Confining and hoop stresses in ring-confined thin-walled concrete-filled steel tube columns

Concrete-filled steel tube (CFST) columns are now widely adopted in many structures due to the superior behaviour provided by composite action. However, this composite action is limited because of the different dilation properties of the constitutive materials at the early elastic stage. Furthermore, due to inelastic outward buckling of the steel tube, CFST columns may suffer serious degradation of the steel confinement. To overcome these problems, external confinement in the form of rings has been recently studied, and test results have shown that such provision can improve the strength, elastic stiffness, ductility and interface bonding of CFST columns. In the work reported in this paper, as a continuation of a previous study of ring-confined CFST columns, ten concrete-filled thin-walled steel tubes were fabricated and tested under uniaxial compression. The main parameters examined were the concrete cylinder strength, the steel tube thickness and the ring spacing. The test results show that external rings are highly effective in improving the uniaxial behaviour of CFST columns. Based on the experimental hoop–axial strain relationships and Prandtl–Reuss theory for the steel tube, the true structural behaviour of unconfined and ring-confined CFST columns, such as the steel–concrete interface bonding condition, the behaviour of the steel tube and core concrete as well as the confining mechanisms, were explored.