The Expected Seismic Fragility of Code-Conforming RC Moment Resisting Frames under Twin Seismic Events

Height of the buildings directly affects the seismic performance and behavior of the structure during the earthquakes. This parameter is also significant when the structure is excited by two consequent ground motions. This paper presents the results of a study on the effect frame height on the seismic performance under two successive strong ground motions. It has been shown that, when the consequence of the first event remain in the structure as damage, the building often shows different dynamic characteristics as well as more vulnerability during the second seismic event. Three RC moment resisting frames with 4, 8 and 15 stories are designed based on the latest version of Iranian code of seismic design of buildings referred as Standard No. 2800 (STD 2800). The frames are then simulated in OpenSees software to perform nonlinear dynamic analysis. Twenty natural ground motion sequences each including two seismic events were selected from previous studies for the purpose of nonlinear incremental dynamic analysis. Maximum inter-story drift was employed as a damage index, to capture the performance of the RC frames under sequences. The accepted performance level for collapse level was taken from Iranian instruction for seismic rehabilitation of existing buildings, PBO-Publication No. 360, 2007, which is similar to ASCE 41-13. Seismic fragility values are calculated for the buildings under the second event when being damaged in first event. Assuming a log-normal distribution for failure probability function, corresponding values of median, μ and standard deviation, β, for each case are calculated and discussed for frames with different heights. The results indicate how the main parameters of the seismic fragility function may differ with frame height.