Abstract
The buckling-restrained knee braced-truss moment frame (BRKB-TMF) is a novel seismic force-resisting system that offers several advantages over conventional truss moment frames. It can be designed per performance-based plastic design (PBPD) methodology to confine inelastic activities to the buckling-restrained braces (BRBs) and column bases, whereas keeping all other structural elements in the elastic range. An innovative BRB with an annealed aluminium core was developed and tested at Indian Institute of Technology Kanpur. It exhibits significantly higher ductility than conventional steel-core BRBs and demonstrates pronounced hardening and energy dissipation capacity. However, this increased hardening behaviour can lead to excessive overstrength in the designed frame, resulting in an uneconomical design. To address this issue, this study proposes a BRB hardening correction factor into the existing PBPD procedure for BRKB-TMFs. The proposed modification reduces frame overstrength and generalizes design procedure, making it applicable regardless of BRB type by accounting for its hardening effects. A four-story prototype building was designed using the modified PBPD procedure, incorporating both aluminium-core and steel-core BRBs and their performance was evaluated and compared. Nonlinear static analysis confirmed that both frames exhibited excellent performance, with inelastic activities confined to the BRBs and column bases. Fragility curves were developed and performance was evaluated using incremental dynamic analysis with three intensity measures: spectral acceleration, peak ground acceleration, and peak ground velocity. The fragility curves for both frames were closely aligned for all intensity measures, indicating their similar dynamic characteristics and validating the general applicability of the proposed design methodology. The adjusted collapse margin ratio was found to be 3.02 for the aluminium-core BRB frame and 1.76 for the steel-core BRB frame, demonstrating the superior collapse resistance of the aluminium-core BRB frame.