LAUSR

Abstract This paper investigates the strength reduction factor of single-degree-of-freedom (SDOF) system subjected to the mainshock–aftershock sequence-type ground motions. Both displacement ductility and cumulative damage are considered in the reduction factor. Records of mainshock-aftershock earthquakes were collected and classified according to site properties. The aftershock ground motions in sequence are scaled to five relative intensity levels. Based on the nonlinear time-history analysis of inelastic SDOF systems, the effects of natural period, ductility factor, damage index and aftershock have been studied statistically. The results indicate that the aftershock ground motion has significant influences on strength reduction factors, and the damage-based strength reduction factor is about 0.6–0.9 times of the ductility-based strength reduction factor. Finally, an empirical expression for strength reduction factor was established by regression analysis.