Response Modification Factor In Intermediate Moment-Resisting Reinforced Concrete Frame System

Abstract

The Response Modification Factor (R) is a critical parameter in seismic design, reflecting a structure’s ability to withstand earthquake-induced forces through controlled inelastic behavior, including ductility, stiffness degradation, and energy dissipation, rather than relying solely on elastic response. Given that R is governed by a complex interaction between structural configuration, material properties, and detailing, its accurate determination is essential for achieving reliable and realistic seismic performance assessments. This study examines the response modification factor (R) of Intermediate Moment Resisting Concrete Frame Systems (IMRCFS) through a comprehensive nonlinear analytical framework. Four three-dimensional reinforced concrete frame models, comprising 1, 3, 6, and 9 stories, were developed with consistent geometric and material properties, while varying in height and stiffness distribution. Each model incorporates four bays in the transverse direction and three bays in the longitudinal direction, enabling a systematic evaluation of height-dependent seismic behavior. The structural systems were initially designed using ETABS in accordance with ACI 318-19, with seismic design parameters defined based on ASCE 7-16 provisions. Subsequently, nonlinear static pushover analyses, incorporating gravity loading and P–Δ effects, were conducted using SAP2000 and ABAQUS to capture the full nonlinear response of the structures. The base shear–displacement capacity curves were obtained from pushover analysis, and the response modification factor (R) was subsequently determined using the Equal Energy Method according to ATC-19 procedures.. The results demonstrate that the base shear corresponding to the yield point provides a consistent and realistic basis for estimating the response modification factor across all investigated configurations. Although R-values exhibit an overall increasing trend with building height, the findings highlight a pronounced sensitivity of R to stiffness distribution and vertical irregularities, particularly in mid- and high- rise systems. Notably, the computed R-values are consistently lower than those prescribed by design codes, indicating that reliance on generalized code values may lead to unconservative estimations. Overall, the study confirms that accurate evaluation of the response modification factor necessitates explicit consideration of nonlinear structural behavior, stiffness irregularities, and system configuration. Furthermore, it validates nonlinear pushover analysis as a robust and reliable approach for assessing the seismic performance of IMRCFS.