Study On The Performance Of Hybrid Columns Of Concretefilled Square Steel Tube With FRP-confined UHPC Core

Steel-concrete composite columns with high-strength stem(Steel-Concrete-FRPUHPC Column,SCF-UHPC Column)is a composite column made by pouring ultra-highperformance-concrete(UHPC)into a fiber reinforced composite(FRP)tube and placing it in a square steel tube and then filling ordinary concrete between interlayers.The core UHPC and the interlayer ordinary concrete are in the three-way compression state when the combined column is axially compressed.Compared with the ordinary Steel-concrete composite columns,the circular section FRP tube effectively improves the uneven constraint of the square steel tube concrete,thereby improving the bearing capacity and ductility of the combined column.In this paper,the experimental preparation of UHPC was first carried out,then UHPC was applied to the high-strength core column of the combined column,and the axial pressure performance test of the SCF-UHPC column composite column was carried out.The main work is as follows:(1)Based on the existing literature,the range of UHPC preparation materials is summarized.According to the closest packing theory of cement-based composites,the UHPC mix design scheme is proposed.Through experiments,the effects of water-tobinder ratio,sand-to-binder ratio,defoaming agent,steel fiber content,feeding agitation sequence and curing system on the strength and flow properties of ultrahigh performance concrete were systematically studied.The optimal ratio of each admixture was determined.The UHPC with compressive strength exceeding 200 MPa was successfully prepared,which laid the foundation for the subsequent research.(2)The prepared UHPC was applied to SCF-UHPC column.The axial pressure test of 22 SCF-UHPC column was carried out,and the influence of FRP thickness,steel tube thickness,section diameter of high strength core column and Slenderness ratio of composite columns,load displacement relation,load strain relation and so on were systematically investigated.The test shows that the failure mode of SCF-UHPC column is mainly FRP tube fracture,the steel tube shows multiple imperfections,and the composite column exhibits ductile failure due to the double restraint of steel tube and FRP tube;The load-displacement curve is mainly divided into four stages,namely the elastic stage,the yield transition stage,the strengthening stage and the post-peak residual bearing capacity stage;the load-hoop strain relationship and the load-vertical strain relationship of the combined column have double line feature.The type feature can be divided into three stages,namely the rising section,the transition section and the breaking section.(3)On the basis of the experimental research,the restraint effect of square steel tube and FRP tube is considered in the analysis of the combined column.The square tube is equivalent to the circular steel tube analysis by the equal area method.Based on the double shear unified strength theory,the axial bearing capacity formula of SCF-UHPC is calculated and compared with the proposed formula,which is in good agreement with the experimental results.(4)According to the composition of SCF-UPHC column section,the theoretical model of the load-strain full curve of SCF-UPHC column is established by superimposing the load-strain curves of steel,FRP tube,sandwich concrete and core concrete and considering the interaction of the above parts.Three key points,namely yield point,peak load point and lower limit point of residual bearing capacity,are analyzed by comparing the model calculation results with the test values,and it is found that they are in good agreement.