Assessment of Seismic Collapse Performance of Low-rise Plan-irregular Reinforced Concrete Buildings with Torsionally-flexible Behavior using FEMA-P695 Framework

A structure is required to show proper performance in regions with a high risk of earthquake, especially when it is an irregular building. In the studies on plan-irregular buildings conducted so far, the seismic collapse performance of torsionally-flexible buildings has not been specifically reported. Generally, a building, which behaves flexibly in torsion, has a fundamental torsional period of vibration that is significantly greater than its fundamental lateral period. However, an adverse situation in terms of vibration periods is observed among the torsionally-stiff types. Compared to the torsionally-flexible buildings, the torsionally-stiff ones show different seismic performance, thus highlighting the necessity of more detailed studies on the torsionally-flexible buildings. The example buildings in the current study were characterized by reinforced concrete special moment-frame systems, including asymmetric stiffness distribution in the plan resulting from changes in the size of the structural elements. The effect of such behavior, i.e., flexibility in torsion, on the collapse performance was then evaluated in different eccentricities. The collapse margin ratio was the proposed performance criteria in this evaluation that quantified how much a building would remain safe during the expected large earthquakes in the area. In this study, OpenSEES software was used to establish and analyze 3D models, and the lumped plastic hinge method was employed to simulate the plastic behavior of elements. According to the results, the plan irregularity corresponding to the asymmetric lateral stiffness distribution had a minor effect on the collapse performance of the torsionally-flexible models. Moreover, the safety margin against the collapse was improved when the mass center displaces on the plan, thus increasing the eccentricity of the studied models. For instance, the safety margin was improved approximately 17 percent when the modeling eccentricity reached 0.3b (b shows the frame spacing). Based on the results, a high plan irregularity did not necessarily mean unfavorable seismic collapse performance in the models with flexible behavior in torsion. In this respect, the code accidental eccentricity provisions were not essential for the proposed building models.