Design Considerations for Precast Structure

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Precast concrete construction offers several key features such as dimensional accuracy, better finishes, and faster erection, which is absent when other concrete construction methods are considered. 

The design of precast structures involves:

1. The design of precast members for all the possible loads during various stages, storage to jointing, and possible loads in the building lifecycle.
2. The design of joints/connections for all possible loads in the building lifecycle.

The following are the important design considerations that are used while designing precast structures.

1. The precast structure should be analyzed as a monolithic structure, and the joints in them shall be designed to take the forces of an equivalent discrete system. 
2. The resistance to horizontal loading shall be provided by placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise.
3. Rotational stiffness is not taken into account if any of the floor-wall joints in the case of precast-bearing wall buildings. 
4. The individual components shall be designed considering the appropriate end conditions and loads at various stages of construction. 
5. The structure's components must be designed for loads in line with the requirements.
6. In addition, members shall be designed for erection, handling, and impact loads that might be expected during handling and erection.
7. Adequate buttressing of external wall panels is essential since these elements are not fully restrained on both sides by floor panels. Adequate design precautions must be taken into account while designing. Multiple studies have shown that the external wall panel connections are the weakest points of a precast panel building.
8. It is important to provide restraint to all load-bearing elements at the corners of the building. These elements and the external ends of cross-wall units should be stiffened, either by introducing columns as connecting units or by jointing them to non-structural wall units, which may support the load in an emergency. Jointing of these units should be designed considering the need for support in an emergency.
9. The potential of gas or other explosions, which might destroy major structural parts and lead to the structure's gradual collapse, must be considered in the design of prefabricated construction. As a result, the potential of progressive collapse must be considered, in which the failure or displacement of one structural element promotes the failure or displacement of another, resulting in the partial or entire collapse of the structure.
10. A provision in the design to reduce the probability of progressive collapse is essential in buildings with more than  six storeys and is of relatively higher priority than for buildings with a lower height.
11. It is necessary to ensure that any local damage to a structure does not spread to other parts of the structure remote from the point of mishap and that the overall stability is not impaired. Still, it may not be necessary to stiffen all parts of the structure against local damage or collapse near a mishap unless the design briefs specifically require this to be done.
12. Additional, protection may be required regarding damage from vehicles; further, it is necessary to consider the effect of damage to or displacement of a load-bearing member by an uncontrolled vehicle. It is strongly recommended that concrete kerbs or similar methods adequately protect important structural members.
13. In all aspects of erection that affect the structural design, the designer must maintain a close liaison with the builder/contractor regarding the erection procedures to be followed.
14. Failures that have occurred during construction appear to be of two types:
    1. Pack-of-Cards Collapse: In this type of failure, the absence of restraining elements, such as partitions, cladding, or shear walls, means that the structure is not stable during the construction/period.
    2. Situational Collapse: In this type of failure, one element falls during erection and lands on an element below. The connections of the lower element then collapses under the load of falling element, both static and dynamic, and a chain reaction of further collapse is initiated.
15. Preventive measures for the above type of failures are:
    1. For pack of cards collapse: The overall stability of a building shall be considered in all its erection stages as well as in its completed state. All joints that may be required to resist moments and shears during the erection- stage only shall be designed with these in mind. Temporary works required to provide stability during construction shall be designed carefully.
    2. For Situational Collapse: To guard against the situational collapse,i.e., the dropping of a unit during erection, particular attention shall be given to the details of all pre-formed units and their seating to ensure that they are sufficiently robust to withstand the maximum stresses that can arise from site conditions. Precast concrete construction generally shall be capable of withstanding the impact forces that can arise from bad workmanship on site.

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