Parametric evaluation of the response modification factor R considering bidirectional ground motions

The current research on evaluating the response modification factor R, related to the lateral strength demand of structures, has been generally based on inelastic single-degree-of-freedom systems. Nevertheless, most structures have more than one main analysis component and will be subjected to bidirectional ground motions. In this study, the results of the parametric evaluation of the response modification factor considering the bidirectional interaction (R-b) of inelastic two-degree-of-freedom systems (2DOF) are presented. The effects on the factor R-b of the vibration period, the ductility capacity, the hysteretic model, the seismic incidence angle, and the period ratio were evaluated. Analysis results show that the bidirectional interaction could increase the lateral strength demand of the 2DOF systems because of the coupling effect of the two components' responses. To make this research useful for improving engineering practice and code provisions, the main contribution is the proposal of a simplified expression for estimating the factor R-b.