Experimental Study And Theoretical Analysis The Behavior Of Tapered Ultra-High Performance Concrete-Filled Double Skin Steel Tubular Under Axial Compression

Tapered ultra-high performance concrete-filled double skin steel tubular(UHPCFDST)refers to two concentric placed ultra-high performance steel tubular members whose yield strength is above 690MPa and whose strength grade is above C100.It is a new type of concrete-filled steel tubular member developed on the basis of solid section concrete-filled steel tubular member.Based on the experimental study,the method of combining the finite element numerical simulation and theoretical analysis,the tapered UHPCFDST axial pressure learning performance are studied,and on the basis of experiment and theoretical analysis,proposed the tapered UHPCFDST calculation method of the axial compression bearing capacity and stiffness of the components.The main research contents are as follows:(1)Axial compression test was carried out on 8 tapered UHPCFDST specimens.The influence of concrete strength,hollow ratio,length of element,D_i/t_i(D_o/t_o),steel ratio and steel tube strength on that failure mode and bear capacity of the concrete are analyzed.The results show that the failure pattern of the specimen is local bulging failure at the upper end.For the specimen with a doubled length,the upper part of the specimen is local bulging at first,and then the specimen has a relatively obvious overall bending deformation.With the increase of concrete strength grade,the ultimate load of the specimen increases.With the increase of the hollow ratio,the bearing capacity of the specimen decreases.When the size of the outer steel tube is the same,increasing the size of the inner steel tube will increase the steel content of the specimen and reduce the bearing capacity.When the size of the inner steel tube is the same,if the size of the outer steel tube decreases,the steel content of the specimen increases and the bearing capacity decreases.At the initial stage of loading,the stiffness of the specimen is basically unchanged.As the load continues to increase,the stiffness of the specimen decreases.When approaching the ultimate bearing capacity,the decreasing rate of specimen stiffness becomes smaller.After entering the strengthening stage,the rate of stiffness reduction increases.When other parameters are the same,the stiffness of the specimen increases with the improvement of concrete grade.(2)The finite element analysis software ABAQUS 6.14 was used for the numerical simulation analysis of the test specimen,and the finite element analysis results were in good agreement with the test results.Based on the test and finite element analysis results,the stress process of the tapered UHPCFDST member under axial compression load is analyzed,and the stress mechanism and failure mechanism of the member are proved.On this basis,the effects of concrete strength,hollow ratio,length of element,D_i/t_i(D_o/t_o),steel ratio and steel tube strength on mechanical properties of member are systematically analyzed by finite element method.The results show that with the increase of concrete strength grade and steel ratio,the bearing capacity and stiffness of the members increase,and the stress between the inner steel pipe and the outer steel pipe increases.With the increase of hollow ratio,length of member and D_i/t_i(D_o/t_o),the bearing capacity and stiffness of member decrease,and the stress between inner and outer steel pipe and concrete decrease.With the increase of the strength of the inner steel tube,the bearing capacity of the component increases,and the stiffness is basically unchanged.The stress between the inner steel tube and concrete increases,while the stress between the outer steel tube and concrete decreases.With the increase of the strength of the external steel pipe,the bearing capacity of the component increases,the stiffness is basically unchanged,and the stress between the internal steel pipe,external steel pipe and concrete increases.With the increase of concrete strength grade,length,D_i/t_i(D_o/t_o),the proportion of load borne by concrete increases.With the increase of hollow ratio,diameter and thickness ratio of D_i/t_i,steel ratio and strength of outer steel tube,the load ratio of outer steel tube increases.With the increase of concrete strength grade and D_i/t_i,the load ratio of outer steel tube decreases.(3)Based on the test results and finite element analysis results,combined with the existing research results,a practical calculation method for the axial bearing capacity and stiffness of the tapered UHPCFDST member is proposed,and the formula calculation results are in good agreement with the test results.