The Phenomenon of Prefabrication

PRE-CASTING IN CONTEMPORARY CONSTRUCTIONS

BACKGROUND
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
This type of construction requires a restructuring of the entire conventional construction  process to enable interaction between the design phase and production planning in order to improve and speed up the construction. One of the key premises for achieving that objective is to design buildings with a regular configuration in plan and elevation.
 Many countries used various precast building systems during the second half of the 20th century to provide low-income housing for the growing urban population.
In general, precast building systems are more economical when compared to conventional multifamily residential construction (apartment buildings) in many countries.  

 CATEGORIES OF PRECAST BUILDING 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
 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 the wall layout, there are three basic configurations of large-panel buildings:

• Cross-wall system.-  The main walls that resist gravity and lateral loads are placed in the short direction of the building.

 • Longitudinal-wall system.-The walls resisting gravity and lateral loads are placed in the longitudinal direction; usually, there is only one longitudinal wall

 • Two-way system. The walls are placed in both directions .

 Thickness of wall panels ranges from 120 mm for interior walls  to 300 mm for exterior walls . Floor panel thickness is 60 mm . Wall panel length is equal to the room length, typically on the order of 2.7 m to 3.6 m.  In some cases, there are no exterior wall panels and the façade walls are made of lightweight concrete  A typical interior wall panel is shown in Figure.

Panel connections represent the key structural components in these systems. Based on their location within a building, these connections can be classified into vertical and horizontal joints. Vertical joints connect the vertical faces of adjoining wall panels and primarily resist vertical seismic shear forces. Horizontal joints connect the horizontal faces of the adjoining wall and floor panels and resist both gravity and seismic loads. 

Depending on the construction method, these joints can be classified as wet and dry. Wet joints are constructed with cast-in-place concrete poured between the precast panels. To ensure structural continuity, protruding reinforcing bars from the panels (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed. Dry joints are constructed by bolting or welding together steel plates or other steel inserts cast into the ends of the precast panels for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points.

Lateral stability of a large-panel building system is provided by the columns tied to the wall panels . Boundary elements are used instead of the columns as “stiffening” elements at the exterior. The unity of wall panels is achieved by means of splice bars welded to the transverse reinforcement of adjacent panels in the vertical joints. Longitudinal dowel bars placed in vertical and horizontal joints provide an increase in bearing area for the transfer of tension across the connections. 

Frame Systems

Precast frames can be constructed using either linear elements or spatial beam-column subassemblages. Precast beam-column subassemblages have the advantage that the connecting faces between the subassemblages 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.. 

Precast reinforced concrete frame with cruciform and linear beam elements is an example of a frame system with precast beam-column subassemblages. The load-bearing structure consists of a precast reinforced concrete space frame and precast floor slabs. The space frame is constructed using two main modular elements: a cruciform element and a linear beam element. The cruciform element consists of the transverse frame joint with half of the adjacent beam and column lengths. The longitudinal frames are constructed by installing the precast beam elements in between the transverse frame joints. The precast elements are joined by welding the projected reinforcement bars (dowels) and casting the concrete in place. 

Joints between the cruciform elements are located at the mid-span of beams and columns, whereas the longitudinal precast beam-column connections are located close to the columns. . 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

 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.

In the connections, the steel bars (dowels) that project from the edges of the slabs are welded to the dowels of the adjacent components and transverse reinforcement bars are installed in place. The connections are then filled with concrete that is poured at the site.

Most buildings of this type have some kind of lateral load-resisting elements, mainly consisting of cast-in-place or precast shear walls, etc. In case lateral load-resisting elements (shear walls, etc.) are not present, the lateral load path depends on the ability of the slab-column connections to transfer bending moments. When the connections have been poorly constructed, this is not possible, and the lateral load path may be incomplete.

Another type of precast system is a slab-column system that 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. 

EARTHQUAKE PERFORMANCE

There is a general concern among the earthquake engineering community regarding the seismic performance of precast construction.  When it comes to earthquake performance, the fact is that “bad news” is more widely publicized than “good news.” However, few engineers are aware of the good seismic performance (no damage) of several large-panel buildings under construction at the same site.

The precast prestressed slab-column system has undergone extensive laboratory testing that predicted excellent resistance under simulated seismic loading. These building have been subjected to several moderate earthquakes without experiencing significant damage.

Due to their large wall density and box-like structure, large-panel buildings are very stiff and are characterized with a rather small fundamental period. For example, a 9-story building in 

India has a fundamental period of 0.35 to 0.4 sec.

 In general, large-panel buildings performed very well in the past earthquakes. It should be noted, however, that large-panel buildings in the area affected by the Bhuj earthquakes were not designed with seismic provisions. Most such buildings performed well in the first earthquake (M 7.0), but more damage was observed in the second earthquake that occurred the same year (M 7.3), as some buildings had been already weakened by the first earthquake .

Destruction of a Framed structure by Bhuj Earthquake

SEISMIC-STRENGTHENING TECHNOLOGIES

According to recent reports, no major efforts have been reported regarding seismic strengthening of precast concrete buildings. However, seismic strengthening of precast frame buildings was done in New Delhi. The techniques used include the installation of steel straps at the column locations  and reinforcing the joints with steel plates to provide additional lateral confinement of the columns.

 Seismic strengthening of precast columns with steel straps 

ENDNOTES

Definition of “Mass Production” in “Industrial Engineering and Production Management” R.k sahni, , 15th Edition, Vol. 21, p. 204, 1989. 

2 UNIDO, 1983. Design and Construction of Prefabricated Reinforced Concrete Frame and Shear-Wall Buildings. Building Construction Under Seismic Conditions in the Gujarat Region. Volume 2.