Effect of beam strength on seismic performance of two-story X-braced frames

摘要

Two-story X-braced frames (TSXBFs), which are the frames having the bracing configurations V- and inverted V-braces in alternate stories creating an X-configuration over two stories have become one of the most commonly used Special Concentrically Braced Frames (SCBFs) in areas with high seismicity. The primary reason that makes TSXBFs attractive to the industry is that the brace-intersected girders of TSXBFs are much lighter than that of the CBFs with the other bracing configurations.According to the current AISC Seismic Provisions, the brace-intersected beams in TSXBFs can be designed considering only the first-mode loading pattern, which reduces the vertical unbalanced loads acting on the brace-intersected girders of TSXBFs as well as the size of the brace-intersected beams substantially. Even though seismic behavior of CBFs has been extensively studied in general, a little work has been done on seismic behavior of the brace-intersected beams in TSXBFs.The purpose of this study is to present the seismic demand on the brace-intersected beams and its impact on seismic performance of TSXBFs. For this purpose, five 6-story TSXBFs and one 6-story CBF with inverted-V bracing are designed and subjected to twenty recorded earthquake ground motions. The results of non-linear time history analyses are presented and discussed in terms of brace ductility, beam ductility and story drift ratio in order to evaluate seismic response of TSXBFs.The study concludes that brace-intersected beams in the TSXBFs with weak beams tend to experience vertical inelastic deformations within beam spans when the structures undergo 2% or larger story drift ratio response and the ductility demand on the braces might increase significantly when the vertical inelastic deformation at the brace-intercepting point of the beam takes place. Also, yielding of the brace-intersected beams lead the beams to deform vertically within their spans, which would result in much more complicated deformation patterns than the first-mode mechanism anticipated by AISC. Furthermore, it is found that even mid-rise frames studied in this work were affected by higher modes. Future work should therefore aim to investigate the impact of the beam responses on brace ductility response as well as overall seismic response of high-rise TSXBFs.