Compressive Behavior Of Steel-Reinforced Circular CFST Stub Columns After Elevated Temperature

Steel-concrete composite columns have been widely used in high-rise buildings,long-span structures and other engineering fields due to its excellent mechanical properties,and have achieved excellent architectural effects and economic benefits.Fire can cause serious damage to building load-bearing components,which is the main potential safety hazard during service of buildings.As a new type of load-bearing column,steel-reinforced concrete-filled steel tubular(SRCFST)column has broad engineering application prospects.However,the research on its mechanical properties after high temperature is very limited at present,which restricts the popularization and application of the new composite column.Therefore,this paper conducts an experimental investigation on the axial compressive mechanical properties of section steel reinforced concrete-filled circular steel tubular(SSR-CFCST)stub columns after elevated temperature,and the parameters considered include steel tube thickness,section steel ratio,concrete compressive strength and temperature.The ultimate load-bearing capacity,failure modes,load-displacement curves,load-strain curves,initial stiffness and ductility of the specimens were mainly analyzed.The research mainly includes the following aspects:(1)A total of 135 SSR-CFCST stub columns were fabricated,and 108 of them were subjected to elevated temperature treatment,then the axial compression static load tests were carried out on all specimens.According to the failure characteristics of specimens,three typical failure modes were summarized.The analysis indicates that when the specimens with the steel tube thickness of 1.79 mm,the failure mode was dominated by the diagonal shear cracks occurred on the steel tube,slight cracks also occur on the corresponding positions of the concrete column,and the end and mid-height of outer steel tube encountered slightly local buckling.When the specimen with thicker steel tube,higher concrete compressive strength but lower section steel ratio,the semi-circular local buckling occurred near the mid-height and one end of the specimen,and there is an obvious shear slip plane between the two buckling parts,serious diagonal shear cracking occurred in the concrete column,and obvious shear deformation of the section steel.When the specimen with thicker steel tube,higher section steel ratio but lower concrete compressive strength,the steel tube had multiple local buckling and the circumferential expansion of the concrete column was obvious,but there was no visible shear crack on the concrete surface,and the shear deformation of the section steel was serious.(2)According to the load-displacement curves,the ultimate bearing capacity,initial stiffness and ductility of each specimen were analyzed.The test results indicated that the steel tube thickness and temperature are the main factors affecting the decreasing amplitude of the ultimate bearing capacity of specimens after elevated temperature.The maximum decrease of ultimate bearing capacity of the specimen with the thickness of 1.79 mm after elevated temperature treatment at 900°C reached 40.31%.With the increase of steel tube thickness,the influence of temperature on the ultimate bearing capacity of the specimen decreased obviously.The ductility of specimens increased remarkably with the increase of section steel ratio and steel tube thickness,but decreased seriously with the increase of concrete compressive strength.In addition,the initial stiffness of the specimen increased with the increase of section steel ratio and concrete compressive strength,but the stiffness degradation of specimens after the elevated temperature treatment above 400°C was very obvious.(3)On the basis of experimental results and the calculation formulas derived by previous scholars for calculating the ultimate bearing capacity of SRCFST columns,the formula for estimating the ultimate load-bearing capacity of SSR-CFCST stub columns at ambient temperature was proposed.Then the formula for estimating the ultimate load-bearing capacity of SSR-CFCST stub columns after elevated temperature was obtained by the SPSS software regression analysis.The accuracy and safety of the formulas were verified by comparing the calculated results with the experimental results,which can provide a theoretical basis and reference value for the popularization and application of SRCFST columns in engineering.