Additional columns

were introduced in the basement, in order to reduce

spans and minimize the slab depth.

Internal columns, on the conventional grid

layout, and perimeter retaining walls were cast up to

ground floor, bypassing the intermediate floor, to

allow an early start on the frame. Reinforcement

couplers were cast into the columns and continuity

strips incorporated in the retaining walls to cater for

subsequent connection to the intermediate slab rein-

forcement.

When the floors of the frame were sufficientlyadvanced and the falsework support to the ground

floor slab removed, the intermediate basement slab

was constructed using conventional formwork.

Frame

During the eight-week tender period for the frame

contract, the frame was changed from in situ to

composite in situ and precast concrete.

This change was made at the

request of the trade

contractor because the spaceoutside the site boundary

was too restricted to fly large

table forms.

The composite floor slab

consisted of precast soffit

slabs and precast rib beams

which spanned 12 m

between the in situ spine and

edge beams. The precast elements

were structurally tied together with an in situ

concrete topping.

The availability of a disused goods yard near the

site made it feasible and economic to set up a

P R O J E C T P R O F I L E

7

Constructing the

double-storey

height columns

Floor

construction

elements

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P R O J E C T P R O F I L E

8

precasting operation more or less on the spot. An

area of 66 x 45 m within the goods yard was

allocated for the prefabrication of reinforcement

cages, two precast production bays and a storage area

for curing and handling the precast rib beams.

In the time taken for the substructure to reachthe ground floor, precast production was in full

swing, with sufficient rib beams stockpiled to meet

programme requirements.

The general building sequence commenced with

column construction followed by the erection of

small table forms for the in situ spine beams and

perimeter beams. After positioning the prefabricated

reinforcement cage for the right-hand perimeter

beam and closing up the side forms, the precast rib

beams for the right-hand bay were then crane-

handled into position. This procedure was repeated

in a similar way for the spine beams and rib beams

for the centre and left-hand spans. Finally, the left-

side perimeter beam cage was positioned and side

forms closed.

Precast soffit slabs - 55 mm or 75 mm thick

spanning 2.4 m with their lattice girder

reinforcement - were craned into position to span

between the ribs. The top layer of reinforcement was

laid and the lapping bars for the spine beams fixed

over the column positions.

Working in a north to south direction this

procedure was repeated across the building from bayto bay. Concreting commenced once an area of

around 1500 m2 was ready - equivalent to 200 m3 of

concrete - dividing the slab into four pours.

The contractor elected to use a C50 grade

concrete for all columns and floors, in order to

obtain high early strength and so allow early removal

of falsework. A pump mix was designed, with a

cement content of 380 kg/m3, giving a 48 hour

strength of 25 N/mm2 - the minimum safe strength

for falsework removal. Back-propping was necessary,

after removal of falsework, and carried down

through three floors.

One of the contributory factors in achieving a

fast rate of construction was the use of the two static

Schwing concrete pumps with long-reaching

booms. Mounted on permanent supports within the

building and rising up level by level with the floor

construction, the pumps reduced quite dramatically

the time taken to place the 200 m3 of concrete for

each pour area. With the use of vibrating screeds of

up to 10 m spans, the deck area was compacted and

finished in a very short time with the minimum of

labour.An adjustable

column form

Compacting in

situ concrete with

a vibrating screed

rail

Precast rib beams

in position on the

third floor

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P R O J E C T P R O F I L E

9

Fast column. construction was achieved using

prefabricated adjustable column forms which

incorporated a built-in concrete platform and access

ladder. These adjustable forms covered the full range

of column sizes required for the project. Kickerless

construction was adopted and helped to reduce theoverall construction time.

To minimize the lead time for reinforcement

supply, the prefabrication of beam cages and column

cages was carried out on site.

Staircases were of precast concrete supported by

dwarf in situ walls located either side of half landings.

Construction of staircases progressed one floor

behind the frame, ensuring safe and easy access and

obviating the need for external access platforms.

Three cranes were provided for the frame

construction and the cladding erection. One was

located centrally, with a 60 m boom capable of lifting

6 tonnes and reaching all parts of the site. Two

smaller cranes, with reaches of 30 m and 35 m, were

located within the building footprint, but at opposite

ends. Thus there were always two cranes available to

service any area of the construction.

Overall, approximately 13 000 m3 of concrete

was placed and 6 miles of precast rib beams used in

the construction of the frame. The average construc-

tion output was just short of 3000 m2 per week,

peaking at 4000 m2. This was achieved at the end of

March 1988. Thereafter a rate of 3500 m2

per weekwas maintained until the frame was topped out.

Precast cladding

An important consideration in letting the cladding

contract was the ability of the cladding contractor to

undertake the design, manufacture and pre-assembly

of the precast concrete cladding panels and window

units, in addition to their erection. Early in the

design stage the architect met the cladding

contractor to standardize and simplify the panel

design, for efficient precast production. Only a fewmodifications to the original details of the precast

profiles were made. These were to ease removal of

the panels from their moulds by adjusting the panel

profiles to give 2.50 taper and by adopting rounded

corners to eliminate sharp arrises.

The terracotta colour of the panels was achieved

by using a mixture of 2.5% yellow and a 2.5% red

pigment by weight, blended with a white cement

mortar. The precast panels, of grade C45 concrete,

were lifted out of their moulds on achieving

20 N/mm2 strength. They were air-cured for two

weeks, under cover, in a specially designated curing

bay protected from the weather. This allowed the

Panels fitted with

windows,

awaiting

transportation

Moulds for

precasting the

curved panels

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P R O J E C T P R O F I L E

10

panels to dry slowly, minimizing efflorescence

problems and maintaining uniformity of the

pigmented colour. During this time, the panels were

lightly sand-blasted to remove surface laitance.

Following the two-week curing period the alumi-

nium window units were installed in the panels,prior to their transportation to site on lorry-mounted

trailers. Once on site the panels were craned off the

trailers directly into their final position.

The panels were separated into 1 m high spandrel

units and 3.6 m high window elevation units. The

panel lengths varied from 3 m to 6 m, with the

occasional 9 m spandrel unit, to suit the trailer length

and to match the perimeter column grid. In addition

to the bay window panel elements and spandrel

units, there were flat window panels, column panels

and special curved panels for the turreted corner.

The precast panel loads were transferred to the

structural frame through stainless steel hangers

attached to the spandrel units, carrying the spandrel

unit and the window elevation panel directly above.

Due to the slenderness of the panel sections,

lateral restraint anchors were provided at the four

corners of each panel, with an additional central

restraint for panels of 6 m or more in length.

Spandrel units were hung approximately in position

by the crane to allow the stainless steel hangers to be

located onto the brackets on the perimeter beam.

The four corner restraint anchors were then fixedbefore the crane was released to fetch the next panel.

The panels were finally positioned to grid (vertically

and horizontally) by adjusting the stainless steel

hanger assembly.

The window panel was then slipped over the

dowel pin located on the spandrel unit, seated on

rubber pads, and shimmed to level, on the levelled

spandrel unit. The four corner anchors of the panel

were then fixed to the frame before the crane was

released.

In all, 1600 panel pieces were cast and fixed to

give an area of 12 000 m2 in elevation. Construction

commenced from mezzanine level upwards in order

to enclose the trading and office floors in advance of

the ground-floor retail units.

It took just 27 weeks to complete the cladding

contract, during which time an average of 60 panels

were erected per week using one, two or all three

cranes as they became available. At the peak of con-

struction, 106 panels were erected per week working

with three site erection teams. The contract was

completed by the end of November 1988, when the

last 100 panels for the ground floor and shop frontareas were installed.

Window panel

fixing assembly

Spandrel panel

fixing assembly

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P R O J E C T P R O F I L E

11

Broadgate House has shown that reinforced

concrete, used for both the frame and cladding, can

be designed and built to provide both clients and

tenants with buildings that are fast and economic toconstruct, and which meet the following critical

requirements of end-users in todays office market.

High net lettable floor areas, created by anefficient floor plan on a 9 x 12 m grid.

Flexibility for vertical and horizontal services,both now and in the future, with a ribbed-slab

floor.

Quality of architecture - the external cladding,which exploited the aesthetic appeal of precast

pigmented concrete, and the interior decorations

enhanced tenant identity and image.

Moreover, reinforced concretes inherent adapt-

ability, short lead-in times and range of construction

options allowed the frame design to be modified

from an in situ to a composite in situ and precast

floor within the eight-week tender period, to take

full advantage of the faster construction method

proposed by the trade contractor. Handed over to

the client in 20 months and framed in 25 weeks,

Broadwalk House is ranked as the fastest-framed

project within the Broadgate development.

This achievement was possible because of close

involvement of the client, the choice of hisprofessional teams, the efficiency of site management

and the co-operation and expertise of the various

trade contractors who carried out the work.

SPEED WITH BUILT-IN QUALITY

Above left: December 1987 -

basement slab under construction,

contiguous piles exposed

Left: April 1988 - frame up to level 6,

cladding erection commenced on

mezzanine level

September 1988 -

cladding complete

to level 5

Broadwalk House was

handed over to the

client in May 1989

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P R O J E C T P R O F I L E

12

Development Manager Rosehaugh Stanhope

Developments plc and

British Rail Property Board

Construction Manager Bovis-Schal Joint VentureArchitect and Structural Skidmore, Owings & Merrill

Engineer

Services Engineer Jaros Baum & Bolles

Frame Contractor Ray ORourke & Son Limited

Precast Contractor Schokbeton-Nijhuis Alkono

- Joint Venture

THE PROJECT TEAM

THE PROGRAMME

A P P E N D I X

CONSTRUCTION COSTS

/m2

Piling 12.04

Substructure 45.84

Frame 92.76

M & E 342.34

Cladding/roofing 171.11

Finishes 102.62

External works 4.34

Sundries 34.89

Total 805.94

m2

Gross floor area 33 500

Net lettable area 27 000Number of storeys 8

SCHEDULE OF AREAS

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P R O J E C T P R O F I L E

13

Trading level 1

Office level 4

Section AA

Section BB

Start September 1987

Finish May 1989

Duration 20 months

CONSTRUCTION TIME

TYPICAL FLOOR

PLANS AND SECTIONS

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PROJECT PROFILE: BROADWALK HOUSE

D.F.H. Bennett and R.W. Gordon

BRITISH CEMENT ASSOCIATION PUBLICATION 97.313