The Effect of Near Field Sequence Earthquake on Concrete Special Moment Frames Equipped with Viscous Dampers

Today, earthquakes are one of the important topics in the seismic design of structures in engineering communities. In seismic areas, in addition to the main earthquake, a series of foreshocks or aftershocks with different intensities usually occur, which occur with a short time interval or a few days and months after the main earthquake. According to past experiences, aftershocks often occur suddenly after the main earthquake and due to static and dynamic stresses during the earthquake process. Successive earthquakes are one of the important topics in the seismic design of structures, which has not been included even in new regulations. Therefore, the study of structures under successive earthquakes and the reinforcement of structures under these earthquakes is inevitable. Therefore, in this study, structures with a concrete special moment frame system with a number of floors of 7, 12 and 20 floors have been considered. SAP2000 software was used to analyze the structures and nonlinear time history analysis was performed for the structures under 7 near-fault earthquake records. The seismic behavior of these structures under single and consecutive earthquakes, without dampers and with the addition of viscous dampers, was investigated, and results such as base shear, roof permanent displacement, relative floor displacement (drift), plastic joint formation process, and hysteresis curve were investigated in all structures with and without viscous dampers under single and consecutive earthquakes. These values ​​were higher in consecutive artificial earthquakes than in single earthquakes, and a greater reduction was created in the corresponding values ​​when the damper was used. The reduction in base shear in single earthquakes was 28% on average and 15% in consecutive earthquakes, which indicated the effect of using viscous dampers. Also, considering that consecutive earthquakes were more intense, this percentage of reduction was lower. Also, according to the results of the permanent displacement of the roof, the reduction in permanent displacement of the roof under single earthquakes was 19.33% and 14% for consecutive earthquakes. The use of dampers has reduced the residual relative displacement between floors, which is greater under single seismic scenarios than in the consecutive case. According to the results obtained, it was determined that there is no direct relationship between the height of the frame and the effect of the seismic sequence phenomenon on the increase in relative displacement between floors. Finally, the results of the sensitivity analysis also showed that the number of frame openings has an effect on the relative displacement of floors in 7-story frames, and with its increase, a decrease in relative displacement is observed in the lower floors (1 to 3) and an increase in relative displacement of floors in the upper floors (4 to 7). This phenomenon has been observed for both single and consecutive earthquake conditions. As a general and final conclusion of this research, it can be said that the response of the structure in the case of a seismic sequence phenomenon is more critical than a single earthquake, and for this reason, the effects of successive earthquakes should be considered in seismic design codes.