Confinement effect of ring-confined concrete-filled-steel-tube columns under uni-axial load

Concrete-filled-steel-tube (CFST) columns show superior strength and ductility performance because of composite action. However, this cannot be fully developed at early elastic stage as steel dilates more than concrete and thereby causing imperfect interface bonding, which reduces the elastic strength and stiffness. To resolve the problem, confinement in the form of rings is proposed in this study to restrict the elastic lateral dilation of concrete. For verification, 62 CFST columns of various dimensions cast with normal-(NSC) or high-strength concrete (HSC) installed with rings were tested under uni-axial compression in this study. From the results, it was evident that: (1) Rings could improve the axial load-carrying capacity, stiffness and decrease the strength degradation rate. (2) The critical confinement index (ξcr) was varied with concrete grade. (3) Rings were effective to limit the lateral deformation of concrete and the steel tube. Based on the obtained results, three analytical models taking into account the confining effects provided by the steel tube and rings developed by the authors are proposed to estimate the axial load-carrying capacity of unconfined and ring-confined CFST columns. The validity of the best proposed model is justified by comparing the predicted theoretical strength with the measured strength in the tests and experimental results by other researchers.