Residual Load Capacity And Experimental Research On Short Steel Tube Infilled With High Strength Concrete Column Post-Fire

Concrete filled steel tubular(CFST)column has many advantages,for example,high capacity,good plasticity and ductility,good seismic resistance,convenient construction,good fire resistance,low cost,and so on.Because of these,it is used more and more in tall buildings and large span structures in the modern society.Also,with the building taller and taller,high strength concrete and ultra-high strength concrete filled steel tubular(UHSCFST)columns are used widely.Fire resistance analysis and whole process analysis of CFST columns under fire was studied by many researchers in the past,but it was rare to study the mechanical properties of high strength concrete filled steel tubular(HSCFST)columns after fire.Fire resistance analysis of HSCFST was done,and then the compressive experiment of HSCFST short columns was performed.The works and achievements are made as follows:(1)Many researches home and abroad about thermal-mechanical properties and constitutive relation of steel and high strength concrete were summarized.Based on this,mechanical models of HSCFST columns with four different types of cross-section were established.Comparing the results of finite analysis with the experimental results,it showed that they were in good agreement.Then parametrical studies on fire resistance were accomplished.It was found that load ratio and relative slenderness ratio were the main factors on fire resistance of HSCFST columns.(2)54 standard core samples of high strength concrete were obtained from circular and square columns.Then compressive experiment of standard core samples was carried out.The temperature which the standard core samples suffered was looked as the main factor of the compression strength,Young's modulus,peak strain and so on.Through regression analysis,the equation of constitutive relation of high strength concrete post-fire was obtained.(3)Based on the different temperatures the columns suffered,16 standard steel test pieces were fetched from circular steel tube and square steel tube.Seen from tensile test results,it was showed that the properties of steel material post-fire was close to the properties of steel material in the room temperature,especially when the temperature the steel tube suffered was within 400?.The relationship between steel material properties(for example,yield strength,Young's modulus)and temperature was found.(4)18 short column specimens were made from circular and square cross-section long columns.It included 10 circular columns,8 square columns.Then compressive test of the short composite columns were conducted.It can be seen from the test results the HSCFST columns post-fire still had high load capacity and high residual stiffness.Compared to the square columns,the circular columns had better ductility.Compared with the circular columns,the square columns were easy to have local buckling on tube and crack on weld line.Local buckling was easy to appear at the both ends and even the middle position for circular columns while it happened at one of the both ends for square columns.The damage of concrete inside and steel tube outside existed already after fire resistance experiment,so the high strength concrete was chipped seriously after compressive test of the short columns.But because of the constraint of the steel tube outside,the integrity of the concrete was still kept well.(5)Based on experimental study of high strength concrete post-fire,the constitutive model of high strength concrete considered the constraint of steel tube was obtained.The constitutive model of steel post-fire was also gained.Non-linear finite element models of circular and square HSCFST columns were established and analyzed.Compared the results of numerical analysis to experimental results,it showed a good agreement.The coefficients of residual load capacity and residual stiffness of HSCFST columns post-fire were defined by comparing many different standards home and abroad.Main factors which influenced the residual load capacity and residual stiffness were found by parametrical studies of these.The simplified equations were presented for calculating the coefficients of residual load capacity and residual stiffness.