Bearing Capacity And Ductility Of Square And Rectangular Concrete-filled Steel Tubular Stub Columns With Binding Bars

Special-shaped (including square, rectangle, L-shaped and T-shaped, et al.) concrete-filled steel tubular (CFT) column with binding bars is a new type structural member for buildings. For the square or rectangle concrete-filled steel tube columns with binding bars (SCFT-WB or RCFT-WB), by setting binding bars at the middle of each cross-sectional side, the distribution of lateral confining stress from the steel tube to the core concrete becomes more uniform, and the corresponding average lateral confining stress to the core concrete becomes larger, as a result, the confinement effect of the steel tube on the core concrete is enhanced, significant enhancement of the compressive strength of core concrete can be found. The behavior of the outward local buckling of the steel plates of the square or rectangle CFT columns is delayed or even avoided before the ultimate strength is reached, because the binding bars act as lateral constraining and constrain the lateral deformation of the tubes.Special-shaped concrete-filled steel tubular (CFT) columns with binding bars have been widely used in practical engineering projects, but researches on the bearing capacity and ductility are not sufficient. As a part of the study on the mechanical behaviors of special-shaped concrete-filled steel tubular (CFT) columns with binding bars, an extensively study on the behaviors of bearing capacity and ductility of the SCFT-WB or RCFT-WB stub columns subjected to axial load, uniaxial compressive load, and biaxial compressive load are conducted, based on the fiber element analysis program with nonlinear constitutional relationship of the core concrete, including the following main aspects.(1) The fiber element analysis technique is used to conduct the load-strain relationship curves of the SCFT-WB and RCFT-WB stub columns subjected to axial compressive load, uniaxial compressive load, and biaxial compressive load, based on the equivalent uniaxial constitutional relationship for the encased concrete of square or rectangle steel tubular with binding bars. The load-strain relationship curves predicted by the program agree well with the experimental ones.(2) The fiber element analysis program developed is used to investigate the effects of varied parameters on the bearing capacity of SCFT-WB or RCFT-WB stub columns subjected to axial load, and simplified formulas to predict the bearing capacities are suggested.(3) The fiber element analysis program developed is used to investigate the effects of ratios of axial compression stress to strength, sectional steel ratios, steel yield strengths, concrete cube strengths, the restraint coefficients of binding bars including spacings and diameters of the binding bars, and sectional aspect ratios on the bearing capacity of SCFT-WB or RCFT-WB stub columns subjected to uniaxial compressive load. The flexural strength increases by increasing the ratio of axial compression stress to strength when it is relatively small, while that decreases by increasing the ratio of axial compression stress to strength when it is relatively large. Furthermore, the flexural strength increases by increasing the sectional steel ratios, steel yield strengths, concrete cube strengths, and restraint coefficients of binding bars, in which the effects of concrete cube strengths and diameters of the binding bars on the flexural strength are inferior to the others. Based on the parameter analysis results, simplified formulas to predict the bearing capacities of SCFT-WB and RCFT-WB stub columns subjected to uniaxial compressive load are suggested, respectively.(4) Significant effects of load angles on the bearing capacity of SCFT-WB and RCFT-WB stub columns subjected to biaxial compressive load can be found. For the SCFT-WB stub columns, the maximum and minimum flexural strength are obtained at load angle 0°(or 90°) and 45°, respectively, and it decreases from the load angle 0°to 45°. For the RCFT-WB stub columns, the maximum and minimum flexural strength are obtained at load angle 0°and 60°, respectively, and it decreases from the load angle 0°to 60°, and increases again from 60°to 90°. Similar effects of varied parameters at different load angles on the N-M and M-φinteraction diagrams of the SCFT-WB and RCFT-WB stub columns can be found, and the M_x/M_x'-M_y/M_y'interaction diagrams are closed curves, symmetric to the x and y axis. According to the features of M_x/M_x'-M_y/M_y'interaction diagrams, simplified formulas to predict the bearing capacities of SCFT-WB and RCFT-WB stub columns subjected to biaxial compressive load are suggested, respectively.(5) The fiber element analysis program developed is used to investigate the effects of ratios of axial compression stress to strength, sectional steel ratios, steel yield strengths, concrete cube strengths, the restraint coefficients of binding bars including spacings and diameters of the binding bars, and sectional aspect ratios on the coefficient of curvature ductility of SCFT-WB or RCFT-WB stub columns subjected to uniaxial compressive load. The coefficient of curvature ductility increases by decreasing the ratio of axial compression stress to strength and the concrete cube strength, increasing the sectional steel ratios, steel yield strengths, and restraint coefficients of binding bars, in which the effect of diameters of the binding bars on the coefficient of curvature ductility is inferior to the others. Based on the parameter analysis results, simplified formulas to predict the coefficients of curvature ductility of SCFT-WB and RCFT-WB stub columns subjected to uniaxial compressive load are suggested, respectively. (6) Significant effects of load angles on the coefficient of curvature ductility of SCFT-WB and RCFT-WB stub columns subjected to biaxial compressive load can be found. The maximum and minimum coefficient of curvature ductility are obtained at load angle 45°and 0°(or 90°), respectively, and it increases from the load angle 0°to 45°. Based on the parameter analysis results, simplified formulas to predict the coefficients of curvature ductility of SCFT-WB and RCFT-WB stub columns subjected to biaxial compressive load are suggested, respectively.