The effect of input motion selection strategies on nonlinear ground response predictions

Ground response prediction is a key tool in the seismic design of earth structures. The prediction is mainly governed by three factors, namely the bedrock motions, the nonlinear characteristics of the soil deposit and the corresponding initial shear wave velocity profile. This study focuses on the first factor by analysing the effect of five different earthquake selection/scaling strategies on the results of nonlinear ground response. The analyses are interpreted in terms of spectral response and engineering demand parameters (EDPs) at ground surface, including the relative horizontal displacement, the peak ground acceleration (PGA) and the spectral acceleration at the first natural period of the soil deposit (Sa(T1)). An ideal 50 m soft clay deposit is analysed using a kinematic hardening soil constitutive model implemented in a fully coupled finite element procedure. The results of the advanced nonlinear ground response analyses indicate that, for two different seismic intensity levels, all the selection strategies lead to similar spectral responses at around T1, with the exception of PGA scaling. Furthermore, the spectral matching strategy leads to a smaller variability in the EDP results. It is also found that the design response spectrum proposed by EC8 for a soft soil deposit does not seem to be fully representative of the possible future earthquake events, thus requiring a revision