Experimental and numerical investigations on eccentric compression behavior of square CFST columns with inner spiral stirrup

In this paper, eccentric compression behavior of square CFST columns with inner spiral stirrup was investigated. First, an eccentric compression test was conducted on 18 CFST columns with influential variables of the spiral pitch, the diameter-width ratio, the slenderness ratio, the eccentricity ratio, the longitudinal rebar ratio and the tube thickness. Test results showed that the spiral stirrup avoided the rapid loss of bearing capacity caused by excessive local deformation of the CFST column under eccentric compression. The performance of CFST columns with inner spiral stirrup can be improved by reducing the spiral pitch and increasing the diameter-width ratio. With the increase of the slenderness ratio and the eccentricity ratio, the ability of spiral stirrup to improve the eccentric compression behavior of CFST columns was significantly weakened. Before the peak load, the strain distribution of the steel tube along the section depth basically conformed to the plane section assumption, and the maximum strain of the spiral stirrup appeared near the corner of the steel tube in the compression zone. Furthermore, a parameter study using the fiber model method was performed to investigate the influence of steel distribution, including a spiral stirrup, longitudinal rebar and steel tube, on the bearing capacity of columns. It is found that the order of contribution efficiency of increasing the steel ratio of each component to the eccentric bearing capacity was spiral stirrup > steel tube > longitudinal rebar. Moreover, a finite element model was established for further analysis of the failure process. Finally, the recommendations on steel distribution and the formulas for calculating the eccentric load-carrying capacity were provided.