Improved behaviour of concrete-filled-stee-tube columns with external confinement

Abstract

In high-strength concrete columns, because of the heavy demand of confining steel to restore the column ductility, it is more efficient to provide the confinement in the form of steel tube to form concrete-filled-steel-tube (CFST) column. Comparing with the transverse steel, CFST columns provide a stronger and more uniform confining pressure to the concrete core, and reduce the steel congestion problem for better concrete placing quality. However, a major shortcoming of CFST columns is the imperfect steel-concrete interface bonding at the elastic stage as steel dilates more than concrete in compression. This adversely affects the confinement of the steel tube and decrease the elastic modulus. To resolve the problem, it is proposed in this study to use external steel confinement in the forms of rings and ties to restrict the dilation of steel tube. For verification, a series of uni-axial compression test was performed on some CFST columns to study the effectiveness of external confinement. From the results, it was found that: (1) Both rings and ties improved the stiffness of the CFST columns; (2) The rings improve significantly the axial strength of the CFST columns while the ties did not improve the axial strength; (3) All externally confined CFST columns can reach a strain of at least 20% before failure occurs.