Research On Compressive Behavior Of Special-shaped Concrete-filled Steel Tubular Stub Co1umns With Binding Bars

Special-shaped (including square, rectangle, L-shaped and T-shaped) concrete-filled steel tubular (CFT) columns are extensively used in engineering structures compared to circular CFT columns for more convenient construction at beam-column joints, larger moment of inertia of cross-section which leads to much higher capacity for resisting lateral load and well agreement with the design need of the architecture plane of buildings. However, the mechanical behavior of special-shaped CFT columns are much inferior to those of circular CFT columns, because much limited confinement effects on core concrete can be expected in special-shaped CFT columns. The confinement effects of special-shaped steel tube on the core concrete reduce beyond the corner regions of the steel plates. Because outward deformation or local buckling in the middle regions of the steel plates are easily occurred under the lateral expansion of core concrete. Furthermore, the local buckling tends to occur earlier and more extensively on the broad faces of the steel tube.It is an effective method to improve the mechanical behavior of special-shaped CFT columns by setting binding bars in the middle position of steel plates at certain spacing along the height of columns. The lateral outward deformation of core concrete and the steel plates are both constrained by setting binding bars, so the local buckling strength of the steel tubes and the confinement effects of the middle regions of the steel plates on the core concrete are enhanced. Based on the analysis above, an extensive study on axial load and eccentric load behavior of L-shaped, T-shaped and cross-shaped CFT stub columns with binding bars (LCFT-WB, TCFT-WB, CCFT-WB) is conducted, including the following main aspects.(1) Tests on TCFT-WB and CCFT-WB, including eleven T-shaped CFT specimens with binding bars and five ones without binding bars, and eleven cross-shaped CFT specimens with binding bars and five ones without binding bars subjected to axial compression were carried out, respectively. The main parameters in tests are the horizontal and longitudinal spacing and diameters of binding bars, the thickness and yield strength of the steel tube, the cross-sectional aspect ratio. The effects of various parameters on the behavior of specimens such as the failure mode, bearing capacity and ductility are analyzed.(2) The stress-strain relationship for the concrete proposed by Cai and Sun is modified to establish the equivalent uniaxial stress-strain relationship for the encased concrete in the L-shaped steel tube with and without binding bars. Then, based on the experiment results of axially loaded TCFT-WB and CCFT-WB, the mechanism of confinement effects on the core concrete by the binding bars and steel tube is discussed, and the constitutional relationship of confined concrete is referred to build the equivalent uniaxial stress-strain relationship for the encased concrete in the T-shaped and cross-shaped steel tube with and without binding bars with clear mechanic concept, respectively. The results of load versus strain curves of the TCFT-WB and CCFT-WB calculated by the proposed constitutional relationship are in good agreement with the experimental ones, respectively.(3) The results predicted by the methods of current design codes for square or rectangle CFT columns are compared with the experiment results of axially loaded LCFT-WB, TCFT-WB and CCFT-WB, and the results show that the maximum strengths predicted by the codes are all scatter from the experimental results remarkably. The equivalent uniaxial stress-strain relationships for the encased concrete in the L-shaped, T-shaped and cross-shaped steel tube are adopted to calculate the axial load bearing capacity of the LCFT-WB, TCFT-WB and CCFT-WB, and the parameter analysis is conducted, then through regression analysis, simplified formulas to predict the axial load bearing capacity are obtained, respectively. The results predicted by the simplified formulas are in good agreement with the experimental ones.(4) Tests on LCFT-WB, including eight L-shaped CFT specimens with binding bars and one without binding bars subjected to eccentric compression were carried out. The main parameters in tests of the LCFT-WB are the horizontal spacing and diameters of binding bars, load eccentricity ratio and load angle. Tests on TCFT-WB and CCFT-WB, including sixteen T-shaped CFT specimens with binding bars and eleven ones without binding bars, and eight cross-shaped CFT specimens with binding bars and five ones without binding bars subjected to eccentric compression were carried out, respectively. The main parameters in tests of the TCFT-WB and CCFT-WB are the spacing of binding bars, load eccentricity ratio and load angle. The effects of various parameters on the behavior of specimens such as the failure mode, bearing capacity and ductility are analyzed.(5) Based on the equivalent uniaxial stress-strain relationships for the encased concrete in the L-shaped, T-shaped and cross-shaped steel tube, the fiber element model is proposed to predict the bearing capacity of the specimens, and the analysis results exhibit that the bearing capacity of the columns can be predicted accurately, respectively. Then parametric analysis using the proposed method is carried out to further study the fundamental behavior of the eccentrically loaded LCFT-WB, TCFT-WB and CCFT-WB, respectively. Finally, simplified interaction formulas are put forward to predict the pure flexural bearing capacity, the eccentrically loaded bearing capacity at certain load angles, and the Mx/Mx'-My/My' curve for the LCFT-WB, TCFT-WB and CCFT-WB, and the results predicted by the simplified formulas are in good agreement with the numerical ones, respectively.(6) Based on the study results about the behaviour of local buckling of the steel plates in the rectangle CFT stub columns with binding bars subjected to axial compression, the analysis on the behaviour of local buckling of the steel plates in the TCFT-WB and CCFT-WB are conducted, and the appropriate spacing of binding bars, the appropriate limitation for aspect ratio, and the corresponding appropriate limitation for width-thickness ratio are suggested, respectively.