Effect of Arrangement of Shear Walls on the Fragility Curves of RC Frames Subjected to Sequential Earthquake Excitations

Observations from field identifications show that, in areas with a high seismicity rate, a structure is exposed to the risk of more than one earthquake in a year. Since after every earthquake, the structure faces a decline in stiffness and resistance, it may not be able to withstand the next earthquakes. In recent years, researchers have conducted many studies on the effect of seismic sequences on the behavior of structures, but the effects of the shear wall placement in the regular and irregular structures subjected to a main shock and its aftershocks has not been investigated. Moreover, the impact of the vertical acceleration component to these buildings has not been studied. In this study, the sequential Christchurch 2010 earthquake was selected to assess the effects of a repeated strong ground motion on the nonlinear behavior of the regular and irregular 3 and 8-story RC frames. In these frames, the location of the shear wall was considered as a variable parameter to investigate the effects of that on the response of the RC frames subjected to a multiple ground excitation for the first time. Incremental dynamic analyses (IDAs) were performed considering mainshock-aftershock sequences. The failure index was considered as the maximum drift between the floors of the structures. The frames were designed as dual systems (moment resisting frame with shear wall) according to the ACI code 318-14. To simulate the FE model of the frames, the Ibarra and Krawinkler concentrated joint model of the OpenSees software was utilized due to its ability to consider the more collapse modes than the other models. The Hazus code was used to provide the fragility curves and also to indicate the failures based on the four drift-based performance levels. The results show that, the placing a thin shear wall in the middle span of the regular 8-story frame creates a lower probability of failure and drift in the all 4 performance levels Slight, Moderate, Extensive, and Complete. However, in the regular 3-story structures, the placement of a thick shear wall at the side span gives desirable results. There greatest effect of the vertical acceleration component of the earthquake was observed in the irregular 8-story structure with a thick shear wall placed at the left span. In the irregular 3 and 8 story structures with walls, the amount of damage and drift in the upper floors is less. Applying the vertical acceleration does not have a significant effect on the irregular 3-story structures with walls in different openings. But for the irregular 8-story concrete frames with thin walls, the effect of vertical acceleration cannot be ignored. Because it significantly increases the drift and base shear force and failure of the structure. The proposed approach yields the best and cost-effective design approach.