Effects of aftershocks on the performance of steel building frames

Damages done by the earthquake excitation have attracted a significant amount of attention on aftershock effects on building structures. Seismic design of buildings is done without paying much attention to the repeated sequence of mainshock-aftershock events. It has been found that a structure may sustain the mainshock but is damaged during the aftershock. In this paper, the performance of steel building frames is evaluated with respect to the aftershocks followed by the mainshock. For this purpose, 4-, 8- and 12-story steel building frames are considered and are subject to synthesized earthquake, response spectrum compatible mainshock-aftershock sequences. For making the sequences, Bath’s law is applied. A nonlinear time history analysis is performed for a variety of synthesized sequences of the mainshock and aftershocks. The variation is implemented by selecting any single mainshock out of an ensemble of 8 mainshocks. In the sequence, a maximum of seven aftershocks followed by a mainshock is used. Time history record used for the analysis is developed by joining the synthesised earthquakes in series keeping a gap of of the 40 s between two events. The performance parameters used in the study are maximum inter-story drift ratio during an event (MIDR), residual inter-story drift ratio after the event (RIDR), maximum residual top story displacement and spread of hinges. The performances of the three building frames are compared in the study. It is seen that the 12-story frame is damaged to collapse level after 3 aftershocks, while the 4- and 8-story building frames survive seven aftershocks.