Probabilistic model for seismic effects on soft soil sites

The seismic response exhibits strong nonlinear amplification on soft soil sites. The common practice in different countries and regions for the seismic design codes is to provide a set of adjustment coefficients that are consistent with their respective site classification criteria to consider the local site effects. Given the significant uncertainties in bedrock input motions, putting forward the site adjustment factors with different levels of exceedance probabilities better meets the performance-based seismic design requirements. 50 type Ⅲ and Ⅳ soft soil sites with covering soil layer thicknesses ranging from 50 to 150 m are collected as numerical models. 100 strong seismic records are selected from 43 reference sites recorded during 73 earthquake events in the ESM database and their peaks are adjusted to 25, 50, 100 and 150 gal as inputs. Using the one-dimensional equivalent linear program SHAKE2000, a total number of 20000 site response calculations are carried out. The distribution features and characteristic parameters of the peak acceleration, peak velocity and peak displacement amplification coefficient, as well as the short-period (T = 0.2 s) and medium-to-long period (T = 1 s) acceleration response spectrum amplification coefficients, namely F_PGA, F_PGV, F_PGD, F_a and F_v, under different intensities of input motions are given. The recommended values for various amplification coefficients are also provided for the exceedance probabilities of 2%, 16% and 50%.