Site-Specific Ground Motion Studies for a Deep Soil Site Near Ahmedabad, Gujarat

Site-Specific Ground Motion Studies for a Deep Soil Site Near Ahmedabad, Gujarat

A. Boominathan (Indian Institute of Technology Madras, India), Krishna Kumar (University of Cambridge, UK) and R. Vijaya (Indian Institute of Technology Madras, India)
DOI: 10.4018/978-1-5225-6948-0.ch002

Abstract

Design ground motions are usually developed by one of two approaches: by performing site-specific analyses or from provisions of building codes. Although contemporary codes consider the site effects to an extent, they provide more conservative estimates. Hence, site-specific analysis, which involves both the seismic hazard analysis and ground response analysis, is a preferred approach to obtain design ground motions. This chapter presents a site-specific analysis for a site near Ahmedabad, Gujarat. The seismic hazard analysis was carried out by DSHA approach. The site is predominantly characterized by deep stiff sandy clay deposits. Extensive shear wave velocity measurement is used for site classification and ground response analysis. The ground response analysis was carried out by using two approaches: the equivalent linear approach using SHAKE2000 and the non-linear approach using FLAC2D. The deep-stiff-soil site is found to amplify the ground motion. The response from nonlinear analysis is found to be considerably higher than those obtained from the equivalent linear approach.
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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, the 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 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 (1997) adopts two basic approaches: a static approach, which considers the effects of ground motions represented by static lateral forces and a 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 when developing the ground motion parameters. Hence, it is necessary to perform site specific ground response analysis for vulnerable sites and critical structures.

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