Nonlinear analysis of square spiral-confined reinforced concrete-filled steel tubular short columns incorporating novel confinement model and interaction local buckling
Square spiral-confined reinforced concrete-filled steel tubular (SCRCFST) columns have recently been proposed as a new form of composite columns. The additional confinement exerted by the spiral reinforcements improves the strength and ductility of a square SCRCFST column compared to a square concrete-filled steel tubular (CFST) column. However, there is a lack of a computational model of SCRCFST columns considering the confinement provided by spirals and the localized interaction buckling of the outer steel tube. Moreover, the application of existing material laws for concrete or the design models originally proposed for square CFST columns may underestimate the performance of SCRCFST columns. This paper investigates the behavior of axially loaded square SCRCFST short columns by utilizing a computationally efficient fiber model developed in this study. Accurate material laws of concrete are developed that incorporate the proposed formulae for the compressive strength of confined concrete and a degradation factor for estimating the residual strength of concrete confined by spirals. The gradual interaction local buckling of the outer steel tube is taken into account in the nonlinear simulation of SCRCFST columns. The computational fiber model is validated against a large test database. The accuracy of various models that determines the lateral pressure on concrete confined by the spiral is evaluated. A parametric study is carried out to investigate the influences of important design parameters on the responses of SCRCFST columns. Moreover, a comparison of the ultimate axial loads of SCRCFST columns calculated using design standards with test results is made. Furthermore, a simple design model is proposed for calculating the axial strengths of short SCRCFST columns. It is demonstrated that the developed fiber modeling technique incorporating the new confinement model simulates well the structural behavior of SCRCFST columns; in addition, the proposed design model can be used to design such innovative composite columns in practice. The parametric study shows that the performance of SCRCFST columns is significantly influenced by the width-to-thickness ratio and yield stress of the steel tube as well as the concrete compressive strength. The ductility of the columns can be improved by using a smaller spacing of spiral reinforcements.
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