Simplified Design Model for Uni-Axially Loaded Double-Skinned Concrete-Filled-Steel-Tubular Columns with External Confinement

One of the significant contributions of using double-skinned concrete-filled-steel-tubular (CFST) columns is that it can extend the maximum limit of concrete strength that can be practically used in the construction industry, by improving the ductility of columns through providing more uniform and continuous confining pressure to the in-filled concrete. However, because of the imperfect interface bonding occurs at early stage, the elastic strength and stiffness will decrease so that the confinement effect provided by the steel tube is not fully utilized. To improve the situation, the authors have proposed to use external confinement in the form of steel rings on the outer steel tube to restrict the dilation of CFST columns and thus restore an intact interface bonding condition. It has been verified by uni-axial compression test that the elastic strength, stiffness and interface bonding were improved. Based on the test results, the authors have developed a theoretical model for predicting the uni-axial load-carrying capacity of doubled-skinned CFST columns. As a continued study, the authors will investigate the most critical parameters affecting the uni-axial strength, and to develop a simplified formula for practical design of doubled-skinned CFST columns through an extensive parametric study.