RISK ANALYSIS OF STEEL FRAME BUILDINGS DUE TO EARTHQUAKE MAINSHOCK-AFTERSHOCK SEQUENCES

Following a large earthquake, numerous aftershocks can be triggered due to the complex stress interaction between and within tectonic plates. The aftershocks have the potential to cause severe weakening of mainshock-damaged buildings, threaten life safety, hamper reoccupation and restoration of buildings, and increase financial loss. The probability of aftershocks has not been included in the description of seismic hazard in performance-based earthquake engineering. This study proposes methodologies and a framework to investigate the performance and risk assessment of steel frame buildings subjected to mainshock-aftershock sequences. The collapse probability of mainshock-damaged steel buildings in aftershocks is investigated based on a calibrated steel frame model which is able to capture the key structural properties of strength and stiffness degradation. A scaling method was proposed to combine the mainshock and the aftershock in incremental dynamic analysis. This research finds that the structural collapse capacity may be reduced significantly when the building is subjected to a high intensity mainshock and the structure is likely to collapse even if a small aftershock follows the mainshock. In an effort to examine the aftershock effect on seismic loss, a framework is developed to evaluate seismic loss of structures subjected to mainshock-aftershock sequences. The framework is applied to examine the effects of aftershocks on seismic loss considering two intensity levels of mainshocks: the Design Earthquake, and the Maximum Considered Earthquake. It is found that aftershocks may have a significant impact on the seismic loss due to the uncertainty of the damage state and cost estimation. Limited study has been performed to investigate the impact of ground motion characteristics on the collapse risk of buildings, especially for the post-mainshock buildings. This research investigates the impact of the mean period on structural collapse. This paper finds that structural collapse capacity decreases and the peak interstory drift as well as peak residual drift before collapse decreases when the mean period increases. The influence of duration and mean period of earthquakes on the collapse risk of postmainshock buildings were also examined accounting for four damage states. This research shows that both duration and mean period of aftershocks play a significant role in structural collapse capacity. The degree of influence of aftershock characteristics on post-mainshock building collapse capacities becomes more significant as the mainshockinduced damage level increases. Additionally, a vector measure of ground motion intensity consisting of the spectral acceleration at the first-mode period, epsilon (an indicator of spectral shape), and duration are employed to investigate collapse risk of mainshock-damaged buildings. The results indicate that a building may be more vulnerable when subjected to a ground motion with smaller epsilon and longer duration. The collapse risk of post-mainshock buildings may be remarkably overestimated when synthesizing the artificial aftershocks by scaling mainshock records, since aftershocks tend to have a larger epsilon and shorter duration than mainshocks