Effects of Earthquake on Structures

During an earthquake, ground motions occur in a random fashion, both horizontally and vertically, in all directions, radiating from the epicentre. The ground accelerations cause structures to vibrate and induce inertial forces on them. Hence, structures to be constructed in earthquake-prone areas need to be suitably designed and detailed to ensure stability, strength and serviceability.

The magnitude of the forces induced in a structure due to a given ground acceleration will depend, amongst other things, on the mass of the structure, the material and type of construction, the damping, the ductility and energy dissipation. Ductility of a material is the ability of a structure or member to undergo inelastic deformations beyond the initial yield deformation with no decrease in the load resistance.

Thus by enhancing ductility and energy dissipation capacity in structure, the induced seismic forces can be reduced and also the probability of collapse reduced (Pillai and Menon, 2012). Further, it is desirable to avoid discontinuities in mass or stiffness in plan or elevation. Torsional effects should particularly be accounted for in buildings with asymmetry in plan or elevation.

29.6.5 General Requirement

Bureau of Indian Standard have specified the minimum design requirements for earthquake–resistance design in IS codes: 1893 (Part I): 2002, IS: 4326: 1993 and IS: 13920: 1993. These requirements have been stipulated after considering the characteristics and probability of occurrence of earthquakes, the characteristics of the structure and the foundation and the amount of damage that is considered tolerable. Codal provisions from other countries are also available.

The criteria adopted by codes for fixing the level of the design seismic loading are generally as follows:

1. Structures should be able to resist minor earthquakes without damages.
2. Structures should be able to resist moderate earthquakes without significant structural damage, and
3. Structures should be able to resist major earthquakes without collapse, but with some structural and non-structural damages.