Influence of the equivalent shear strain reduction factor on soil layer seismic response

The equivalent shear strain reduction factor (α) is central to the equivalent linearisation method for soil layer seismic response analysis, yet its commonly used empirical value of 0.65 lacks rigorous scientific basis. To investigate the influence of α, the El Centro wave, Kobe wave and an artificial wave with varying amplitudes as were selected as inputs, α was adjusted systematically and the seismic responses of 70 different types of engineering sites were calculated. The effects of α and ground motion intensity on surface peak acceleration, response spectrum peak (RSP) and spectral peak period were comprehensively analysed and discussed. The results showed the following. The peak period increased with α and larger site periods (T_s) corresponded to greater change amplitudes. For sites with T_s < 0.15 s, the peak ground acceleration (PGA) and RSP increased with α. For sites with T_s = 0.15–0.20 s, the PGA and RSP first increased and then decreased. For sites with T_s > 0.2 s, the PGA and RSP both decreased with increasing α. Based on these findings, recommended values of α are proposed: under strong ground motion, α = 0.8 for sites with T_s < 0.2 s and α = 0.5 for sites with T_s ≥ 0.2 s; under weak ground motion, α remains as the commonly used empirical value of 0.65.