Experimental Study On Flexural Stiffness Of Concrete Filled Steel Tubes Subjected To Debonding

In the normal service stage of concrete-filled steel tube(CFST),the bond between the steel tube and the core concrete will have a great influence on the stiffness and the creep under long-term load.In this paper,the flexural stiffness and the long-term flexural stiffness of CFST after the debonding between the steel tube and core concrete are systematically studied.The major work are as follows:(1)The flexural stiffness,failure mode and ultimate bearing capacity of the CFST members with different bonding conditions were obtained through the bending stiffness test.The variation rule of the deflection of CFST and the strain state of steel tube in the whole loading process of the CFST with different bond conditions show that the flexural stiffness of CFST members will be reduced by debonding..(2)The complete processes of loading on CFST flexural members with different bonding scenarios were tested by acoustic emission technology,and the damage development of core concrete under different bonding conditions was obtained.In addition,the influence of debonding on the bending mechanical behavior of concrete filled steel tubular members was further revealed.The contribution of core concrete to the stiffness and bearing capacity of the section is reduced.(3)Based on the large general finite element program ABAQUS,the finite-element model of CFST members with different bond states is established.By selecting the reasonable element type,the constitutive relation of the steel tube and concrete material,and setting up the reasonable and effective contact type and boundary condition of the outsourced steel tube and the core concrete,the flexural behavior of the concrete filled steel tube is simulated and calculated effectively.On this basis,the bending stiffness and key influence parameters of CFST members under different bonding conditions are studied.The results show that the larger the debonding range is,the more the bending stiffness is reduced.The lower the steel containing ratio is,the higher the concrete strength is,and the greater the debonding effect is on the bending stiffness.(4)Through the creep test of 3 different bonding conditions of CFST flexural members and the core concrete test blocks,the development regulation of the deflection and the strain change regulation of the steel pipe are obtained during the whole loading process,and the creep development mechanism of the CFST flexural member with debonding is revealed.Debonding results in increased mid-span deflection of steel pipe concrete flexural members and increased stress in steel pipes.Combined with the effect of debonding on flexural stiffness of CFST members,the long-term flexural stiffness of CFST members is preliminarily discussed.(5)Based on the B3 model,the creep data of core concrete are fitted to obtain the creep degree function of core concrete.Based on the Fortran language,the ABAQUS is developed two times,and the creep function of core concrete is written into the subprogram of calculating concrete creep.The numerical analysis model for the creep of the flexural member of the CFST after debonding is obtained.This method is used to analyze the creep of the flexural members of the concrete filled steel tube with different debonding degree and steel content.The results show that the greater the debonding range is,the greater the creep is,the lower the steel content of the section,the greater the effect of debonding on the creep.