Abstract: The performance-based seismic design requires engineers to design structures with desired seismic performance for a specific level of earthquake ground motions. One of the key factors is the determination of seismic effect, which depends on the local site condition. This paper collected 956 borehole profiles and shear-wave velocity profiles of soil lithology in Beijing, Suzhou and Tangshan urban areas, China. The correlations of the time-averaged shear-wave velocities VS20 and VS30 for the corresponding top 20 m and 30 m depths for sites are established. Cyclic triaxial tests were performed on typical samples of undisturbed soils at the depth of 105 m taken from Beijing urban area in this study, and the shear modulus reduction and damping increasing curves with increasing shear strain for various types of soils are given. Nonlinear site responses of soil column models for 170 borehole profiles in Beijing urban area were computed, and the site fundamental periods TS for each soil column model were estimated through the Nakamura’s HVSR method and the Chen’s weak-motion method. Subsequently, combined with the site classifications of the current seismic design codes at home and abroad and the recent research results of some scholars on site classification, two alternative seismic site classification schemes were proposed, namely the two-proxy method based on the H (soil depth from the surface to bedrock with shear-wave velocity ≥ 500 m/s) and VSE (time-averaged shear-wave velocity for the smaller value of both the upper 30 m and H), as well as the three-proxy method based on the H, VSE and TS. The two proposed site classification schemes in this study provide a beneficial reference to modify the site classification in the current seismic codes in China.

Key words: seismic site effect, site classification, soil depth from the surface to seismic bedrock, travel time averaged shear wave-velocity, site fundamental period