Precast Concrete Structures 1
Transcript of 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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