Behavior of circular-steel-tube-confined square CFST short columns under axial compression

This paper proposes circular-steel-tube-confined square concrete-filled steel tube (CFST) to improve the mechanical property of the newly built and existing deficient square CFST. The external adding circular steel tube does not enter the beam-column connection area and mainly functions as lateral confinement to the inner square CFST. Twelve circular-steel-tube-confined square CFST short column specimens and twelve conventional square CFST short column specimens were designed and tested under axial compressive load. The main variables of the test include column type, steel ratio, outer circular steel tube size, and concrete strength. The overall failure modes, axial load-displacement curves, macroscopic deformation characteristics, and the inner and outer circular steel tube confinement were analyzed and compared systematically. The study revealed that the outer circular steel tube yielded horizontally in tensile strength and provided effective confinement to the inner square CFST. Therefore, the bearing capacity of the square stub column was improved significantly, and the brittle failure in the confined area was prevented. Due to the effective confinement by the outer circular steel tube, the stress state in the square steel tube was much more uniform. The column configuration led to an effective but uneven confinement in the outer circular steel tube, concentrating near the square steel tube corner. Based on the superposition method, a suitable design method is proposed to predict the compressive strength of circular-steel-tube-confined square CFST, and the theoretical predictions are in good agreement with the test results of this paper.