Torsion Effect on the RC Structures using Fragility Curves Concerning Soil-Structure Interaction

The existence of torsion, as well as consideration of the Soil-Structure Interaction (SSI), increase the natural periods of the structure resulting from a subsequent decrease in the seismic demand of the system. This paper summarizes the probabilistic assessment for evaluation of collapse fragility curves in concrete moment resisting structure with different mass center eccentricities. A 12-story, 3-D, moment resisting concrete structure with fixed-base and considering SSI, both types of one- and two-way eccentricities is employed to estimate the collapse fragility curve by the IM-based approach. According to the obtained results, increasing the torsion due to shifting the mass centers decreases the median of the collapse fragility curve. In addition, it was observed that the SSI consideration for soil type D with shear wave velocity of 180m/s to 360m/s leads to reduction of the median of collapse capacity by 30%-40% in the presence of torsion effect due to one- and two-way mass center eccentricities in range of 0-20% of the building's plan dimensions respectively. In other words, the fixed-base assumption overestimates the median of collapse capacity and leads to unsafe design. Moreover, shifting the mass centers of all the stories up to 20% of the building's plan dimensions, with or without the consideration of the SSI, decreases the median of collapse capacities and increases the seismic vulnerability of the building. Accordingly, the fixed-base assumption can be underestimated the dispersion range of the collapse fragility curve. The result shows that the mentioned differences cannot be neglected.