Case history of nonlinear site response analysis and its significance in Northern California, USA

抄録

This paper presents a case history of one-dimensional time-domain nonlinear site response analysis for a major infrastructure project in the San José area, USA. The subsurface material of the study site consists of Holocene alluvial deposits with interbedded and crosscut clays and silts, sands, and gravels. Two levels of design earthquakes were considered herein associated with the return periods of 225 and 975 years. For each design level, site response analyses were performed to propagate a suite of eleven pairs of horizontal input ground motions at the reference soil horizon (a depth of about 75 m or 240 ft; V_S30 of about 490 m/s or 1600 ft/s) through a site-specific ground model. The site-specific ground model also accounts for strain rate effects on clay layers. A series of sensitivity evaluations were also completed for verifying and enhancing understanding of dynamic response of the study site. The site response analysis results show the site exhibits high nonlinearity, especially in clay layers, de-amplifying short-to-intermediate period content (0.01 to 1 s) while amplifying long period content. Also, a comparison of surface response spectra from the site response analysis versus ground motion models is presented herein highlighting the design implications of conducting site response analyses at soft sites, where high nonlinearity is anticipated.