Behavior Of Eccentrically Loaded Spiral-Confined Ultra-High-Strength Concrete-Filled Steel Tubular Columns

Square concrete-filled steel tube columns are commonly used in super high-rise buildings.The application of ultra-high strength concrete in concrete-filled steel tuber columns can further reduce the cross-section size of columns,reduce the weight of the structure and increase the effective use area of the building.However,due to the remarkable brittleness of ultra-high strength concrete,the ductility requirement can’t be guaranteed only by the restraint of square steel tube.In order to solve this problem,the spiral stirrups can be used to restrain the ultra-high strength concrete,square spiral stirrup-confined ultra-high strength concrete-filled steel tube columns.In this paper,the eccentric compressive behavior of square spiral stirrup-confined ultra-high strength concrete-filled steel tube columns is studied.The main results are as follows:(1)Six square ultra-high strength concrete-filled steel tube columns with spiral stirrups and three square ultra-high strength concrete-filled steel tube columns without spiral stirrups were designed and conducted under eccentric loading.The main parameters of the specimens including: eccentricity and stirrup characteristic values.Local buckling occurred in all specimens and cracking occurred in the corner vertical weld of all specimens(except EC-H-0.4).Spiral stirrups were ruptured at the later stage of loading,and the load-bearing capacity of the specimens with high-strength spiral stirrups decreased obviously after the stirrups rupture.The load-axial displacement curve,load-lateral deflection curve and bending moment-curvature curve of the specimens were analyzed.By the analysis and comparison of the curves,the following conclusions were gained: ○1 the bearing capacity and ductility of the column were increased with the spiral stirrups in the small eccentricity;○2 the effects of spiral stirrups will be reduced by the increase of eccentricity;○3 the difference of the effect of spiral stirrups using high-strength steel or normal-strength steel can be ignored when the eccentricity was 0.6.(2)Using the test results,the uniaxial stress-strain relationship of concrete for the analysis of the section of the square spiral stirrup-confined ultra-high strength concrete-filled steel tube columns was proposed,and the bending capacity calculated by this uniaxial stress-strain relationship is agree well with the measured results.Furthermore,a formula of bending capacity was proposed by this study for the square spiral stirrup-confined ultra-high strength concrete-filled steel tube columns.(3)The nonlinear finite element analysis for the square spiral stirrup-confined ultra-highstrength concrete-filled steel tube columns was carried out using ABAQUS software.The finite element model developed by this study can reasonably estimate the load-displacement curves and failure models of all specimens.Based on this model,the factors which influence the behavior of eccentric compressive columns were investigated,including eccentricity,thickness of steel tube,and volumetric hoop ratio and material strength of spiral stirrup.Above analysis shows that: increasing the thickness of the steel tube,the volume hoop ratio of the spiral stirrup or the reinforcement strength will increase the bearing capacity of the column and the contribution of increasing the thickness of the steel tube to is the greatest.