عنوان مقاله [English]
Building structures damaged by a seismic event may be exposed to the risk of aftershocks or another event within a certain period. Major earthquakes that are considered as ‘main shocks’ are typically followed by smaller earthquakes known as ‘aftershocks’, which originate at or near the rupture zone of the larger earthquake. In order to gain insight into steel frame building behavior under main shock - aftershock events, it is required to simulate the building subjected to possible seismic hazards considering beam to column connections. In current research, a moderate moment resisting steel frame has been subjected to multiple earthquakes to emphasize the importance of decision-making on destruction or repair of damaged structures. The research has focused on frame damaged connections and has hypothesized that the damages to the connections are prior to any damage to the structure. The purpose of the research is to investigate and mechanize the behavior of steel frame connections in both main and aftershock quakes. Since beam-to-column connections in steel moment resisting frames may suffer an extremely low cycle fatigue in both main shock and aftershock, the focus of this research is based on the low cycle fatigue (LCF) parameters. For achieving a comparable study between main shock and aftershock events, the finite element method has been adopted. To verify the finite element model with an experimental program, a steel connection with accessible details has been numerically analyzed under the same load algorithm as in the experiments. In order to describe the analysis results, the appropriate parameters to low cycle fatigue (LCF) have been adopted. Plastic and rupture strain represents local buckling and damage in welding near the access hole and the trend of the mentioned parameters along the beam width represented a promising agreement between numerical and experimental outputs. Meanwhile, hysteresis graph related to cyclic loading at the beam tip and displacement have been compared between experimental and numerical results. The equivalent plastic and rupture indices can be quantitatively utilized to predict low cycle fatigue and local rupture mechanisms of the connection (beam and column). A plastic strain of 1.8 and a rupture index of 0.045 represent local yielding. In loading cycles in accordance with the experiments, in the cycle 21, the top flange yields sooner, and LCF parameters in welding are more than the metal. The trend of output graphs indicates reduction and increase of yielding indices due to stiffener plate in beam web and access hole respectively. Subsequently, the suitable properties of the nonlinear rotational springs have been acquired from hysteresis graph and IMK model. These springs stand for beam-to-column connections and accelerate solution process. Furthermore, instead of considering all possible damage mechanisms that make it time-consuming to investigate the connection under the aftershock, the change of these nonlinear springs properties include simplified damages. The selected main shock record belongs to Bam acceleration time history. This record has been induced to the frame. Based on the obtained results, the critical connection was detected that is susceptible to fracture in following shakes. The prone connection has been zoomed in again, and LCF parameters (plastic and rupture indices) for the connection components have been represented. The connection components for the column are composed of left and right flange, top and bottom stiffness plates and panel zone. The beam components includes top and bottom flanges, beam web and bolts. After gaining the hysteresis graph and related envelope points, the nonlinear spring properties have been reassigned. This process ensures considering connection damages in the frame. The frame was subjected to an aftershock event that occurs with a time delay of 13 seconds after the main shock. The aftershock record was selected from a time period of the main shock acceleration record which has the highest amounts. In order to consider the effects of maximum aftershock acceleration on the connection behavior, the maximum values had been selected varying from 1/2 to 1/6 proportional to the main shock. The results of the analyses represent that during the aftershock, the column reacts considerably as well as the beam web, compared to the main earthquake case. During induction of the main shock, maximum values of plastic and rupture strain in beam web were 1.89 and 0.033, respectively. These parameters have been increased to 3.88 and 0.689 in an aftershock of 1/2 of the main shock peak acceleration. The column suffers from local buckling in the flange next to the connection. The maximum values of plastic and rupture indices are 1.35 and 0.031 and have been increased to 3.31 and 0.241 for a 1/2 main shock acceleration earthquake. The LCF parameters of the connection under aftershock have been reduced due to the reduction of peak acceleration. The location of peak parameter occurrence changes and is not constant. In some cases, the panel zone, and in some other, stiffness plates react the most to the loading.