Impact of Sequential Ground Motion Pairing on Mainshock-Aftershock Structural Response and Collapse Performance Assessment

Earthquake engineers lack well-founded consensus guidelines for selecting ground motion time series for sequential mainshock-aftershock events for use in seismic performance assessment. Past practice has seen sequences formed by coupling as-recorded mainshock and aftershock records and by using repeated mainshock records for both event types. Using mainshock-mainshock versus mainshock-aftershock record pairs, this paper assesses the structural performance of five ductile reinforced concrete frames with varying heights using sequential nonlinear response history analyses. Systematic differences are found in the frequency content of mainshock and aftershock records, which in turn produce expected differences in structural responses conditional on the dynamic characteristics of each structure. The outcome is measurable differences in the structural response, with mainshock-mainshock sequences potentially overestimating or underestimating seismic demand and risk relative to the use of more-appropriate mainshock-aftershock record pairs. This finding holds true even when mainshock-mainshock sequences are formed by preserving the magnitude and distance relationships between as-recorded mainshocks and aftershocks. The correlation between event terms of mainshock and aftershock ground motions recorded from the same sequence is found to have a significant impact on maximum story drift ratio. This paper provides recommendations for aftershock record selection that draw upon these results.