Seismic analysis and design verification

Seismic analysis and design verification

Usually, the structures are designed for gravity loads and checked for earthquake loads. This ensures

•       elastic response under moderate earthquake;

•       collapse is precluded under a severe earthquake.

As discussed already, the important factors that influence earthquake-resistant design are the geographical location of the structure, the site soil and foundation conditions, the importance of the structure, the dynamic characteristics of the structure such as natural periods and properties of the structure such as strength, stiffness, ductility and energy dissipation capacity. These factors are considered directly or indirectly in all the methods of analysis.

1Elastic response analysis

Usually elastic response analysis is performed as a part of the usual design procedure. In the seismic coefficient method, also known as equivalent static load method, a certain proportion of (gravity + % live load) is assumed to act as a static horizontal load and the structure is analysed for it. For simple and regular structures, the seismic coefficient method is normally used. Structures as multi-storey buildings, overhead water tanks and bridge piers are usually designed by the response spectrum method while for important structures such as nuclear reactors, time history response analysis is usually adopted.

2Response spectrum method

Although the response spectrum method requires more calculations than the seismic coefficient method, it has the advantage that it can account for irregularities as well as higher mode contributions and gives more accurate results. Therefore, this is the most widely used method in seismic analysis.

3Time history method

For important structures both linear and nonlinear response can be obtained by carrying out a detailed time history analysis for one or more design accelerograms These design accelerograms may be either natural accelerograms recorded at site or at similar sites or they can be artificial accelerograms generated such a way as to be compatible with the design response spectrum. A variety of numerical time stepping methods  are available for calculation response of time history.