Study of steel buildings with LCF system under critical mainshock-aftershock sequence: Evaluation of fragility curves and estimation of the response modification factor by artificial intelligence

In the seismic active zones, the stored energy in faults is continuously released which can lead to the seismic sequence phenomenon. So that, strong main shocks may involve numerous foreshocks and aftershocks with a magnitude close to the main shock. Hence, it is expected that the behavior of buildings under successive shocks would differ compared to one earthquake. Thus, this paper examined the effects of the seismic sequence phenomenon on the performance of steel buildings with the Linked Column Frame (LCF) system under critical successive earthquakes. As LCF is relatively new lateral force resisting system and considering the successive scenarios still seems necessary, this paper estimates the Response modification factors (R factors) by artificial intelligence, besides evaluating the fragility curves under critical mainshock-aftershock sequence. Therefore, building frames with 3, 5, 7, 9, and 11 stories equipped with the LCF lateral load resisting system had been analyzed in the OpenSees software consist of incremental dynamic, nonlinear static, and linear and nonlinear dynamic analyses. R factors were calculated under the seismic scenarios with/without sequence after performing more than 18 thousand nonlinear dynamic analyses. For more comprehensive examination of R factors, in addition to the number of stories, the effect of other parameters, including the length and the behavior of link beams (shear/flexural) were studied. An ideal artificial neural network was moreover designed based on the parameters, and experimental equations were proposed to determine the R factor under consecutive earthquakes with acceptable accuracy. The results indicated that the designed buildings for the R factor based on single earthquake could not provide the expected performance when experiencing successive shocks because the R factors caused by consecutive earthquakes was 26% lower than the single case.