Collapse Fragility of Steel Structures Subjected to Earthquake Mainshock-Aftershock Sequences

This paper investigates the collapse probability of mainshock-damaged steel buildings in aftershocks, as an essential part of developing a framework to integrate aftershock seismic hazard into performance-based engineering (PBE). Analytical studies were conducted utilizing structural degradation models derived from existing publicly available NEEShub data. During earthquake events, aftershocks have the potential to cause severe damage to buildings and threaten life safety even when only minor damage is present from the mainshock. While aftershocks are normally somewhat smaller in magnitude, their ground motion intensity is not always smaller. Aftershocks may have a higher peak ground acceleration than the mainshock, even longer duration, and significantly different energy content as a result of the change in their location relative to the site. To date, the description of seismic hazard in PBE has not included the probability of aftershocks. In this study, the structural degradation model of a four-story code-compliant steel moment-resisting frame is calibrated using existing publicly available NEEShub data. Three approaches to generate collapse fragility for the steel building that sustain a certain state of damage from a mainshock are used to investigate the effect of damage states from mainshocks on the structural collapse capacity. It is found that structural collapse capacity may reduce significantly when the building is subjected to a high intensity mainshock. As a result, the structure is likely to collapse even if only a small aftershock follows the mainshock. In addition, the effects of mainshock records, fault types and spectral shapes of aftershocks on the structural collapse capacity, are evaluated, respectively.