Fier And Post-fire Behaviours Of Circular Steel Tube Confined Reinforced Concrete Columns

The steel tube confined reinforced concrete(STCRC) column has been applied to high-rise buildings and large-span structures, due to its high load bearing capacity, excellent seismic performance and ease in construction of the connection to reinforced concrete beams. However, no study has been reported on the fire or post-fire behaviours of STCRC columns. This thesis intends to investigate the effects of fire on the behaviours of STCRC columns, and formulate simplified methods to predict fire resistance and residual capacities of STCRC columns in fire and after fire. The main rearch work and achievements are as follows:(1) Four full-scale STCRC columns and one full-scale concrete-filled steel tubular(CFST) column were tested to failure, under the ISO 834 standard fire condition. The influences of key parameters on the deformation and fire resistance of STCRC columns were investigated, including load ratio and modes of members. The cross-sectional temperatures, axial displacements, fire resistance and failure modes were recorded and discussed. The results show that the fire resistance of STCRC columns decreases significantly with the increase of load ratio. And STCRC columns possess higher fire resistance than CFST columns.(2) A sequentially coupled thermal-stress finite element model was developed using the program ABAQUS, featuring heat transfer analysis and stress analysis. Extensive parametric analysis was carried out with the FE model to investigate influences of key parameters on thermal distributions and fire resistance. The considered parameters included exposure time, cross-section dimension and steel tube to concrete area ratio for thermal analysis, and load ratio, cross-section dimension, slenderness ratio, material strengths, steel tube to concrete area ratio and reinforcement ratio for fire resistance analysis. It was found that the temperature of steel tube mainly depends on the exposure time, while the temperature of concrete is affected by both exposure time and cross-section diameter. The simplified methods for predicting temperatures of steel tube, reinforcing bars and concrete were proposed. By inducing the detrimental effect of temperature, a method was proposed for evaluating load bearing capacities of STCRC columns subjected to fire, which can be used to calculate the fire resistance.(3) Experiments of twenty-six STCRC columns after exposure to ISO 834 standard fire were conducted to investigate their residual behaviours. The key parameters, including heating time, slenderness ratio, load eccentricity, concrete compressive strength and cross-section dimension, were studied to identify their influences on the residual behaviours. The temperatures in specimens were measured during the heating and cooling phases, while the load-displacement relationships, strains in steel tube and residual capacities were recorded during the subsequent compressive tests. The delay of temperature rise was observed for concrete. The load bearing capacity of specimens increases with the increase of cross-section diameter and strength of concrete, whereas decreases with the increase of heating time, slenderness ratio and load eccentricity. All specimens failed by global buckling and exhibited faily ductile behaviour.(4) A finite element models was developed using the program ABAQUS to investigate the post-fire performance of STCRC columns. The FE model was validated against tests and used to identify influences of key parameters on the residual capacity and stiffness of STCRC columns. The considered parameters included heating time, cross-section dimension, slenderness ratio, load eccentricity, material strengths, steel tube to concrete area ratio and reinforcement ratio. The stiffness of STCRC columns was found to be more sensitive to high temperature than the load bearing capacity. Simplified methods were proposed for predicting cross-sectional capacities, buckling capacities, capacities under combined axial and flexural loading and compressive stiffness of STCRC columns after standard fire exposure, which can be used to evaluate the residual performance of STCRC columns at the stage of assessment of fire damage.