Mechanical Behavior Of Rectangular CFST Columns Under Biaxial Compression-Bending Loading Conditions

The rectangular concrete filled steel tube?CFST?column has been widely applied in practice because of its advantages in structural mechanics.Rectangular CFST columns are often subjected to biaxial compression-bending load,accompanied by transverse shear load.This paper analyses the mechanical behavior of rectangular CFST columns under biaxial compression-bending loading condition and biaxial compression-bending-shearing loading condition by using finite element method?F EM?.The main work and achievements can be summarized below.Firstly,a finite element analysis?FEA?model for simulation of the mechanical behaviors was established,which was verified by some test results.Based on this FEA model,the full range anaysis of rectangular CFST columns was conducted and the influence of parameters on mechanical behaviors was also analyzed.The parameters include load eccentricity angle,load eccentricity and loading path.The results shows that the cross-sectional area of core concrete tension zone,under the ultimate state of biaxial compression-bending bearing capacity,decreases with the increase of load eccentricity and the decrease of load eccentricity angle.And the ultimate biaxial compression-bending capacity increases with the the increase of load eccentricity and the decrease of load eccentricity angle.Loading path has little influence on mechanical behavior of rectangular CFST columns.Then,the influence of transverse shear load on the mechanical behaviors of rectangular CFST beam columns was analyzed.It shows that the maximum transverse shear force of unidirectional compression-bending-shearing members in strong axis direction is the largest,while that of unidirectional compression-bending-shearing members in weak axis direction is the smallest,and that of biaxial compression-bending-shearing members is between the above two.The maximum transverse shear force of rectangular CFST columns under biaxial compression-bending-shearing loading condition increases with the increase of the axial force when the axial compression ratio is small,but decreases when the axial compression ratio is large.The ratio of height to width has little effect on the maximum transverse shear force in the direction of strong axis,but great influence on that in the direction of weak axis.The maximum transverse shear force of rectangular CFST columns increases with the increase of steel strength,the increase of concrete strength,the increase of steel ratio and the decrease of the shear span ratio.The elastic stiffness of V-?35?curve does not change with steel strength.But it increases with the increase of concrete strength,the increase of steel ratio and the decrease of the shear span ratio.Finally,a parametric study on biaxial compression-bending bearing capacity and N-M curve of rectangular CFST columns was conducted based on FEA model.The results shows that the equilibrium point also exist on N/N_u-M_x/M_ux-M_y/M_uy curved surface of rectangular CFST columns subjected to biaxial compression-bending load.The equilibrium point moves outward with the increase of concrete strength,the decrease of steel strength,the decrease of steel ratio and the decrease of slenderness ratio.When the ratio of height to width increases,the equilibrium point moves outward in the direction o f strong axis,but it moves inward in the direction of weak axis.Based on the parametric study,two simplified calculation methods were verified,which were used to calculate the bearing capacity of rectangular CFST columns under biaxial compression-bending loading condition and compression-bending-shear loading condition.