Study On The Ultra-high Performance Concrete Filled Square Steel Tube Short Columns Under Axial Loading

Ultra-High Performance Concrete(UHPC)as a new high strength cement-based composite materials,since its launch,has been widespread concerned by researchers for its high strength and superior durability,become the development direction of new Concrete materials in the 21st century.UHPC is poured into steel tube to form the composite component of steel tube.on the one hand,the steel tube restrains the deformation of the core UHPC,improve brittleness and further enhance strength,on the other hand,The concrete filled with steel tube can delay or avoid the premature partial buckling of steel tube,give full play to the mechanical properties of steel.The Ultra-High Performance Concrete filled steel tube can adapt to the modern engineering structure to develop in the direction of towering,large-span,overloading and bear the needs of the poor conditions,particularly applicable to high-rise buildings,long-span Bridges and tall structures,antiknock structure,thin wall structure and the structure of high abrasion,high corrosive environment.At present,the research on Ultra-High Performance Concrete filled steel tube component is mainly focused on circular section,there are few reports on the research of square section,the axial compressive mechanical properties and working mechanism is unclear,the calculation method of ultimate bearing capacity is not mature enough.Based on the national natural sciences foundation(51738011),in this paper,through the combination of experimental research and numerical simulation,the compression performance,working mechanism and bearing capacity of the steel tube UHPC short column were studied in depth.The main research work and results are as follows:(1)By axial compression test,the load-deformation curve and load-strain curve were measured,the failure form of specimens was investigated.The research shows that the damage type is mainly related to the thickness ratio and the confining factor,both are the strength damage.When the thickness ratio is greater than 14.3 and the confining factor is between 1.149 and 2.258,the test piece is damaged by the waist drum,when the width ratio is less than 10,and the confining factor is between 2.495 and 6.070,the specimen is coarsely rounded.All specimens show good deformation ability,and the brittleness problem of UHPC is well solved.(2)By finite element analysis,the stress phase and the stress variation of the steel tube and concrete at the feature point are analyzed,the working mechanism was clarified.The research shows that the whole process is divided into four stages:elastic stage,elastic-plastic stages,load drop stage and stable stage.When the ultimate bearing capacity was reached,the total section stress of the steel tube reached its yield stress,the longitudinal stress of concrete reached its axial compressive strength.The elastic stage is longer than that of ordinary steel tube,and the interaction between steel tube and core UHPC is weaker than that of ordinary steel tube.(3)Based on the experiment,the finite element software ABAQUS was used to expand the analysis,and the influence of concrete strength,section width ratio and steel strength on the bearing capacity was analyzed.The research shows that,when the compressive strength of core concrete was increased from 80MPa to 200MPa,and the ultimate bearing capacity increased by 5.92%,14.0%,22.70%and 41.75%respectively,it's a linear growth trend;when the thickness ratio decreased from 33.3 to 6.3,and the ultimate bearing capacity increased by 33.37%,55.28%,70.05%and 106.13%respectively,the increase was gradually increased;when the yield strength of steel was increased from Q235 to Q460,and the ultimate bearing capacity increased by 21.31%,30.06%,35.88%and 43.63%respectively,it's a linear growth trend too.(4)Based on the comprehensive analysis of test results and finite element simulation,combined with domestic and foreign relevant specifications,the calculation formula of bearing capacity of the ultra-high performance concrete filled square steel tube short columns under axial compression was presented,which can provide reference for engineering design.