Modelling the out-of-plane buckling behaviour of BRBFs

Abstract

The objective of this study is to assess the impact of modelling the out-of-plane buckling behaviour of buckling-restrained braced frames (BRBFs) on their estimated seismic performance. A number of recent studies have highlighted the possibility of gusset plates and brace end-zones buckling out-of-plane before the buckling-restrained braces (BRBs) themselves yield. Very few of these studies have attempted to characterise the importance of capturing this failure mode in nonlinear models of BRBFs used to estimate their global seismic performance. The NZS 1170.5 requirements and NHERP design guidelines for BRBFs were used to design and analyse a BRBF building in this study. Non-linear BRBF models were developed in OpenSees using two numerical modelling approaches: 1) a conventional BRBF model without out-of-plane imperfections, incapable of capturing the out-of-plane buckling of braces; and 2) a 3d planar BRBF model with flexible gusset plates and brace end-zones, and out-of-plane imperfections capable of simulating the out-of-plane buckling failure mode. According to the results of the pushover analyses, the gusset plates appear to be undersized when designed according to the current design practice adopted in New Zealand. Furthermore, the conventional BRBF modelling approach, which is incapable of simulating the out-of-plane buckling failure mode, overestimates the BRBF's base shear and deformation capacities by 6.95% and 516%, respectively. To improve the performance of the BRBF, the gusset plates were redesigned following suggestions adopted from the literature. Pushover analysis conducted on the revised BRBF model indicate enhanced BRBF performance and no out-of-plane buckling of the gusset plates.