Ductility Demand on Reduced-Length Buckling Restrained Braces in Braced Frames

Concentrically braced frames (CBFs) experience strength and stiffness degradation due to the compression buckling of conventional braces in the event of seismic loading. This effectively reduces the ductility potential of conventional buckling-type braces (BTBs). On the other hand, the inhibition of global buckling instability of buckling-restrained braces (BRBs) results in nearly symmetric load displacement response with enhanced energy absorption ability and excellent low-cycle fatigue capacity in the non-elastic regions. The reduced axial stiffness and the preferred moment-released end connections of BRBs in buckling-restrained braced frames (BRBFs) sequels in an increased drift demand requirements as compared to the CBFs with BTBs. A hybrid brace with a combination of elastic BTB and yielding RLBRB for a smaller length in line can limit the drift response while maintaining the superior hysteretic behavior as that of the BRB. For such hybrid braces during the seismic action, RLBRB component can undergo balanced inelastic cyclic displacement response while the longer elastic BTB can provide the desired stiffness to reduce the drift response. However the ductility demand requirement in the RLBRB segment of the composite brace will be increased in proportion with the decrease in yield length of RLBRB in comparison with the full length yielding brace. The present study is focused on the ductility demand requirement in a low-rise 3-story building frame fitted with the proposed hybrid braces in all story levels. The study frame is designed considering design parameters similar to conventional BRBFs. Inverted-V (chevron) arrangement of braces were considered for the study frame in which the yielding length of the brace is considered to be one-third of the distance between the work points. The seismic performance of the hybrid braced frame is analyzed for design base earthquake (DBE) hazard level. The maximum inter story drift response, residual drift response, maximum ductility demand in RLBRB of the hybrid brace and the distribution of brace ductility demand over the height of the frame were studied. The results are compared with the response of a low rise conventional BRBF designed with the same parameters.