PSI - Issue 70

Pradeep Ushakumari Abhinand et al. / Procedia Structural Integrity 70 (2025) 129–136

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1. Introduction In earthquake-resistant design, it is often impractical and uneconomical to design structures to withstand the full elastic seismic forces generated during design-level ground motions. To address this, seismic design codes incorporate a reduction factor, commonly referred to as the seismic response reduction factor ( R ), which lowers the elastic base shear to a more feasible design-level force (Uang, 1991; Kappos, 1999; Mwafy and Elnashai, 2002; Elnashai Mwafy, 2002; Balendra and Huang, 2003; Ferraioli et al ., 2014; Hassan, 2002). This approach is a cornerstone of modern seismic design philosophy, as adopted in various national and international codes (ASCE/SEI 7-22, 2022; ANSI/AISC 341-22, 2022; ANSI/AISC 360-22, 2022; FEMA P695, 2009; IS 800, 2007; IS1893(1), 2016; IS 18168, 2023). However, the numerical values assigned to R are predominantly based on engineering judgment, empirical finding based on the previous earthquakes, and a limited body of experimental research. Most seismic codes specify constant R values for different structural system types, without explicitly accounting for variations arising from parameters such as structural period, geometry, or detailing practices (Maheri and Akbari, 2003; Kim and Choi, 2005; Asgarian and Shokrgozar, 2009; Kalapodis et al ., 2018; Navvar and Pandikkadavath, 2023). Consequently, the use of a uniform R factor may not ensure uniform safety levels across different structural configurations. Observations from past seismic events suggest that well-detailed structural frames can perform satisfactorily, even when designed with reduced base shear demands, provided that, capacity design principles, adequate ductility, and sufficient overstrength are incorporated. These insights underscore the critical need for accurate evaluation of R factors and an enhanced understanding of the parameters affecting them, to balance safety and economy in seismic design. Special Steel Moment Resisting Frames (SMRFs) are popular lateral load-resisting arrangements for earthquake applications. The R -factor suggested for SMRFs is comparable to that of other high-ductility systems, showing their potential to dissipate seismic energy efficiently. SMRFs are identified by its moment-carrying column-beam regions and meticulously detailed cross-sectional dimensions, especially with respect to component width-to-thickness ratios, to ensure acceptable nonlinear behaviour under seismic disturbances (ASCE/SEI 7-22, 2022; ANSI/AISC 341-22, 2022; ANSI/AISC 360-22, 2022; FEMA P695, 2009; IS 800, 2007; IS1893(1), 2016; IS 18168, 2023). The design of SMRFs follows the strong-column – weak-beam capacity limit strategy, along with the reduced beam section (RBS) at the beam ends but away from the connection regions. This arrangement enables the formation of major plastic deformations in beams compared to columns, facilitating repeated cyclic inelastic deformations without major instabilities during the seismic events. Such detailing enhances the strength and stiffness related characteristics of the SMRFs, to achieve the desired seismic response as per the assigned R -factor in the design codes. Just like the other seismic force-resisting systems, the R -factor of SMRFs also primarily depends on the structural overstrength and ductility (Maheri & Akbari, 2003; Kim & Choi, 2005; Asgarian & Shokrgozar, 2009; Kalapodis et al ., 2018; Navvar & Pandikkadavath, 2023). Overstrength in SMRFs derived from multiple sources, including material over strength, reserve cross-sectional area regions, structural redundancy, and contributions from non-structural components. On the other hand, structural ductility is predominantly derived from the inherent material plasticity and hardening behaviour of the structural steel, as well as from the stipulated detailing requirements that enables the stable hinge formation and energy dissipation. Based on the above background, this study aims to evaluate the seismic response reduction factor or R -factor for a SMRF designed in accordance with Indian seismic design standards (IS 800 2007; IS1893(1), 2016; IS 18168, 2023). Unlike the seismic design norms in the United States, which recommend relatively higher R -values for SMRFs, the Indian standards (IS) suggest a comparatively lower R -value of 5. To investigate the effect of this recommendation, a 4-storey SMRF building is selected and designed based on the applicable Indian codes. The inelastic behaviour of the structural components is captured by incorporating suitable material and sectional properties into numerical models using the OpenSees framework. A set of seismic ground motion records is selected to carryout both the linear and nonlinear time history analyses, ensuring a comprehensive evaluation of the nonlinear response of the SMRF due to the seismic excitation. From the analysis results, relevant base shear values are obtained and used to compute the seismic response parameters, including the R -factor. The obtained results are evaluated and compared against the provisions specified in Indian seismic design codes. This comparison offers an insight into the adequacy and conservativeness of the currently recommended Indian standard SMRF R -value.