Experimental study on axially compressed circular CFST columns with improved confinement effect

The concrete-filled-steel-tube (CFST) column has been widely used in construction due to the benefit of composite action between inner concrete and exterior steel tube. Under axial compression, the steel tube is subjected to biaxial stress state in the hoop and axial direction, and the hoop stress component provides confinement for the inner concrete. However, the existence of axial stress component accelerates the buckling of steel tube, which hinders its confinement effect to inner concrete and accordingly column's axial compressive strength and ductility. This paper aims to enhance the axial compressive strength and ductility of CFST stub columns by improving its confinement effect provided by the steel tube. CFST stub columns axially loaded/unloaded were experimentally studied for different cases: a) varying concrete strength, b) lubrication on the contact between steel tube and concrete and c) corrugations in the steel tube that are introduced to intentionally weaken the tube vertically. It was found that axial compressive strength of CFST stub columns was effectively enhanced by reducing the axial stress in the steel tube. The introduction of corrugations in the steel tube led to largely concentrated axial deformation which reduced axial stress in other portions of the steel tube and consequently resulted in “tighter” hoop confinement to the inner concrete; both axial compressive strength and ductility were enhanced.