The torsion induced by seismic excitations leads an excess storey-displacements and ductility demands causing distress in the lateral load resisting elements located at the extreme edges of structures, especially when the structure is designed to be strained into the inelastic range during a design earthquake. While the crucial step of performance-based design procedures is the determination of the displacement demands of the structures under the seismic actions, taking the torsion into account in design of structures is a significant issue for practical engineers. Several researches have focused on simplified nonlinear analysis procedures to predict earthquake responses of multi-story asymmetric buildings with simple lateral load resistant shear-beam elements. In these procedures, the seismic performance is estimated through nonlinear static analyses (such as pushover analysis) of MDOF models. Although, practical engineers intend to continue using these simplistic methods in which a lateral force distribution proportional to only the first mode of vibration of the building is used for estimation of seismic demands, however, where the effect of higher modes is considerable, more rigorous seismic-performance design procedures such as Modal Pushover Analysis should be employed. The objective of the current study is to evaluate MPA method (one directional and two directional MPA methods) for evaluation of seismic performance of R/C frames with reinforced concrete shear walls located on asymmetric plans (the concepts of stiffness eccentricity and mass eccentricity were used) subjected to an assemblage of ordinary and near-fault ground motions. Symmetric frame basis of these models were developed using the generic structures algorithm. Then asymmetric buildings were created by adding shear walls in both two principal directions. The nonlinear behavior of beam elements is idealized using moment-curvature theory, while fiber model is utilized to represent the interaction among tension/compression axial force and biaxial bending in column elements and shear walls modeled by panel elements. This approach of modeling is more accurate for multistory dual lateral load resistant structures, because it redefines the stiffness from the strength in each time step. This research investigates the accuracy of MPA procedure in estimating seismic demands (story drifts) of mass center, the flexible side and the stiff side of the structure’s plan compared with those from nonlinear RHA to assess the torsional effects in multistory structures.
