Response Characteristics And Prediction Method Of Ultimate Bearing Capacity Of Concrete-Filled Steel Tube Column Exposed To Fire

Structural column is one of the most important components in structure.Concrete-filled steel tube(CFST)columns have higher compressive,flexural and shear capacities,which play an significant role in modern buildings with high(ultra-high),large(large span)and strong(strong seismic capacity)qualities.In case of fire,because the core concrete absorbs heat,the heating of the external steel tube has a hysteresis effect compared with the pure steel structure,thus improving the fire-resistance performance of the components.In fire,once the column loses its bearing capacity,the structure will partially collapse or even collapse completely.Therefore,the research on the response characteristics and ultimate bearing capacity of CFST columns subjected to fire is of great importance for structural safety.At present,the research on the fire resistance of single member of CFST has been extensive,but the boundary conditions of the test model mostly fail to reflect the actual restraint stiffness.At the same time,it also ignores the high fluidity in the preparation process of core concrete and the aggravation of various properties of high-performance pumped concrete under the high temperature brought by the addition of pumping agent.Existing studies focus on describing the failure modes,heating modes and axial deformation of CFST columns under fire,but there is a lack of research on failure mechanism of CFST columns with certain restraint stiffness under fire.Additional although the fire resistance of CFST column has been deeply studied at home and abroad,most of them focus on the study of its fire resistance limit(time),but how the ultimate bearing capacity of CFST column under fire evolves and how to predict it is still lacking.Therefore,this paper conducts some researches on the above.Specifically,the main work and research results of this paper as following:(1)14 CFST columns with high slump core concrete at a predetermined boundary restraint stiffness under the condition of fire are tested,and the test results were analyzed.It is found that the scale actions affects the fire behaviour of CFST columns.A large size specimen absorbs more heat and thus heating up slowly because of its large heat capacity,and causing a large temperature gradient between outer and core.At the same time,with the same eccentricity and restraint type,the restraint stiffness has a significant influence on the axial displacement of CFST specimens.When other factors are the same,the end rotation angle of the specimenunder the condition of large restraint stiffness is greater than that under the condition of small restraint stiffness.At the same time,with the increase of eccentricity,the end rotation angle of each test column increases correspondingly.After the test,it was found in the anatomical diagram of the sample in the fire zone that the external steel tube and the core concrete were no longer in the initial tight fitting state,and there was an obvious gap between them.(2)Based on the summary of experimental phenomena and results of the analysis of(1),by means of finite element analysis software ABAQUS6.13 to maximize to carry out simulation and to fulfill the parameter studies.On the basis of the failure characteristics and fire response characteristics basically consistent with the actual test,the complex changes inside the CFST column during the fire were captured,and the failure mechanism was analyzed.Studies showed that the failure of CFST column is caused by separation of steel tube and core concrete.As a result,the combination of the initial effect disappeared.The results usually characterized by the occurrence of local buckling of steel tube in advance and the the high rate of deformation of column during the compression stage.The effect of slenderness ratio on failure mechanism of CFST column in fire is larger than that of load ratio and restraint stiffness.(3)Based on(1)and(2),considering the degradation of mechanical properties of steel and core concrete in the process of fire and the influence of slenderness ratio.These indexes is incorporated into the formula of normal temperature ultimate bearing capacity of Japanese standard,and the relevant influence coefficient is added to avoid the inhomogeneity of material strength distribution caused by the internal temperature gradient of core concrete,so as to obtain the deformation formula of ultimate bearing capacity of CFST column in fire.In addition,two typical calculation examples are selected to compare the calculation results of the predicted ultimate bearing capacity of CFST column under fire with those of finite element simulation.The numerical simulation and formula calculation results of the two calculation examples are slightly different,but in general,the formula calculation results and numerical simulation results fit well.