Seismic Behavior of Steel Buildings with Hybrid Braced Frames

Abstract

This paper presents the seismic performance of buildings with a hybrid bracing system in which buckling-restrained braces (BRBs) are used at the lower stories with the higher level of ductility demands, and conventional buckling-type braces consisting of steel hollow structural sections (HSS) are used in the upper stories with relatively smaller ductility demands to minimize the probability of their fracture under reversed cyclic displacements. This type of hybrid braced frame (HBF) could prove economical, especially for retrofitted buildings in seismically active regions. A series of nonlinear time-history analyses was conducted to investigate the seismic performance of 3- and 6-story buildings with the hybrid bracing system. The main parameters studied are the seismic design parameters, maximum interstory drift ratios, fracture response of the HSS, and the optimal deployment of the BRBs. The seismic performance of the HBFs was compared with conventional concentrically braced frames and buckling-restrained braced frames (BRBFs) designed according to the current code provisions. Results showed that using BRBs at only a few lower levels of the HBFs gives similar performance to that of BRBFs. The incident of brace fractures in conventional HSS can be delayed or completely eliminated by using the proposed hybrid bracing systems.