Behaviour Of Steel Beam To Concrete-Filled Steel Tubular Column Connections After Exposure To Fire

With the increased use of concrete-filled steel tubular columns in building structures, it is becoming more and more significant to further the research studies on the mechanical behaviour and potential damage after exposure to fire. But there is very little research work on postfire behavior of this kind of structure at home and overseas. The paper describes the research studies on the mechanical behaviours and practical calculation on concrete-filled steel tubular structural connections after exposure to fire. The main achievements can be summarized as follows: (1) The paper presents theoretical and experimental studies on the axially loading and flexural bending behaviours of CFST members after expure to fire. The load versus deformation relations, which is on the whole in good agreement with tested results, of the CFST members under axial loading or flexural bending moment after fire was analyzed theoretically. The theoretical model was used to analyze the potential factors influencing the compression resistance, the bending moment capacity, axial stiffness and flexural stiffness of CFST after fire. On the base of the parametric analysis, the practical calculation of the postfire compression resistance, bending moment capacity, axial stiffness and flexural stiffness was obtained. (2) A series of experimental study on the hysteretic behaviour of steel beam to concrete-filled steel tubular column connections after exposure to fire were conducted for the first time under combined constant axial load and cyclically increasing flexural load. The specimens was heated following as closely as possible the ISO-834 standard fire curve or the standard fire curve prescribed by the Regulation of GB9978-88 (Structural Member Fire-Resistant Experimental Method). According to the experimental results, comparisons was made with the changing mechanical behaviours of the joints between at ambient temperature and after fire, under different beam-column stiffness ratios and different axial load levels, and between at ambient temperature and after repaired. (3) Based on the determinated stress-strain relations for steel and concrete after high temperatures and the temperature fields in the structural member, a finite element model, in which geometrical and material's nonlinearity are taken into account silmultaneously, was presented to calculate the load versus deformation relations of CFST connections after exposure to fire under monotonic and cyclic loading. With the validation of relative exemples, the calculated results using the FEA model are on the whole in good agreement with the experimental results. (4) Based on the theoretical model, the influences of the slenderness ratio, sectional dimension, steel ratio, beam-column stiffness ratio, beam-column moment ratio, axial load level and fire duration time on the P-? relation of the steel beam to concrete-filled steel tubular column connection after exposure to fire are discussed. The load bearing capacity and the simplified method for calculation of the P-? hysteretic model of the joint after exposure to fire is put forward based on the parametric analysis results.