Effect of Mainshock-Aftershock Sequences on Woodframe Building Damage Fragilities

Abstract

Although aftershocks have the potential to cause severe damage to buildings and threaten life safety, their effect in seismic risk analysis is not explicitly accounted for in modern building design codes, nor in emerging methodologies such as performance-based seismic design. The ultimate objective of this study is to systematically integrate aftershock hazard into performance-based earthquake engineering (PBEE) through analytical studies with structural degradation models derived from publicly available experimental data. In this paper, the first step is made by introducing a procedure to compute the probability of a mainshock-damaged woodframe building entering different damage states as a result of aftershock. Aftershock fragilities are developed by performing incremental dynamic analysis (IDA) using a sequence of mainshock-aftershock ground motions. To compute the seismic response of the damaged building, IDA is performed using a sequence of mainshocks of different intensity levels combined with random aftershocks. The variation in aftershock fragilities for each of the damage states for several different mainshock intensities is presented. The effect of the mainshock damage is to alter the fragilities and is quantified for the building investigated. It is observed that the fragility curves for collapse risk have a similar shape for the mainshock-aftershock sequences for different levels of mainshock. As well as that there was an unexpected result that, if the building model survives the mainshock, the additional collapse risk because of aftershocks may not be as critical as originally thought for engineered woodframe construction. The effect of aftershocks on damage states appears to be more significant, relatively speaking, than on collapse of low-rise woodframe buildings, indicating that including aftershock hazard on performance-based seismic design will be significant.