Study On Axial Compression Capacity Of Square Steel Tube Confined UHSC Short Column

UHSC columns constrained by square steel tubes have the characteristics of high bearing capacity,strong cross-section adaptability and convenient construction.The constraint effect of steel tubes can effectively improve the brittle failure of UHSC.However,at present,researches on mechanical properties of UHSC Ultra-high Strength Concrete(UHSC)short columns confined by square steel tubes are scarce,and the calculation method of bearing capacity is not clear.Therefore,in this paper,the mechanical properties of such members are studied by means of experimental research,finite element simulation and theoretical analysis.Based on the analysis results,the calculation method of axial compression bearing capacity is proposed.The main work contents are as follows:1)The experimental study of square steel tube constrained UHSC short column was carried out,and the failure mode,load-displacement,load-strain and load-Poisson ratio curves were compared and analyzed with the concrete compressive strength and steel content as parameters.The test results show that the failure model of UHSC short column constrained by square steel tube is mainly affected by the compressive strength and binding force of concrete.The deformation curves of the specimens filled with UHSC and ordinary concrete have the same stress stage,but when the hoop coefficient is the same,the elastoplastic stage of the specimens filled with ordinary concrete is longer.The bearing capacity of UHSC short column constrained by square steel tube with lower steel content decreases obviously and suddenly after ultimate load.When the binding force or sheath coefficient is the same,the Poisson’s ratio of the specimens filled with UHSC increases less before the ultimate load,and the binding force is not fully exerted,and the constraint effect of steel tubes on the core UHSC is worse.2)verified finite element model was used to compressive strength of concrete,chamfering ratio,steel ratio and section length ratio,steel yield strength for the parameter constraint effect of each other constraints UHSC filled steel tubular short column were analyzed,and the analysis shows that: as the steel ratio,fills in UHSC artifacts within the increase of the ultimate bearing capacity is lower than ordinary concrete short column,fill the constraint effect of UHSC components within the edge length ratio,the influence degree of the chamfering rate is low.When the hoop coefficient is the same,the constraint effect of square steel tube on UHSC short column is significantly lower than that of square steel tube on ordinary concrete short column,and the difference of the constraint effect is more obvious with the increase of the hoop coefficient.The results of test and finite element analysis have the same variation rule for this phenomenon.After further analysis of the constraint mechanism,it is found that the root cause of the different constraint effect of the specimens filled with UHSC and filled with ordinary concrete is the different compression ratio of the core concrete.3)The national standard calculation method and the calculation method based on Mander formula are introduced,and the accuracy is evaluated.The calculation method with the smallest error is selected for correction.The constraint reduction coefficient is proposed according to the analysis results above.The results show that,with the expansion of material selection range,neither of the two formulas can accurately predict the ultimate bearing capacity of UHSC short column constrained by steel tube,and the hoop coefficient increases,and the error gradually increases.According to the difference of restraint effect between UHSC and ordinary concrete,the constraint reduction coefficient is introduced into the calculation formula,and the revised calculation method can accurately predict the ultimate bearing capacity of all members within the parameter range in this paper.