Performance Assessment of Structures due to Mainshock-Aftershock Seismic Sequences by Endurance Time Method

Due to the stress interaction within and between tectonic plates, it is very natural to happen many aftershocks after any large earthquake. Because of the seismic hazard of aftershocks, they can cause sever additional damage to mainshock-damaged buildings and threat life safety, increase financial loss, prevent reoccupation and restoration of buildings. The probability of aftershocks has not included in the definition of seismic hazard in performance based earthquake engineering, however, seismic performance assessment is especially important for the rapid decision making process associated with buildings under mainshock-aftershock sequences. This study investigates the seismic performance of steel moment resisting frame buildings subjected to mainshock-aftershock sequences by endurance time method which is a new and fast method to estimate the response of various structures in both linear and nonlinear analysis. The case studies correspond to two SAC steel MRF buildings of three and nine stories, which were designed to post-Northridge codes and standards. Two-dimensional nonlinear finite element models of the buildings are developed with the OpenSees, using a finite length plastic hinge beam model and a bilinear constitutive law with deterioration, and are subjected to multiple mainshock aftershock seismic sequences. It is found that a rational combination of mainshock and aftershocks should be considered for analysis and design of structures and for any given mainshock, it needs to impose aftershocks in two opposite directions and use the critical state to evaluate the final status of structures. Also, the cumulative response of structure should be considered in assessment of structural performance in seismic sequences.