Influence of time-dependent seismic hazard on structural design

Recent works on seismic hazard have introduced the concept of time-dependent seismic hazard and different models have been proposed to predict the inter-arrival time between consecutive events. Currently, the reliability assessment of structures and relevant design rules proposed by the codes are based on the Poisson recursive model, for which the frequency of the occurrence of seismic events does not change over time. This paper presents results on the impact of Time-Dependent Seismic Hazard on structural design, by evaluating the strength required by the structure (seismic capacity) for different time intervals elapsing from the last event. "Seismic capacity" is understood here as the capacity required to provide a fixed reliability level, measured by the failure rate. Two different seismic scenarios have been investigated and results concerning different site-to-source distance, capacity dispersion of the structure and different recurrence properties of the time-dependent source have been discussed. Finally, the impact of recursive properties of time-dependent model is analysed and discussed. The results obtained from the analyses highlight a significant influence of time-dependent hazard properties on the structural capacity required to attain a target reliability, and give evidence to the different roles played by the parameters considered in the analysis. Within the set of the considered case studies, the ratios between seismic capacities evaluated by the time-dependent and non-time-dependent model span the range [0,1.32] in the first scenario, where inter-arrival time varies from 0 to two times the mean return period. The second scenario involves multiple sources and observed ratios were in the range [0.84,1.23], extreme values are relevant to inter-arrival times equal to 139 y and 371 y, respectively.