Nonlinear seismic response of soft soil site in Shanghai

Site seismic effects are an important factor affecting the design ground motion input. It is well known that for a deep-covered soft-soil site similar to the Shanghai area, the site will exhibit nonlinear characteristics even under relatively small bedrock ground motion input. Therefore, corresponding to different probability levels of ground seismic motion, the seismic effect of the site is also quite different. In the current design codes, the differences in seismic effects are considered by adjusting the maximum value of the design response spectrum and the characteristic period, and the rationality of this adjustment needs further verification. For this reason, the paper takes the standard site model of 280 m cover thickness commonly used in current research as the benchmark, and utilizes the nonlinear time course analysis to study the seismic response characteristics and the variation law of the response spectra of the site under the frequent earthquakes, design earthquakes and rare earthquakes. The results show that: with the increase of the peak value of the input bedrock seismic wave, the amplification coefficient of the surface response wave of the deep-covered soft-soil site decreases gradually; With the increase of the bedrock input, the acceleration amplification coefficient of the deep soil layer increases obviously, which needs to be taken into account when designing the deep underground structure. The long period amplification effect of the case with larger bedrock input (the design earthquake and rare earthquakes) is obvious, and the amplification effect of the acceleration on the surface due to the nonlinearities of the soil is smaller; The current specification adjusts the maximum value and characteristic period of the design response spectrum to achieve the adjustment of different levels of seismic action, which may lead to the underestimation of the long-period response spectrum.