Seismic Design Method And Fragility Analysis Of Assembly CFT Composite Frames With BRBs

In recent years,the state and local governments have been vigorously developing the application of assembly steel structure buildings and popularizing the promotion of new shock-absorbing technology.However,traditional steel structures have presented some drawbacks,such as large amount of field welding,lower degree of assembly and quality assurance,and it is improper to earthquake and wind resistance of high-rise buildings.Since Wenchuan earthquake,buckling-restrained brace(BRB)has been received extensive attention from the academic and engineering circles,but there is still a lack of experimental and theoretical investigations on BRBs with various gusset connections.Meanwhile,many researches were related to the rigid concrete filled steel tubular(CFT)composite frames,however,in-depth study on the cooperative seismic performance of blind bolted assembly CFT frames and BRBs has been scarcely reported.Therefore,a novel composite structure composed of blind bolted end plate(BE)CFT composite frames and BRBs(BECFT-BRB)was proposed and presented in this dissertation.For the novel composite structure,BRBs could effectively improve the lateral stiffness and energy dissipation capacity of the structure,CFT columns exhibit superior vertical bearing capacity,and BE connections would realize reliable joint assembly,which will be maximizing the advantages of each part of the structure.This is also consistent with the development concept of national assembly steel structure buildings.In order to explore the seismic behavior,design method and seismic fragility of the novel composite structure,research works were conducted and summarized as follows:(1)Five specimens on BRBs with various gusset connections were carried out under cyclic loading test and numerical analysis.The failure modes,axial elastic stiffness,core plate strain,cumulative plastic deformation etc.were evaluated.Meanwhile,the relationship between the core plate strain demand and the structural story drift was established.Additionally,the formulas of equivalent stiffness of BRBs with various gusset connections were proposed for the first time and verified by experimental and numerical results.It is clear that the contribution of the gusset stiffness to the overall BRB stiffness should be considered in the process of structural design.(2)A series of pseudo-dynamic tests were conducted on two specimens of assembly BECFT-BRB structures.The seismic behavior of both specimens at different loading levels were explored by the experimental failure modes,iner-story shear force-drift curves,stiffness degradation,ductility coefficient and energy dissipation responses.Test results demonstrated that both specimens were in elastic stage and BRBs provided greater lateral stiffness to the BECFT frame at the frequent occurrence earthquake(FOE)level;BRBs began to yield to dissipate energy at the design base earthquake(DBE)level;moreover,they absorbed most of seismic energy to prevent the main frame from premature failure at maximum considered earthquake(MCE)or super-MCE levels.(3)On the basis of the fibre model theory,the elastic-plastic finite element analysis(FEA)model of the novel composite structure was developed by OpenSees program.It is well known that the shear force-deformation model of panel zones and moment-rotation model of composite joints are key points to accurately predict seismic behavior of the novel composite structure.Therefore,the shear force formulas of square or circular CFT panel zone were modified.Subsequently,optimal formulas of three key parameters,including initial stiffness,plastic moment capacity and rotation capacity,of a BECFT composite joint under positive and negative moments were proposed.Meanwhile,the mathematical moment-rotation model considering the ultimate moment capatiy of the joint was also developed to account for post-yield strain hardening effect.The accuracy of the FEA model established by above theoretical methods was experimentally verified.The influence of the gusset stiffness on the seismic performance of the novel composite structure was discussed.(4)The modified plastic design method of energy balance-based and global failure mode was proposed for the novel composite structure.The energy balance equation considering post-yield strain hardening effect was derived based on the design target of structural global failure mode.Meanwhile,the formulas of overall yield drift of the novel composite structure was suggested.Moreover,relevant formulas for avoiding three kinds of adverse failure modes and realizing the global failure mode were derived,especially the specific joint rotation values were assumed to BE joints under FOE,DBE and MCE hazard levels.It effectively solves the problem of performance-based control of semi-rigid joints.Then 6-,9-,12-and 20-story BECFT-BRB archetype structures were respectively established and explored by nonlinear time history analysis.The reliability and validity of the proposed plastic design method were verified in terms of inter-story drift,residual inter-story drift,joint rotation and BRB displacement ductility.(5)The IDA-based probabilistic seismic fragility analysis method for the BECFT-BRB archetype structures was developed and extended.The non-collapse fragility curves of all archetype structures were generated by the probabilistic seismic demand and capacity analysis.Then the probabilities of reaching or exceeding various limit states were calculated for all archetype structures.Moreover,an alternative collapse evaluation procedure was suggested by using the geometric mean spectral acceleration as its earthquake intensity measure,which led to simplification of evaluation process.Furthermore,the record-by-record successive accumulation method was derived to systematically carried out probabilistic damage analysis.The probabilistic distribution functions of a structure at different damage states were presented in terms of the system level,subsystem level,and component level.It shoud be noted that the uncertainty of determining potential collapse points was considered to incorporate the epistemic uncertainty.The 6-and 12-story archetype structures were selected to assess their exceeding probability for different damage scenarios,which is beneficial to being targeted on plans of earthquake prevention and disaster mitigation,and resulting in the reduction of casualties and property losses.