Experimental study on failure mechanism of square steel tube and spiral reinforcement multiple confined concrete stub columns under axial compression

In order to reveal the failure mechanism of square steel tube and spiral reinforcement multiple confined concrete stub columns under axial compression, twelve specimens subjected to axial compressive load were tested. The failure modes of outer steel tube, inner concrete and spiral reinforcement were observed. The inner forces and stress state of spiral reinforcement, longitudinal reinforcement and outer steel tube were also studied during the experimental research. The test results indicate that the square steel tube, spiral reinforcement, longitudinal reinforcement and inner concrete can work well with each other, and the strength of the materials can be well mobilized. The spiral reinforcement in the elastic stage plays a limited role in delaying the onset of square steel tube local buckling. However, it can provide significantly confinement to the inner concrete in the inelastic and flow-plastic stage and therefore improve the bearing capacity and deformation ability. The square steel tube and spiral reinforcement form a multiple confined mechanism to the inner concrete. In other words, the spiral reinforcement can effectively offer extra confinement in addition to that provided by the square steel tube after the occurrence of local buckling, and can address the issue of uneven confinement provided by square steel tube.