Seismic Performance Of Fire-Exposed Concrete-Filled Steel Tubular Column-Steel Beam Blind Bolt Joints

Concrete-filled steel tubular(CFST)structures might be subjected to single or coupled hazards during their life-cycle,such as fire and earthquake.However,there are no specific provisions on post-fire seismic assessment of CFST structures in the domestic and foreign designing codes.Therefore,it is of essential importance to study the mechanical properties of this kind of structure under full-range fire,which lays the foundation of scientific post-fire seismic assessment.The post-fire seismic behavior of CFST columns and CFST column-steel beam blind bolt joints under combined full-range loading process and full-range fire were systematically investigated in this paper.The research work conducted is as follows:1.Twenty groups of push-out tests were first carried out to study the bond behavior between steel tube and core concrete after exposed to constant high temperature,so as to provide the range of bond strength s between steel tube and concrete under multiple loading cases.The influence of sectional shape,high temperature,long-term sustained loading,and cyclic loading on the bond behavior was studied.A detailed high-fidelity finite element analysis(FEA)model was developed considering the stress-strain relationship,thermal strain,loading and unloading criterion of material under different stages of full-range fire.The influence of interactions between steel tube and core concrete and shear-span ratios on the post-fire seismic behavior was investigated.The modified post-fire hysteretic relationship model for CFST columns was developed.2.Ten groups of tests were conducted to study the seismic behavior of CFST column-steel beam blind bolt plane joints with reinforced concrete slab after exposed to ISO-834 standard fire.Temperature field of the joints under fire,load-displacement relationship,crack propagation of concrete slab,strength degradation,stiffness degradation,ductility and energy-dissipating capacity were studied.There were two typical failure modes of joints in the tests including connection zone failure and column failure.3.A FEA model of joint was developed and verified according to the test results.Proper interactions and constraints were selected among blind bolts,end plates,steel tube and core concrete in the model.General agreement was achieved between the test results and the results predicted by the FEA model.On the basis of the verified model,the seismic behavior of joints after fire exposure was studied,including failure modes,internal force distribution,deformation characteristic,stress distribution,et al.4.Parametric analysis was carried out to study the main influence factors on the post-fire seismic behavior of joints,including heating time,number of blind bolts in the connection zone,et al.A simplified post-fire hysteretic relationship model of joints was illustrated.Calculation methods for ultimate strength,stiffness,rotation capacity,loading and unloading criterion were given in the model.The model laid the foundation of post-fire seismic analyses of structures.