Development of post-mainshock behavioral models for seismically damaged steel columns subjected to aftershock loading

This study develops behavioral curves for steel columns subjected to earthquake damage. Damage in lateral load-resisting members can substantially degrade stiffness, strength, ductility, and energy-dissipation capacity. To reduce the subjectivity inherent in visual damage assessment, two objective damage indices were adopted for eight steel columns: the maximum displacement attained during seismic loading and the residual displacement remaining after loading ceased. Incremental dynamic analysis (IDA) was conducted to characterize the response of damaged columns, and probabilistic procedures were used to derive fragility curves under mainshock-aftershock record sequences. In this framework, the mainshock record was scaled to impose a prescribed damage state, after which the aftershock record was applied following a 10-s interval. After scaling the aftershock, the columns were subjected to IDA until failure. Fragility curves were then developed for both damaged and undamaged columns, and adjustment factors were introduced to transform the response of an undamaged column into that of a damaged column. Strength reduction ranged from 11% to 29%, whereas stiffness reduction ranged from 28% to 65%, depending on damage level, ductility, and residual displacement.