Mechanical behavior of spiral stirrup reinforced concrete filled square steel tubular columns under compression

Stirrup-confined concrete-filled square steel tubular (CFSST) columns with spiral stirrup alleviates the local buckling of square steel tube, thereby improving both ultimate bearing capacity and ductility. This paper presents an experimental study on compressive behavior of spiral stirrup reinforced concrete-filled square steel tubular (SSRCFSST) columns. Twenty-nine axial compression specimens and eighteen eccentric compression specimens were tested for experimental investigation of the effects of the sample size, the concrete strength, the thickness of steel tube, the ratio of diameter to width, the spacing of spiral stirrup and the steel ratio. In addition, the effect of eccentricity ratio on the eccentric compression specimens was also investigated. The test results show that the spiral stirrup has excellent cooperation ability with square steel tube and concrete, can effectively improve the non-uniform confine of square steel tube and enhance the confine effect on concrete. Furthermore, decreasing the spacing of spiral stirrup can enhance the axial bearing capacity and ductility of SSRCFSST columns, but has little effect on the eccentric bearing capacity. A composite model confined by square steel tube and spiral stirrup is established, the calculation formulas to estimate the bearing capacity of SSRCFSST columns are developed and close correlations are found between the calculated results and the experimental results.