A Site Specific Study on Evaluation of Design Ground Motion Parameters

Significance Of Site-Specific Analysis

The characteristics of the design ground motion at a particular site are influenced by the location of the site relative to potential seismic sources, the seismicity of those sources, and nature of rupture at the source, travel path effects, and the importance of the structure or facility for which the ground motion is to be used. Design ground motions are usually developed in one of two approaches: from site-specific analyses or from the provisions of building codes and standards. Although contemporary codes do consider site effects, they usually do so by lumping groups of similar soil profiles together so that their provisions apply to broad ranges of soil conditions within which the local conditions of a particular site are expected to fall. Because of this, the design ground motions developed from code provisions are usually more conservative (i.e. correspond to stronger levels of shaking) than those developed from site-specific analyses. The UBC adopts two basic approaches: static approach which considers the effects of ground motions represented by static lateral forces and dynamic approach in which ground motion is characterized by a design response spectrum. These approaches are based on developed hazard maps and provide zone factors that reflect to an extent the local site conditions. However these maps do not consider local variations at the site in developing the ground motion parameters, deeming it necessary to perform site specific ground response analysis for vulnerable sites and critical structures.

Site specific design ground motions reflect the detailed effects of the subsurface conditions at the sites of interest. The process involves development of site-specific ground motions by seismic hazard analysis and ground response analysis. The seismic hazard analysis will produce a set of ground motion parameters that may or may not correspond to the conditions at the site of interest. Seismic hazard analyses are based on a set of attenuation equations which usually correspond to a fairly narrow range of subsurface conditions. If the site of interest is located on a similar profile the parameters developed shall be taken as ground motion parameter. If not, the parameters developed should be suitably modified for the effects of local site conditions. The parameters can be modified by analytical or empirical approach. The former involves deconvolution and conventional ground response analysis (Kramer, 1996).