Experimental Research On Cyclic Behavior Of Concrete-filled Steel Tubular Columns With Sustained Axial Load After Exposure To Fire

With the increased widespread use of concrete-filled steel tube (CFST) in civil engineering, especially in high-rise buildings, the fire safety of CFST has attracted engineers'attention. It is imperative to make systematic researches on the fire resistance and post-fire behaviors of CFST. Against this background, researches on the effect of sustained axial load and cooling phase on post-fire cyclic performance of CFST columns have been carried out in this thesis. The main works are summarized as follows:(1) A new hybrid heating method, namely heating using an electric furnace and blowing liquefied petroleum gas fire into the the furnace simultaneously, was used to heat the CFST columns and to simulate the real fire attack. The furnace heating can be controlled to make the average temperature in the furnace to follow as closely as possible the ISO-834 standard fire curve. During the cooling phase, the furnace cooling was also controlled according to ISO-834 fire standard.(2) Tests on temperature distributions of five CFST specimens, including two heated by electric furnace and three heated by the hybrid heating method, was carried out to experimentally study the characteristics of temperature field of CFST during the fire exposure and cooling phase. The test results were also used to verify the thermal analysis model using ABAQUS program.(3) A finite element model was developed using ABAQUS to predict the temperature distributions of CFST columns heated by different methonds, i.e. gas fire heating, electricity heating and hybrid heating methods, during the fire exposure and cooling phase. The thermal modeling can provide a basis for the coupled thermal-stress analysis of CFST in fire and the post-fire hehavior analysis of the fire-damaged CFST.(4) Axial deformations of six full-scale CFST columns during the whole period of fire were measured to discover the axial deformation versus time history of CFST columns under the coupled fire exposure and sustained axial load. The experimental parameters including the fire duration time and the axial load level were investigated.(5) The hysteretic behavior of nine beam-columns including one under room temperature and eight after the whole period of fire, was experimentally studied. The specimens were heated by the new hybrid heating method to make the average temperature in the furnace to follow as closely as the ISO-834 standard fire curve. Effects of the fire duration time and the axial load level on the ultimate lateral strength, stiffness, dissipated energy and ductility of the columns were analyzed.(6) A finite element model using ABAQUS program was developed to analyze CFST's post-fire hysteretic behavior.