Experimental Study On Statiic Performance Of Concrete Filled Steel Tube Reinforced Concrete Columns After Fire(High Temperature)

Concrete filled steel tube reinforced concrete(CFSTRC)column is a composite member with concrete-filled steel tube as its core and steel bars tied outside the steel tube and poured concrete.It has many advantages,such as excellent bearing capacity,stiffness and seismic performance.At present,building fires occur frequently,and the research on various structural components after fire has been carried out one after another,while the research on CFSTRC columns after fire are very rare.Therefore,in order to improve the post-fire assessment system of building structures and promote the development of fire damage assessment of CFSTRC columns,the static performance of CFSTRC columns after fire(high temperature)was studied in depth from the material level to the component level in the main way of experimental research.Firstly,a large number of steel specimens,48 high-strength concrete cubic specimens and 48 cylindrical specimens were tested after high temperature.The physical and mechanical properties of steel and high-strength concrete were obtained,and the law of temperature variation was analyzed.The formulas for calculating steel strength,residual compressive strength and modulus of elasticity of high-strength concrete after high temperature were proposed.Secondly,25 CFSTRC columns were designed to conduct fire high temperature test with steel tube diameter,stirrup cover thickness and heating temperature as parameters,and the apparent shape and mass loss rate of the specimens after high temperature were obtained.14 of the specimens were tested under axial compression after high temperature fire.The failure process and morphology under axial compression were observed,and the load-deformation characteristics,cracking load,ultimate bearing capacity,peak displacement,stiffness,ductility and damage as well as their variation with temperature were analyzed.Results showed that with the increase of temperature,the cracking load,the bearing capacity and stiffness of specimens under axial compression decrease,and the peak displacement and ductility decrease first and then increase.The deterioration of mechanical properties is mainly affected by the diameter of steel tube,the thickness of stirrup protective layer and the heating temperature.Thirdly,considering different axial compression ratios,shear tests of 11 CFSTRC columns after high temperature fire were carried out.The shear failure process and morphology were observed.The load-mid-span deflection characteristics,cracking load,shear capacity,mid-span peak deflection and shear ductility were analyzed.Results showed that with the increase of temperature,the cracking load and shear capacity of specimens decrease,and the mid-span peak deflection and shear ductility increase.The deterioration of mechanical properties is obviously influenced by the parameters of steel tube diameter,stirrup cover thickness,heating temperature and axial compression ratios.Finally,two formulas for calculating the axial compressive bearing capacity of CFSTRC columns after high temperature and two formulas for calculating the shear bearing capacity after high temperature were deduced and put forward by introducing the material strength correction coefficient and the overall temperature influence coefficient of bearing capacity.Based on the results of axial compression and shear tests,the damage regularity of CFSTRC columns under high temperature was analyzed.It was found that the damage speed of axial compression stiffness of CFSTRC columns after high temperature is the largest,the bearing capacity under axial compression is the second,and the shear capacity is the smallest.And the post-fire evaluation method of CFSTRC columns were preliminarily discussed.