Precast Concrete Structures 1

8/2/2019 Precast Concrete Structures 1

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SUBMITTED BY:

RENU RANI

2010PST117


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INTRODUCTION

The concept of precast construction includes those buildingswhere the majority of structural components arestandardized and produced in plants in a location away fromthe building, and then transported to the site for assembly.These components are manufactured by industrial methodsbased on bunch production in order to build a large numberof buildings in a short time at low cost.

Precast concrete construction requires a restructuring of theentire conventional construction process to enable

interaction between the design phase and productionplanning in order to improve and speed up the construction.One of the key premises for achieving that objective is todesign buildings with a regular configuration in plan and

elevation


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Main features of this construction process are as

follows:

The division and specialization of the human workforce.

The use of tools, machinery, and other equipment, usually

automated, in the production of standard, interchangeable parts and

products.

Compared to site-cast concrete, precast concrete erection is faster

and less affected by adverse weather conditions.

Plant casting allows increased efficiency, high quality control and

greater control on finishes.


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Benefits of Precast Structures

Inherent Fire Properties - Concrete has its own inbuilt fire resistance which is

present during all construction phases. Fire resistance is typically achieved without

the application of additional sprays or linings. This is an important inherent

advantage over steel and timber solutions.

Health & Safety: Precast floor slabs they provide a safe working platform for site

operatives. Simultaneously installing precast stairs offers safe and easy access

between floors.

Reduced Construction Programme: Precast concrete increases speed of

construction, which gives earlier return on investment, freeing up the project critical

path and allowing earlier completion. It is estimated that a precast structure takes up

to 20% less time to construct than a similar cast in situ structure.


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Build ability: Precast frames can greatly improve buildability because the

sensitive parts of the operation can be moved from the site to the factory.Larger Clear Spans - Reducing the number of columns is critically

important in developments such as sports stadia and car parks. Longer spans and

shallower construction depths can be obtained by using pre stressed concrete

beams and floors.

Composite ActionPre stressed precast elements act compositely with an in

situ structural screed (topping), combining the benefits of precast and in situ

construction.Air tightness: Air infiltration in precast buildings is minimal because of the

relatively small number of joints in the construction. This factor combined with

the thermal mass of concrete gives excellent thermal performance.


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PRECAST BUILDING SYSTEM

Depending on the load bearing, precast

building system can be divided into following

categories:

Precast concrete frame.

Precast concrete wall.

Precast concrete floor.


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PRECAST CONCRETE FRAME

Precast frames can be constructed using either linear elements

or spatial beam-column sub assemblages .Precast beam-column

sub assemblages have the advantage that the connecting faces

between the sub assemblages can be placed away from the

critical frame regions.

Types of precast frame construction:

(1)Portal Frames.

(2)Column and Beam (single storey and multi-storey).

(3)Cross Wall Construction.


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PORTAL FRAME

A Portal Frame is a frame consisting of precast columns connected at the top by a

pitched or sloping horizontal beam Portal Frames are economical to produce and

are often associated with industrial or warehouse buildings where a clear span of

up to 35m or more is required.

Industrial Portal Frame


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MULTI-STOREY COLUMN AND BEAM

Single storey columns:

Single storey column are jointed at each floor level are more often located inside

the perimeter of the building. Spine beams are typically erected over these columns

and are connected using high strength dowel bars in grouted dowel tubes, cast into

the beams and With this method of precast construction, buildings are erected one

floor at a time with beams placed at the head of columns at one level before the

upper level columns are erected and connected through the beams to the columns

below columns.

Figure 2

Continuous spine beams


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MULTI-STOREY PRECAST BUILDINGS

Multi-storey precast buildings consist of a combination of Column and Beam and

Cross Wall construction where the benefits of each are used to optimise structural

and cost efficiencies. Lift shafts and gable walls are common cross wall elements

used in conjunction with Column and Beam structures, adding significantly to the

structural stability of the building.

Figure 3

Typical multi-storey building


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Cross Wall Construction

Cross wall multi-storey structures consist of precast floors and load bearing walls,

where the walls are designed as the means of primary support. Longitudinal stability

is achieved by external wall panels and/or diaphragm action involving the floors and

roof, which are connected, back to lift cores or staircases, which are also formed by

precast wall panels.

Figure 4

Cross wall


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PRECAST CONCRETE WALL

Precast wall are used for internal & external walls, lift shafts, central cores

etc. Precast wall system is mostly used in domestic construction, both for

individual housing and for apartments. The precast walls can be load

bearing or only partition walls. The surface of the elements is smooth on

both sides & ready for painting or wall papering.

Figure 5: Wall panel


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Depending on the wall layout, there are three basic

configurations of large-panel Buildings:

(1)Cross-wall system: The main walls that resist gravity and

lateral loads are placed in the short direction of the

building.

(2)Longitudinal-wall system: The walls resisting gravity and

lateral loads are placed in the longitudinal direction.

(3)Two-way system: The walls are placed in both directions.


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PRECAST CONCRETE FLOOR

Precast concrete flooring offers an economic and versatile

solution to ground and suspended floors in any type of building

construction.

Types of Floor unit:

(1) Hollow core floor.

(2)Double tee floor slabs.

(3)Solid wide slab floors.

(4)Composite plank.

(5)Bubble floor


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Hollow core floor slabs: Hollow core slabs derive their name from thecircular voids or cores which run from end to end of the slab. The cores can

function as cable services ducts and significantly reduce the self-weight of theslabs, maximising structural efficiency. Units are available in standard 1200mm

widths and in depths of 150mm to 500mm


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Double tee floor slab:Double-Tee floor units are produced in standard widths of2400mm and in depths of between 250mm and 1000mm The system offers greater

structural capacity at longer spans than hollow core or wide slabs, but often requires a

deeper floor zone. The Double-Tee system is the only system which offers a solution for

spans over16m


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Solid wide slab floors

Solid Wide slab is also referred to as plate flooring which is

generally used in residential developments. Wide slab flooring

contains an internal mesh/strand which facilitates notching.


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Types of wide slab

In situ screeded slabs: are 2400mm wide and in depths of 65mm to

200mm deep with spans of up to 7.5m. These slabs are generally for

upper floors and have smooth self-finishing soffits.Generally a

75mm structural screed is required..

Pre-screeded slabs: are 150mm to 200mm deep and are delivered

self-finished for ground floor applications. These slabs are

particularly suited to poor ground conditions or where cut and fill is

required. Maximum spans of 7.5m can be achieved.


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Composite beam and plank floor unit

This is a tertiary system in which a composite floor is produced by primarybeams (R.C., precast, steel etc.) support long span beams, reinforced or pre

stressed depending on structural requirements and manufacturing capability

as shown below:


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Advantages of precast floor systems :

(1)Reduction in Frame Size

Precast floors can be designed to act compositely with the structure of the

building to reduce frame sizes: e.g. main support beams supporting solid slab

and screed can be designed as T-beams.

(2)Progressive Collapse

Pre stressed composite floors can be tied-in to the main structure and are

therefore particularly suited to buildings where progressive collapse is a

consideration.

(3)Diaphragm Action

Precast floor slabs together with the structural screed provide a structural deck

with full diaphragm action where required in multi-storey buildings.


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Flooring arrangements

A floor slab comprises of a large number of individual units, each

designed to serve for specified loads, moments etc., or it may

comprise a complete slab field where the loads are shared between

the precast units according to the structural response of each

component.

Floor unit: a discrete element designed in isolation of other units.

Floor slab: several floor units structurally tied together to form a

floor area.

Floor field: a floor slab where each floor unit is designed as part of

the whole floor.


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Design of Pre cast concrete

Structural design of precast concrete is performed in stageswhen architectural design and other general requirement are

considered.

Following aspects should be considered:

(1)Precast Concrete Components

(2)Joints.

(3)Stability.

(4)Structural Integrity.


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Precast concrete components

Columns

The following is relevant to column design:

(1) Columns are designed for combined axial compression and

bending.

(2) Column bending moments at beam level are determined from the

eccentric loading at the connection.

(3) In the case of slender unbraced columns, the additional moments

due to slenderness effects are added to the total design moment at

each floor level.


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Beams

Reinforced or pre stressed concrete beams in a precast concrete frame aredesigned for the specified loadings and support conditions. The beam may

be composite or non-composite. Composite beams act with the floor,

column and screed to form a monolithic structure. Beams are typically

designed as simply supported with a characteristic concrete strength of

between40N/mm2 and 50N/mm2.

Figure: Inverted tee-beam


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Joints and connection:

The design and construction of joints is the most important aspect of precast

construction. Joints provides strength/robustness and transmit forces

between the structural components. There are a number of different methods

for connecting units including tie-bars, steel billets or plates, in conjunction

with the use of high strength grouts.

Column to Foundation Fixing Detail

A number of factors determine which type of column to foundation fixing

detail should be used. These factors include:

(1)Design requirements, i.e. fixed or pinned base.

(2)Ground conditions.

(3)Presence of services around the column base, e.g. drainage, gas mains, etc.


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Methods of fixing precast columns to the in situ

foundation

There are three main methods of fixing precast columns to

the in situ foundation:

(1). Projecting Starter-Bars.

(2). Bolt or Base plate Connections.

(3). In situ Pocket Foundation.


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Projecting Starter-Bars

The number and size of projecting starter-bars is determined by the project

engineer to suit design requirements. . This method of fixing a precast

column to an in situ base is extensively used in medium to high rise

buildings where the complete structure is designed as a braced frame and

thus a pinned connection is required at the base.

Figure: projecting starter-bars


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Bolted or Base plate Connections

Mild steel bars can be welded to the base plate and cast onto the foot of the column

during the curing process. The column is then fixed to the in situ foundation using

cast-in holding down bolts to form a pinned base connection.


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In situ Pocket Foundation

The in situ Pocket Foundation will provide a fixed base connection to theprecast column, which is particularly useful in low rise precast industrial

units.


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CASE STUDY

The Hub, 5 Piccadilly Place (Manchester)


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Continued

The Hub is a 10 storey U shaped development, wrapped around a hard

landscaped plaza, of 167 new apartments in the heart of a Manchester mixed

use development area, by Argent Group in Manchester City centre.

The building structure is formed entirely from precast elements, which has

led to each floor level being erected and clad within a 4 week period .

A wide range of precast sections: Architectural sandwich cladding panels,

Pre cast stairs, Twin wall, Solid wall, Hollow-core planks, Solid wall lift

shafts are the precast elements used for the construction.


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Alameda Mid-Corridor Trench (LOS ANGELES)


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Continued

Alameda Mid-Corridor Trench is a 10-mile-long freight rail expressway

built in a 33-foot-deep, below-grade trench lying adjacent to Alameda

Street. The trench is a midway section of a 20-mile-long rail corridor that

links the ports of Los Angeles and Long Beach with other rail systems.

The top of the trench is permanently braced with 1,500 53-foot-longs, 39-

inch-deep precast, pre stressed concrete struts. Key bridge components

across the trench consist of double tees, inverted tee beams, box girders and

roadway slabs. The use of precast concrete components allowed the pieces

to be cast as trenching began rather than having to wait until a length was

dug out to begin pouring concrete. This minimized the amount of time

streets had to be closed and detours erected around the construction


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CONCLUSION

Pre-cast structures have a great potential to respond to new market

demands.

Precast concrete has considerable advantages as a construction

material, including inherent fireproofing, sound and durabilitycharacteristics.

Precast concrete increases speed of construction, which gives earlier

return on investment, freeing up the project critical path and allowingearlier completion and it was found that precast structure takes up to

20% less time to construct than a similar cast in situ structure


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Precast Concrete Structures 1

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