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PRECAST CONCRETE STRUCTURES
1. INTRODUCTION
The concept of precast (also known as “prefabricated”) construction includes
those buildings, where the majority of structural components are standardized and
produced in plants in a location away from the building, and then transported to the site
for assembly. These components are manufactured by industrial methods based on mass
production in order to build a large number of buildings in a short time at low cost.
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The 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..
This type of construction requires a restructuring of entire conventional construction
process to enable interaction between design phase and production planning in order to
improve and speed up construction.
1.1 TYPES OF PRECAST SYSTEMS
Depending on the load-bearing structure, precast systems can be divided into the following
categories:

Large-panel systems

Frame systems

Slab-column systems with walls

Mixed systems
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1. 1. 1
LARGE PANEL SYSTEMS
The designation “large-panel system” refers to multistory structures composed of
large wall and floor concrete panels connected in the vertical and horizontal directions so
that the wall panels enclose appropriate spaces for the rooms within a building. These
panels form a box-like structure. Both vertical and horizontal panels resist gravity load.
Wall panels are usually one story high. Horizontal floor and roof panels span either as
one-way or two-way slabs. When properly joined together, these horizontal elements
act as diaphragms that transfer the lateral loads to the walls.
A large-panel concrete building under construction
Depending on wall layout , there are three basic configurations of large-panel buildings:
 Cross-wall systems
 Longitudinal wall systems
 Two-way systems
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1. 1. 2
FRAME SYSTEMS
Precast frames can be constructed using either linear elements or spatial beamcolumn 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; however, linear elements are generally preferred because of the
difficulties associated with forming, handling, and erecting spatial elements. The use of
linear elements generally means placing the connecting faces at the beam-column
junctions. The beams can be seated on corbels at the columns, for ease of construction
and to aid the shear transfer from the beam to the column. The beam-column joints
accomplished in this way are hinged. However, rigid beam-column connections are used in
some cases, when the continuity of longitudinal reinforcement through the beam-column
joint needs to be ensured. The components of a precast reinforced concrete frame are
shown in Figure
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1. 1. 3
SLAB-COLUMN SYSTEMS WITH SHEAR WALLS
These systems rely on shear walls to sustain lateral load effects, whereas the
slab-column
structure resists mainly gravity loads. There are two main systems in this category:
• Lift-slab system with walls
• Prestressed slab-column system
In the Lift –slab system, the load-bearing structure consists of precast
reinforced concrete columns and slabs,. Precast columns are usually two stories high. All
precast structural elements are assembled by means of special joints. Reinforced
concrete slabs are poured on the ground in forms, one on top of the other. Precast
concrete floor slabs are lifted from the ground up to the final height by lifting cranes.
The slab panels are lifted to the top of the column and then moved downwards to the
final position. Temporary supports are used to keep the slabs in the position until the
connection with the columns has been achieved.
A lift-slab building
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The prestressed slab-column system uses horizontal prestressing in two
orthogonal directions to achieve continuity. The precast concrete column elements are 1
to 3 stories high. The reinforced concrete floor slabs fit the clear span between
columns. After erecting the slabs and columns of a story, the columns and floor slabs are
prestressed by means of prestressing tendons that pass through ducts in the columns at
the floor level and along the gaps left between adjacent slabs. After prestressing, the
gaps between the slabs are filled with in situ concrete and the tendons then become
bonded with the spans. Seismic loads are resisted mainly by the shear walls (precast or
cast-in-place) positioned between the columns at appropriate locations.
Post-tensioned slab-column connection
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2. PRECAST CONCRETE STRUCTURAL ELEMENTS
1.1 Precast Slabs
Hollow core slabs
1.2 Precast Beam & Girders
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1.3 Precast Columns
Precast columns
1.4 Precast Walls
Inverted Tee beams supported on precast columns
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1.5 Other Elements
2.
Precast concrete Stairs
Uniquely shaped structural elements for a sports stadium
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DESIGN CONCEPTS FOR PRECAST CONCRETE BUILDINGS
The design concept of the precast buildings is based on the buildability,
economy and standardization of precast components.
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In design of precast members and connections, all loading and restraint conditions from
casting to end use of the structure should be considered. The stresses developed in precast
elements during the period from casting to final connection may be more critical than the service
load stresses. Special attention should be given to the methods of stripping, storing,
transporting, and erecting precast elements.
When precast members are incorporated into a structural system, the forces and
deformations occurring in and adjacent to connections (in adjoining members and in the entire
structure) should be considered. The structural behavior of precast elements may differ
substantially from that of similar members that are monolithically cast in place. Design of
connections to transmit forces due to shrinkage, creep, temperature change, elastic
deformation, wind forces, and earthquake forces require special attention. Details of such
connections are especially important to insure adequate performance of precast structures.
Precast members and connections should be designed to meet tolerance requirements. The
behavior of precast members and connections is sensitive to tolerances. Design should provide
for the effects of adverse ccombinations of fabrication and erection tolerances. Tolerance
requirements should be listed on contract documents, and may be specified by reference to
accepted standards. Tolerances that deviate from accepted standards should be so indicated.
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All details of reinforcement, connections, bearing elements, inserts, anchors, concrete
cover, openings and lifting devices, and specified strength of concrete at critical stages of
fabrication and construction, should be shown on either the contract documents prepared by the
architect/engineer ofrecord or on the shop drawings furnished by the contractor. Whether this
information is to be shown on the contract documents or shop drawings depends on the provisions
of the contract documents. The shop drawings should show, as a minimum, all details of the
precast concrete members and embedded items. The contract documents may specify that
portions of connections exterior to the member are also to be shown on the shop drawings. The
contract documents may also require the contractor to provide designs for the members and/or
connections.
The contract documents should show the loads to be considered in design of the precast
concrete elements of the structure, and they should indicate any special requirements or
functions (for example: seismic loads, allowance for movements, etc.) that should be considered
in design assigned to the contractor. In this case, the shop drawings should include complete
details of the connections involved.
Precast concrete structure consisting of solid wall panels and hollow core slabs.
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A single story warehouse consisting of double tees supported by insulated sandwich wall panels.
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3. TYPICAL CONNECTION OF PRECAST CONCRETE ELEMENTS
COLUMN TO COLUMN CONNECTION
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BEAM TO COLUMN CONNECTION
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SLAB TO BEAM CONNECTION
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WALLPANEL CONNECTED TO INSITU CONCRETE
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CONNECTION BETWEEN SLABS
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CORNER CONNECTIONS OF WALL PANELS
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CONNECTION OF WALL PANELS TO COLUMNS
4. PRECAST CONCRETE CONSTRUCTION AND SEISMIC DESIGN
There is a general concern regarding the seismic performance of precast construction.
It is noticed that large panel construction performs better than frame system.
However, in areas of high seismic risk, structures must be designed to respond safely to
the dynamic forces imparted into the structure. Innovations in joint design are improving the
connection systems in precast concrete structures and making them increasingly suitable for use in such
areas.
Information, pictures, photographs etc of the paper have been taken from

www.bca.gov.sg/publications/BuildabilitySeries/others/bsl_cp3.pdf

www.cpci.ca

Precast industrial buildings detailing manual by National Precast Concrete Association Australia


www.world-housing.net/uploads/precast__concrete.pdf
Precast construction by Svetlana Brzev, British Columbia Institute of Technology, Canada ,Teresa Guevara-Perez,
Architect, Venezuela

ACI 550R-96, Design recommendations of precast Concrete Structures.

Precast.ppt by Fundamental of Building construction , Materials & Methods, 5th Edition