Development of site factor for surface spectral acceleration calculation at the alluvial soils

Abstract

Calculation of surface spectral acceleration (SA) is one of the important steps in seismic design. The SA value can be obtained from bedrock spectral acceleration and multiplied by the site factor. According to ASCE/SEI 07-16, for short-period spectral acceleration (0.2 sec / SS) greater than 1g and long-period (1 sec / S1) greater than 0.2g, Site-Specific Propagation Analysis (SSA) shall be used for site factor calculation. SSA can be performed using three different data, bedrock elevation, dynamic soil profile, and acceleration time histories. The site factor can be calculated by comparing the surface to bedrock spectral acceleration obtained from SSA. This paper describes the development of the site factor at the alluvial area (soft soil area) in Semarang city, Indonesia. The site factor was calculated at 23 boring positions. Microtremor tests were conducted in this area to predict the soil dynamic profile. Five different acceleration time histories having magnitude from 6.5 Mw to 6.8 Mw and epicenter distance less than 10 km were collected for SSA. The average FPGA and Fa values developed at 23 boring positions using SSA in this area were almost equal compared to the same site factor developed using the 2019 Indonesian Seismic Code. However, the average Fv calculated using SSA is lower than the same site factor calculated based on 2019 Code.