This paper presents a trilinear restoring force model based on the test results of two square concrete-filled steel tubular (CFT) stub columns without binding bars and eight square CFT stub columns with binding bars subjected to constant axial load and cyclic lateral load. The effects of binding bars and axial load level on the specimens are considered to determine the feature points of the skeleton curves. Calculation formulas for the characteristic parameters of the model, including the yield load, the peak load and the ultimate load, are given by analyzing the influence of the confinement factor of steel tube, the confinement coefficient of binding bars and the axial load level. Additionally, the hysteretic rule under cyclic lateral load is confirmed. The predicted hysteretic cures and skeleton curves based on the proposed model are in good agreement with the experimental results.Keywords: Binding bars, hysteretic rule, restoring force model, skeleton curve, square CFT columns.
INSTRUCTIONConcrete-filled steel tubular (CFT) columns have been used for more than 100 years in the construction of the building structures and bridges [1,2]. The CFT columns can utilize adequately the tensile characteristics of steel tube and the compressive property of core concrete, respectively. In recent years, the mechanical behavior and the seismic performance of square CFT columns have been studied [3 -8]. Additionally, the binding bars have been set to improve the mechanical behavior of square CFT columns. The static performance of square CFT columns with binding bars have been investigated and discussed in the literature [9]. Previous studies confirmed that the local buckling of steel tube can be delayed or restricted, and the core concrete can be effectively confined by both the steel tube and binding bars [10]. Thus, the ultimate strength and ductility of the square CFT columns with binding bars are highly improved. However, the seismic behavior of square CFT columns with binding bars has not yet been studied and discussed.The restoring force, which refers to the ability to restore the deformation of the structures, is the important basis to analyze the elastic plastic analysis of CFT columns [11]. Usually, the restoring force model is proposed by experimental results or parametric analysis, and it is the relationship between the force and the displacement. Several restoring force models for square CFT columns have been proposed [12,13]. However, the effect of binding bars has not been considered in these models.In this paper, ten square CFT columns, including two without binding bars and eight with binding bars, were tested under constant axial load and cyclic lateral load. A simplified restoring force model, including the skeleton curve and hysteretic rule, for the square CFT columns with binding bars was proposed based on the theoretical model and * Address correspondence to this author at the School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510640, China. Tel: +8...