Seismic Resilience Assessment Of RC Frame Structure Considering Mainshock-aftershock Effect

With the development of social economy,‘seismic resilience’ has been put forward and developed,and the requirements for seismic design of structures have changed from ‘life safety’ to ‘life,property and function safety’.In addition,it is shown that more than 85% mainshocks are followed by a large number of aftershocks.Numerous earthquake data and studies have shown that aftershocks can cause additional damage to the structure,thus causing additional seismic losses and increasing the recovery time of the structure.However,the effects of aftershocks are ignored in the current structural seismic resilience studies.Therefore,this paper takes reinforced concrete(RC)frame structures,which exist in large numbers in cities in China,as the object to study their economic losses and recovery times under mainshock aftershock sequences,and proposes a seismic resilience assessment method that considers the effects of mainshock aftershock sequences.The main research contents are as follows:(1)For the 4-story and 8-story RC frame structures,equivalent single strut model was used to simulate the infill walls,and a 2D bare frame model and infilled frame model were built by Open Sees.50 real mainshock-aftershock records were selected.The engineering demand parameters required for damage assessment of the structure,infill wall,ceiling,critical medical equipment and interior contents and the thresholds for different damage states were determined.The fragility under aftershock of structures or elements with different damage levels due to mainshock is obtained based on double incremental dynamic analysis,which can be used to consider the effects of multiple aftershocks in structural risk analysis.It is found that the bare frame model underestimates the acceleration response and overestimates the displacement response.(2)On the basis of aftershock probabilistic seismic hazard analysis method,the seismic losses of the structure under mainshock and multiple aftershocks were studied,and the effects of the closest distance from the site to the fault,mainshock magnitude,building use,and the existence or nonexistence of infill walls in the finite element model were analyzed.It is noticed that the larger the magnitude of the mainshock and the closer the site is to the fault,the more destructive the aftershock is.The damage state of the structure or element will shift to a more severe damage state with the increment of the number of aftershocks.(3)According to the results of the risk analysis,the seismic resilience of the structure under mainshock-aftershock sequences is studied,and the concept of aftershock impact time window is proposed to describe the effect of multiple aftershocks on the repair time of the structure,the increase of direct economic loss per day is used as the standard for assessing the aftershock impact time window,which determines the economic loss and repair time of the condition under mainshock-aftershock sequences.Considering multiple aftershock effects was found to increase the total structural recovery time by more than 20%.Considering only the mainshock effects significantly underestimates the total economic loss by about 20% on average when the replacement conditions are not reached within the aftershock impact time window,and by at least 90%when the replacement conditions are reached.(4)The ratio of direct economic loss to building replacement cost is used to quantify the loss of building function during the aftershock impact time window,while the recovery of building function is determined according to the repair time and linear function recovery function,the seismic resilience loss factor was calculated for each condition.The seismic resilience of the model was found to be worse for the closer distance to the fault and the larger magnitude of mainshock,and for the model in this paper,it has better seismic resilience when used as a hospital.