Improving strength, stiffness and ductility of CFDST columns by external confinement

• Use of external steel ring to confine the outer tube of concrete-filled double skin tubular (CFDST) columns.
• Confinement effectiveness of rings studied by the improvement in load-carrying capacity, elastic stiffness, Poisson's ratio and ductility.
• A theoretical model for evaluating the load-carrying capacity of ring-confined CFDST columns for practical design purpose was proposed.

Concrete-filled double-skin tubular (CFDST) column is one of the most efficient forms of column construction in which the steel tubes provides both axial strength and confining pressure to enhance the strength and ductility of the in-filled concrete. Compared with confined reinforced concrete columns, CFDST columns had stronger and more uniform confining pressure provided to the in-filled concrete by the steel tubes, which reduces the steel congestion problem for better concrete placing quality. However, a major shortcoming of the CFDST columns is the imperfect interface bonding that occurred at the elastic stage that reduces the elastic strength and stiffness of columns. To improve the situation, it is proposed in this study to use external steel rings to restrict the dilation of outer steel tube of CFDST columns. For verification, a series of uni-axial compression test was performed on some CFDST columns with external steel rings. From the results, it was found that the elastic strength, elastic stiffness and ductility were enhanced by installing the steel rings as external confinement. Lastly, a theoretical model for predicting the axial strength of confined CFDST columns has been developed.