7.1 Design Considerations
7.1.1 Precast structures could be analyzed either as an emulative systems or as a jointed system. However, emulative analysis is typically preferred where the structure is detailed such that the overall behaviour of the building in its service life will be similar to a RCC building constructed in-situ. In emulative approach, the precast structure is analyzed as a monolithic one and the joints in them designed to take the forces of an equivalent discrete system. Resistance to horizontal loading shall be provided by having appropriate moment and shear resisting joints or placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise. No account is to be taken of rotational stiffness, if any, of the floor-wall joint in case of precast bearing wall buildings. The individual components shall be designed, taking into consideration the appropriate end conditions and loads at various stages of construction. The components of the structure shall be designed for loads and reactions in accordance with Part 6 'Structural Design, Section 1 Loads, Forces and Effects' of the Code. In addition, members shall be designed for handling, erection and impact loads that might be expected during handling and erection.
NOTE — Rotational stiffness can be accounted for long/short term deflection calculations provided that the approach is mutually agreed by the design team and approved by the Authority.
7.1.2 In some conventional forms of construction, experience has shown that structures are capable of safely sustaining abnormal conditions of loading and remaining stable after the removal of primary structural members. It has been shown that some forms of building structure and particularly some industrialized large
panel systems have little reserve strength to resist forces not specifically catered for in the design. In light of this, therefore, recommendations made in 7.1.3 to 7.1.10 should be kept in mind for ensuring stability of such structure.
7.1.3 Adequate buttressing of external wall panels is important since these elements are not fully restrained on both sides by floor panels. Adequate design precautions may be taken by the designer. External wall panel connections are likely to be the weakest points of a precast panel building, if not designed properly.
7.1.4 It is equally 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 in emergency may support the load. Jointing of these units should be done bearing in mind the need for load support in an emergency.
7.1.5 In prefabricated construction, the possibility of gas or other explosions which can remove primary structural elements leading to progressive collapse of the structure shall be taken into account. It is, therefore, necessary to consider the possibility of progressive collapse in which the failure or displacement of one element of a structure causes the failure or displacement of another element and results in the partial or total collapse of the building.
7.1.6 Provision in the design to reduce the probability of progressive collapse is essential in buildings of over six storeys and is of relatively higher priority than for buildings of lower height.
7.1.7 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, but it may not be necessary to stiffen all parts of the structure against local damage or collapse in the immediate vicinity of a mishap, unless the design briefs specifically requires this to be done. The requirements as specified at 7.2 shall be followed to prevent progressive collapse of the structure.
7.1.8 Additional protection may be required in respect of 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 important structural members are adequately protected by concrete kerbs or similar method.
In areas with possibility of passenger vehicular impact, the precast components should be designed for a horizontal load of 27 kN, in any direction, acting at heights between 450 mm and 700 mm on an area not
larger than 300 mm × 300 mm to produce maximum load effect. This load is not required to act concurrently with any handrail or guard rail system loads.
7.1.9 In all aspects of erection that affect structural design, it is essential that the designer should maintain a close liaison with the builder/contractor regarding the erection procedures to be followed.
7.1.10 Failures that have occurred during construction appear to be of two types. The first of these is the pack-of-cards type of collapse in which the absence of restraining elements, such as partitions, cladding or shear walls, means that the structure is not stable during the construction period. The second is the situation in which one element falls during erection and lands on an element below. The connections of the lower element then give way under the loading, both static and dynamic, and a chain reaction of further collapse is set up.
7.1.10.1 A precaution against the first form of failure is that 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.
7.1.10.2 To guard against the second form of failure, that is, the dropping of a unit during erection, particular attention shall be given to the details of all pre-formed units and their 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.
7.1.11 The design shall take care of requirements owing to erection given at 10.11.
7.1.12 In case of buildings whose floors are capable of providing rigid horizontal diaphragm action, the total shear in any horizontal plane shall be distributed to the various vertical elements of lateral forces resisting system, assuming the floors to be infinitely rigid in the horizontal plane.
In case of building whose floor diaphragms cannot be treated as infinitely rigid in their own plane, the lateral shear at each floor shall be distributed to the vertical elements resisting the lateral forces, considering the in-plane flexibility of the diaphragms.
1 A floor diaphragm shall be considered to be flexible, if it deforms such that the maximum lateral displacement measured from the chord of the deformed shape at any point of the diaphragm is more than 1.5 times the average displacement of the entire diaphragm.
PART 6 STRUCTURAL DESIGN — SECTION 7 PREFABRICATION, SYSTEMS BUILDING AND MIXED/COMPOSITE CONSTRUCTION: 7A PREFABRICATED CONCRETE
2 Reinforced concrete monolithic slab-beam floors or those consisting of prefabricated/precast elements with topping reinforced screed can be taken as a rigid diaphragm.
7.1.12.1 Floor systems play a key role in the lateral resistance of building structures by providing diaphragm action. Diaphragm action serves to,
a) transfer lateral loads at each level to the lateral load-resisting elements (walls, frames); and
b) unite individual lateral load-resisting elements into a single lateral load-resisting system.
These actions are crucial in allowing the lateral (load-resisting) system to perform as intended. Diaphragm action occurs by virtue of the in-plane stiffness inherent in the floor system. Accordingly, significant in-plane forces may develop in the diaphragm during lateral loading. In this regard, the panelled nature of floor systems in precast concrete construction can present challenges in the seismic design of diaphragms. These design challenges may pertain to providing adequate strength, stiffness and in cases, ductility in the diaphragm and diaphragm connections.
7.1.12.2 Standard analysis methods for diaphragm design
The magnitude of the inertia force at each floor level is estimated using a capacity design approach or a diaphragm design force distribution based on equivalent lateral loads. A key design step is to transform the diaphragm design forces into internal actions. For regular floor plans, a 'horizontal plate girder' analogy is typically used in design. In this approach, diaphragm design forces are typically applied at each level as a distributed in-plane load along the length of the diaphragm. The resulting girder internal forces are used to determine the amount of flexural reinforcement (chord steel), shear (web) reinforcement, and lateral system reaction load path reinforcement (collector steel). An irregular mass distribution in plan may require the design forces to take on a smeared load configuration as the mass distribution with respect to diaphragm depth has relevance in determining the proper web reinforcement.
For more complex diaphragm configurations, a strut-and-tie method can be used. However, there is compelling evidence that the strut-and-tie method accurately predicts the behaviour of floor diaphragms in general and may become the preferred method of diaphragm analysis. See Fig. 1 and Fig. 2.