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 categories: Depending on the load-bearing structure, precast systems can be divided into the following 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. 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
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
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 Structural Engineering & Geospatial Consultants 1.1 Precast Slabs 1.2 Precast Beam & Girders Hollow core slabs
1.3 Precast Columns Precast columns Inverted Tee beams supported on precast columns 1.4 Precast Walls
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.
Structural Engineering & Geospatial Consultants A single story warehouse consisting of double tees supported by insulated sandwich wall panels.
Structural Engineering & Geospatial Consultants 3. TYPICAL CONNECTION OF PRECAST CONCRETE ELEMENTS COLUMN TO COLUMN CONNECTION
Structural Engineering & Geospatial Consultants BEAM TO COLUMN CONNECTION
Structural Engineering & Geospatial Consultants SLAB TO BEAM CONNECTION
Structural Engineering & Geospatial Consultants WALLPANEL CONNECTED TO INSITU CONCRETE
Structural Engineering & Geospatial Consultants
Structural Engineering & Geospatial Consultants CONNECTION BETWEEN SLABS
Structural Engineering & Geospatial Consultants CORNER CONNECTIONS OF WALL PANELS
Structural Engineering & Geospatial Consultants 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