Development of fragility surfaces for pile-supported structures under mainshock-aftershock sequences

The seismic damage investigations indicated the high probability for the pile-supported structure to suffer further destruction due to the effects of strong aftershocks, but the seismic fragility assessment of pile-supported structures considering aftershocks rarely attracted notice. In this study, the seismic fragility of a pile-supported structure under the mainshock-aftershock sequence is systematically assessed through the simulation results based on a solid-fluid coupling finite element model. Firstly, the numerical model is validated by the test date got from a centrifuge model in order to check its effectiveness. Then the strong motion records recorded at liquefied sites are selected for synthesizing the seismic sequences through scaling and combination. And the Engineering Demand Parameter (EDP) is dertemined as the residual displacement, representing the cumulative damage caused by seismic sequences. Additionally, the quantitative limit states of EDP corresponding to four damage states considering pile-soil interaction is defined by a push-over analysis. Furthermore, since the choice of optimal Intensity Measures for seismic sequences is beneficial for enhancing the credibility, 24 sets of Intensity Measures are assessed from three sides including efficiency, practicality, and proficiency. Finally, the mainshock-aftershock fragility surfaces with two Intensity Measures are proposed, considering the random uncertainty. And the results indicate that mainshock aftershock sequences could induce higher exceedance probabilities of a limit state when comparing to under the excitation of mainshock only. The research results could offer a basis for the seismic performance evaluation of the kind of pile-supported structure.