Seismic response modification factors evaluation of a 4-storey buckling-restrained braced frame (BRBF)

Abstract

The availability of adequate strength and ductility to frames facilitate use of seismic response modification coefficient (R) for their design. The subcomponents of this R coefficients, namely the force reduction component due to ductility (Rμ) and over strength factor (Ω) have relevance in this context. The seismic code, such as ASCE 7-22 given values to R and Ω (considering the type of seismic lateral load resisting system) are mostly empirical in nature. That is, for a given type of lateral load resisting frame the code provided values may not ensure same level of safety as it do not account the variations in geometry and other parameters. Hence further investigations are needed in that direction. The case of buckling-restrained braced frames (BRBFs) are also not different from this. This study tries to assess the seismic response modification factors of a 4-storey BRBF designed as per the current United States code norms. For that, past study suggested design magnitude earthquake records are adopted. The case study frame is modelled in OpenSees and nonlinear incremental dynamic analyses (IDAs) are carried out in iterative manner to get the fully yielded base shear (Vy). Here 2% inter-story drift ratio (IDR) is set as the criteria for complete yielding. From the IDAs obtained 2% IDR corresponding peak ground acceleration, linear dynamic analyses are performed to get the relevant elastic base shear (Ve). Relating Ve, Vy and design base shear (Vs), the seismic response modification factors (Rμ, Ω and R) are evaluated. The results showed that the R coefficient assessed is 1.49 times larger than the code given value whereas Ω evaluated is 0.86 times less than the code stipulated value for the considered study frame.