SEISMIC FRAGILITY ANALYSIS OF LOW-DUCTILE RC FRAME ACCOUNTING FOR THE INFLUENCE OF MAINSHOCK-AFTERSHOCK SEQUENCES

Under the action of mainshock-aftershock sequences, the aftershock can cause additional damage to a damaged structure by the mainshock, or even cause it to collapse, especially for a low ductility structure. Currently, the analysis and evaluation of aseismic performance of low-ductile structures under the mainshock-aftershock sequences are not sufficient. According to the failure characteristics of a low-ductile reinforced concrete (RC) frame, the shear failure of beam-column joints, the flexure-shear failure of column, the strength and stiffness degradation, and the bond-slip failure of longitudinal reinforcement at the end of beam were taken into consideration for the numerical simulation of a low-ductile RC frame. The refined analytical model of a 6-story 3-span low-ductile RC frame structure was established via OpenSees finite element software. The real mainshock-aftershock sequences and the mainshock-aftershock sequences based on repeated method were selected. The effects of aftershock directionality and the number of aftershocks were also considered. Taking different mainshock-aftershock sequences as input, the dynamic analysis of the low-ductile RC frame model was carried out and the seismic fragility curves corresponding to different damage states were obtained. Then the effect of different mainshock-aftershock sequences on its aseismic performance was analyzed. The results show that the refined analytical model of a low-ductile RC frame presented can simulate the degradation of stiffness and strength of beam-column joints due to shearing actions. Comparing with the mainshock input only, the exceedance probability of the damage state of the low-ductile RC frame under the influence of the mainshock-aftershock sequences is higher, and with the increase of the damage extent under the mainshock, the increase is much more obvious. The exceedance probability of the corresponding damage state under the influence of the mainshock-aftershock sequences based on the repeated method is higher than that under the influence of the real mainshock-aftershock sequences. The different direction of aftershock has certain influence on the exceedance probability of corresponding damage states. Increasing the number of aftershocks will also increase the exceedance probability of corresponding damage states.