Seismic Behavior Of Rectangular Concrete-filled Steel Tubular Stub Columns With Binding Bars

Concrete-filled steel tubular(CFT) columns with binding bars, which are setting in the cross section of steel tube at certain spacing along the height of columns, are effectively a new type of composites. The outword deformation of the steel tube can be confined by setting binding bars. The local buckling of steel tube can been delayed or prevented as the binding bars can act as the lateral inward constraining. Meanwhile, the concrete confinement is obviously enhanced due to the lateral deformation of the teel tube is restricted. Furthermore, both the beraing capacity and the ductility of CFT columns are improved due to the mechanical property of core concrete and steel tube are sufficiently displayed. However, the seismic performance of CFT columns with binding bars has not been reported thus far in the literature. It is necessary to research the seismic behavior of columns with binding bars for the requirement of engineering application. The seismic behavior of square and rectangular CFT columns with binding bars are experimented under constant axial load level and cyclic lateral loads, and the numerical simulation is used to study the seismic behavior and mechanical behavior of the columns with binding bars. The main research work is carried out as follows.(1) The specimens of square and rectangular CFT columns with binding bars were designed and carried out under cyclic lateral load with different spacing of binding bars, axial load ratios and the depth-width ratio. The effects of parameters on the behaviors of failure patterns, hysteretic properties, skeleton curves, rigidity degeneration, deformability, energy dissipation capacity were discussed. The results indicate that the seismic beavior of the columns with binding bars are remarkably improved. The strength capacity, the deformation capacity and the energy-dissipating capacity are improved, especially for the specimens with small spacing of binding bars.(2) Considering the changing of Poisson ratio of materials using the USDFLD subroutine, the damage of core concrete, the interaction between steel tube and core concrete, the residual stress of steel tube and initial imperfection of steel tube, the finite element(FE) is modeled by the softwar ABAQUS. The FE model is validated by the experimental data. The mechanical behavior of rectangular CFT columns with binding bars is discussed subjected to cyclic lateral load. Then, the stress distribution of core concrete and the contact stress are studied particularly. Furtherly, the length of the confined zone to set binding bars for the members is estimated.(3) The hysteretic curves of the specimens are simulated basing on the software OpenSees. Then, the influence of axial compression ratio, thickness of steel tube, concrete strength, yield strength of steel tube, spacing of binding bars and slenderness ratio on the resilience model of skeleton curves were discussed. The restoring force model of square and rectangular CFT columns with binding bars is proposed by the characteristic parameters and the theory. The unloadind rigidity and hysteresis rule are built for the model. Result of hysteretic cures from the suggested model shows good agreement with the experimental results.(4) Based on the energy principle of deformation theory, the formulate for the elastoplastic local buckling of steel tube in the CFT columns are derived, assuming that the unloaded edges of steel plate are elastically restrained against rotation and the loaded edges are clamped. The formula is developed to study the elastoplastic local buckling of the steel tube for the columns with bindind bars. Finally, the appropriate spacing of binding bars, appropriate limitation for aspect ratio and corresponding appropriate limitation for width-thickness ratio are suggested due to the elastoplastic local buckling of steel plate.