Site-Specific Seismic Analyses Procedures for Framed Buildings for Scenario Earthquakes Including the Effect of Depth of Soil Stratum

Abstract

The importance of the effect of sediments above bedrock in modifying the strong ground motion has been long recognized (Boore, 2004; Boore & Joyner, 1997; Idriss & Seed, 1970; Seed & Idriss, 1969; Lam et al., 2001; Govindarajulu et al., 2004; Tezcan et al., 2002; Bakir et al., 2005; Kamatchi et al., 2007) in literature. The nature of soil that changes the amplitude and frequency content has a major influence on damaging effects of earthquake. To account for these effects, most of the seismic codes, for example the Indian code (IS 1893 (Part 1) 2002) has defined response spectra for three types of soil viz., hard soil, medium soil and soft soil. As an improvement over this approach, amplification factors based on empirical and theoretical data (Borcherdt, 1994) have been introduced in International Building Codes (IBC, 2009; ASCE 7 2005) for site classes A to E for the short period range and long period range based on the average shear wave velocity of top 30 m soil stratum. For site class F (soft soil) it has been recommended that site-specific analysis need to be carried out. However, Sun et al. (2005) showed that the site coefficients specified in IBC 2000 (IBC 2000) are not valid for Korean Peninsula due to the large difference in the depth of bedrock and the soil stiffness profile. Further, building codes are highly simplified tools and do not adequately represent any single earthquake event from a probable source for the site under consideration. Recently, it is being suggested (Heuze et al., 2004 ) that in addition to use of seismic code provisions, site-specific analysis which includes generation of strong ground motion at bedrock level and propagating it through soil layers (Heuze et al., 2004; Mammo 2005; Balendra et al., 2002) and arriving at the design ground motions and response spectra at surface should also be carried out. In this chapter the procedure to carry out site-specific seismic analysis of framed buildings is illustrated with examples for Delhi city. Rock outcrop motions are generated for the scenario earthquakes of magnitude, Mw = 7.5, Mw = 8.0 and Mw = 8.5. Three actual soil sites of Delhi have been modeled and the free field surface motions and the response spectra are obtained. It has been observed that the PGA amplifications and the response spectra of the three sites are quite different for the earthquakes considered. The same has reflected in considerable