Mechanism and load-bearing capacity of square steel tubular and spiral stirrup composite confined-concrete stub columns under axial compression

To investigate the axial compressive behavior of square steel tubular and spiral stirrup composite confined-concrete (STSCC) stub columns, a total of 29 specimens were tested under axial loads. The compressive mechanical of STSCC were analyzed based on the apparent evolution process, the failure modes of core concrete and spiral stirrup, the load-displacement and load-strain curves. The experimental results show that the spiral stirrups can effectively improve the uneven characteristics of confinement provided by the square steel tube. The deformation compatibility among spiral stirrup, longitudinal reinforcement and square steel tube is very good, and the yield strength of spiral stirrup can be fully developed within the scope of reinforcement ratio. The axial compressive bearing capacity and deformation performances of STSCC columns are better than those of normal square CFST columns. The compression performance increases with the spacing of the spiral stirrups, while decreases with the diameter and the ratio diameter to width. A axial compressive capacity formula with clear physical meaning and simple form is proposed based on the composite confinement model. The test results agree well with the theoretical ones, which validates the effectiveness of the proposed formula and can provide reference for engineering application.