Study On Eccentric Compression Performance Of Built-in High-strength Spiral Stirrup Square Concrete Filled Stell Tube Columns

Square steel tube concrete column is a widely used steel-hybrid structure in composite structures,especially in super high-rise buildings,which has the dual advantages of steel structure and concrete structure,and has the advantages of simple construction rules,convenient connection,good compression and bending and seismic performance.However,compared with round steel tube,the constraint of square steel tube on internal concrete is mainly concentrated in the core concrete and four corners,and there are many non-constrained areas,so the constraint effect of square steel tube on internal concrete is far less than that of round steel tube.In order to solve the problem of uneven effect of square steel tube on internal concrete,a new combination structure with built-in high-strength spiral stirrups in the steel tube is proposed to consolidate the constraining effect of square steel tube on concrete.In this paper,the mechanical properties of eccentric compression of square steel tube concrete columns with built-in high-strength spiral stirrups are systematically studied,and the specific work is as follows:1.The eccentric compression test of 15 specimens was carried out,including 9built-in high-strength spiral stirrup square steel tube concrete columns,3 built-in ordinary spiral stirrup square steel tube concrete columns,and 3 square steel tube concrete columns.The main variables include spiral stirrup yield strength,spiral stirrup spacing,and load eccentricity.The results show that:(1)The built-in high-strength spiral stirrup in square steel tube concrete can significantly improve the ultimate bearing capacity and ductility of square steel tube concrete column.For specimens with small eccentricities,the effect is more significant;(2)The yield strength of the spiral stirrup has almost no effect on the failure form of the specimen.All the high-strength spiral stirrups were broken or necked,indicating that the spiral stirrups completely promoted the confining pressure effect.(3)The buckling of the concrete column with built-in high-strength spiral stirrup is more significant,which shows that the built-in high-strength spiral stirrup makes the specimen better bending and failure,so that the specimen shows better deformation ability.(4)For the same number of internal spiral stirrups,a smaller spiral spacing is more conducive to improving the strength and ductility of square steel tube concrete columns.2.The eccentric compression test of 6 built-in high-strength spiral stirrup square steel tube recycled concrete columns and 2 square steel tube recycled concrete columns was carried out.The study shows that:(1)The built-in high-strength spiral stirrups of square steel tube have the same degree of restraint effect on recycled concrete,which significantly improves the strength and ductility of the specimens.(2)In the recycled concrete,all the high-strength spiral stirrups have also broken or necked.(3)The axial and lateral deformation of the steel tube measured by the built-in high-strength spiral stirrup column is more obvious than that of the steel cube column without internal spiral stirrup,and the drum part of the steel tube is more symmetrical up and down,and the built-in high-strength spiral stirrup makes the specimen better bending and failure.(4)In the case of the same eccentricity,with the increase of the reinforcement rate of the built-in high-strength spiral stirrup,the strength,ductility and stiffness of the specimen also increase.(5)The 100% replacement of recycled aggregate has a greater impact on the early stiffness of the load-deflection curve of the specimen,and has little effect on the bearing capacity.3.Through the test,the failure process and final failure form of the specimen were observed,and the load-deflection curve,load-strain curve,spiral stirrup circumferential strain curve,etc.were obtained through experimental data analysis.The results show that the high-strength spiral stirrup has a significant restraining effect on the internal concrete,the ultimate strength of the specimen is significantly improved,and the ultimate strain of the steel tube is also improved to a certain extent.According to the results of this paper,in the actual engineering design,the load eccentricity should be set between 0-0.15,and the spacing of spiral stirrups should be controlled within 20 mm.4.In the existing specifications do not have a specific built-in high-strength spiral stirrup square steel tube concrete column bias calculation formula,first through the steel tube concrete column calculation formula to calculate the ultimate bearing capacity of the specimen,the results show that the bearing capacity provided by the high-strength spiral stirrup to the column can not be ignored;Then,the calculation formula of axial compressive bearing capacity of built-in high-strength spiral stirrup square steel tube concrete column proposed in this project is combined with the eccentric bearing capacity calculation formula in the specification of steel tube concrete column,a new calculation model of bias bearing capacity of built-in high-strength spiral stirrup square steel tube concrete column is proposed,the bearing capacity of the test piece in the test is calculated,and the accuracy of the calculation model is verified by verifying the bearing capacity of the built-in spiral stirrup specimen of other similar studies.