Research On Behavious Of Square Thin-walled Concrete-filled Steel Tubes With Steel Bar Stiffeners Under Fire

Concrete filled steel tubular columns have been extensively used in multi-story and high-rise building for their high bearing capacity,good plasticity and toughness,easy to construct and fire resistance performance.Applying stiffener in the ordinary concrete filled steel tubular and decreasing the thickness of the steel tube at the same time will lower the construction costs and make the structures more artistic compared to the former kind.In normal operation condition,core concrete still has a restriction from the thin-walled steel tube still.However,high temperature can cause the strength reduction of materials in the fire.When steel tube becomes thinner,it will be more likely to occur local buckling on the surface of the steel tube which could decrease the restriction to the core concrete.In order to avoid the adverse effect,the cross stiffeners have been used to postpone the buckling of the steel tube which could reduce the mechanical property of the whole structure and enhance its fire resistance performance.The entire structure,using stiffener in CFST columns couldn’t only deduct the loss of bearing capacity but also prevent the structure from collapsed.Therefore,the recovery of the structure will be easier and lower cost.The study mainly research on the fire resistance of concrete filled square steel tubular with stiffeners mid-height columns in experiment and analysis with the method of finite element.The main works have been introduced briefly below:(1)Fire resistance test of 5 full-scale concrete filled square thin-walled steel hollow section columns has been done under the control of ISO-834 heating curve.In the experiment the temperature of feature points on the cross section,axial load-displacement curve,lateral displacement curve and the failure mode of the components have been tested.Based on the experimental results,the study analysis the effect of loading ratio,load eccentricity ratio,space between stiffening rib on the regularty of fire resistant.(2)The mechanical-thermal coupled models of CFST with stiffener mid-height columns by the software ABAQUS have been verified based on the experimental statistic.After that,further research on mechanics mechanism has been taken that include the temperature distribution,failure mode,stress-strain distribution and its evolution of the concrete filled square thin-walled steel tubular columns.(3)After parameter analysis of the heating up time,the length of side,thickness of the steel tube,space between stiffening bars,the influence of temperature field has been obtained.The loading ratio,load eccentricity ratio,the length of side,calculated length,thickness of steel tube have been taken into account to analyze their effect on the fire resistance.Based on the influencing disciplinarian of factors,the fire resistance simplified calculation formula of concrete filed square thin-walled steel tubular with stiffener columns has been set up which can provide a reference to the fire resistant design of relative engineering.