Research On Damage Control Based Multi-Index Evaluation And Design Method Of Buckling-Restrained Braced Frames

The post-yielding stiffness of a buckling-restrained brace (BRB) is low and BRBs at all stories cannot simultaneously enter the yield stage under an earthquake. The yield of the BRBs at one story or certain stories will cause a story stiffness mutation when the structure is at the elastic-plastic stage. Therefore, structures equipped with BRBs as a main lateral resistance system were prone to lateral deformations and damage concentration at certain stories of the structures, which leads to a larger residual deformation in the structures, and then inducing an obvious damage concentration effect (DCE). At the structural level, taking the buckling-restrained braced frames (BRBF) as the research object, the paper carries out a systemic research on multi-index evaluation and design method for the research target of mitigating the DCE.Taking the drift concentration factor (DCF) as the main index to measure the damage concentration, the paper compares the DCE of various structural systems and structures with different beam-to-column connections. The influence rules of BRB-to-frame stiffness ratio, post-yielding stiffness ratio of BRBs, beam-to-column bending strength ratio and BRB area ratio between stories on the DCE of the BRBFs are investigated. The results show that the BRBFs behave an obvious DCE after a yield of the BRBs at one story or certain stories and the increased BRB-to-frame stiffness ratio, beam-to-column bending strength ratio or the difference of BRB area between stories, and the decreased post-yielding stiffness ratio of BRBs can magnify the DCE of BRBFs.Based on the incremental dynamic analysis (IDA), a multi-index seismic capacity evaluation method is proposed and used to further reveal the DCE of BRBFs and its influence factors. A strong column-week beam calculation formula with different moment ratio of column ends is derived and the moment distribution rule of the frame in BRBF after the BRB yields is theoretically analyzed. The results show that the yielding of the BRB in BRBF can lead to the moments of column ends of the joints at the story reverse and increase, where the frame easily behave column hinge mechanism. Therefore, the aseismic measures of the frame in BRBF should be more rigorous than those in frame structure.Damage control for the BRBFs is studied at the structural layout level. Taking the displacement and the DCF respectively as the indices to measure the structural seismic damage and the damage concentration, the paper analyzes the optimal, economic and practical brace layout principle between the braces and the frames as well as those among different stories. Using the performance target that the yielding of the upper BRBs should occur before the adjacent lower frame yielding, a theoretical formula determining the upper limit value of the BRB-area-ratio among different stories is deduced. Based on the above simplified layout principle, the suggested force-based design process of the BRBFs is given.The paper summaries the performance-based methods of BRBF design, such as design methods based on energy, those based on the effective damping ratio and those based on the elasto-plastic displacement response spectrum. The equal displacement rule of the single-degree-of-freedom systems is verified. A simplified displacement-based design method is proposed in this paper, which can determine directly the section area of BRBs. The overall yield displacement of the main frame is taken as the target displacement under major earthquake to control the structural DCE. The elastic displacement design spectrum is used to solve the target period of BRBFs. A 6-story 3-bay buckling-restrained braced steel frame is designed using the proposed method, and a series of nonlinear response history analyses (RHAs) are performed to verify the design result and check the method’s accuracy. Finally, the influence of the yield strength of the BRB on the method’s accuracy and seismic structural response is analyzed in detail.Based on the design concept of the damage uniform, this paper puts forward a rocking wall buckling-restrained braced frame (RW-BRBF). A set of formulas for structural internal force and lateral displacement under the action of typical horizontal forces are derived according to the continuous computational model to obtain approximate solution formulas determining the stiffness and strength demand of the rocking wall under major earthquake. A simplified seismic design process is presented in this paper, which can directly determine the sectional area of the BRBs, the stiffness and strength demand of the rocking wall, and an example analysis verifies the process’s accuracy. Then, the paper compares the seismic performance of the BRBF, moment resisting frame, rocking wall frame and RW-BRBF.The paper puts forward a hinged wall with BRBs in base (HWBB), which can be regarded as a type of high ductility and damage-controlled shear wall. The design principle of the HWBB is stated, its equivalent calculation model is derived and the seismic performance is investigated. On this basis, a simplified seismic design process is presented for the HWBB frame (HWBBF) based on the design concept of the energy dissipation concentration, which can directly determine the sectional area of the BRBs in base, the stiffness and strength demand of the rocking wall. Then, the advantages of the HWBBF are illustrated by the mean of comparative analyses. The results show that the propsed RW-BRBF and HWBBF at the structural level merge properly the two kinds of the structural characteristics, which are the energy dissipation of the BRBs and the deformation control of the rocking wall. Both of the RW-BRBF and HWBBF can effectively reduce the DCE of the BRBFs, and the latter is easier to repair after the earthquake.