Report - PEER - University of California, Berkeley

INTRODUCTIONThe effect of local site conditions on the amplification of ground motions has longbeen recognized (e.g., Seed and Idriss 1982). Recent earthquakes, such as the 1985Mexico City, 1989 Loma Prieta, 1994 Northridge, and 1995 Kobe earthquakes haveresulted in significant damage associated with amplification effects due to local geologicconditions (e.g., Seed et al 1987, Chang et al. 1996). While potentially other factors leadto damage (such as topographic and basin effects, liquefaction, ground failure, orstructural deficiencies), these events emphasize the need to characterize the potentialeffect of local soil deposits on the amplification of ground motions.Extensive studies of seismic site response have been performed over the last thirtyyears. Recently, Borcherdt (1994) developed intensity-dependent, short and long periodamplification factors based on the average shear wave velocity measured over the upper100 feet of a site. Concurrently, Seed et al. (1991) developed a geotechnical siteclassification system based on shear wave velocity, depth to bedrock, and generalgeotechnical descriptions of the soil deposits at a site. Seed et al. (1991) then developedintensity-dependent site amplification factors to modify the baseline "rock" peak groundacceleration (PGA) to account for site effects. With this site PGA value and a sitedependentnormalized acceleration response spectra, a site-dependent design spectra canbe developed. Work by these researchers along with work by Dobry (Dobry et al. 1994)has been incorporated into the 1997 Uniform Building Code (UBC) based primarily onthe site classification system and amplification factors developed by Borcherdt (1994). Ashear wave velocity based classification system, however, has two important limitations:1