5) precast

Structural Engineering & Geospatial Consultants

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.


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


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.

Depending on wall layout , there are three basic configurations of large-panel buildings:

Cross-wall systems

Longitudinal wall systems

Two-way systems

A large-panel concrete building under construction


1. 1. 2 FRAME SYSTEMS

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; 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


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


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


2. PRECAST CONCRETE STRUCTURAL ELEMENTS

1.1 Precast Slabs

1.2 Precast Beam & Girders

Hollow core slabs


1.3 Precast Columns

1.4 Precast Walls

Inverted Tee beams supported on precast columnsPrecast columns


1.5 Other Elements

2.

Precast concrete Stairs Uniquely shaped structural elements for a sports stadium


DESIGN CONCEPTS FOR PRECAST CONCRETE BUILDINGS

The design concept of the precast buildings is based on the buildability,

economy and standardization of precast components.


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.


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.


A single story warehouse consisting of double tees supported by insulated sandwich wall panels.


3. TYPICAL CONNECTION OF PRECAST CONCRETE ELEMENTS

COLUMN TO COLUMN CONNECTION


BEAM TO COLUMN CONNECTION


SLAB TO BEAM CONNECTION


WALLPANEL CONNECTED TO INSITU CONCRETE


CONNECTION BETWEEN SLABS


CORNER CONNECTIONS OF WALL PANELS


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

5) precast

Documents

structural engineering

hollow core slabs

precast concrete structures

shop drawings

wall panels

precast elements

linear elements

contract documents