Uni-axial Compression Test of Concrete-filled-steel-tube Columns Confined by Tie Bars

Concrete-filled-steel-tube (CFST) columns have been adopted due to its superior strength and ductility performance contributed by the composite action. However, this beneficial composite action cannot be fully developed at early elastic stage as steel dilates more than concrete and thereby causing imperfect interface bonding. Hence, it reduces the elastic strength and stiffness of the CFST columns. To resolve the problem, confinement in the form of tie bars is proposed in this study to restrict the lateral dilation of concrete and steel at initial elastic stage. In this paper, CFST columns of various dimensions, concrete strength and installed with tie bars were tested under uni-axial compression. From the results, it was evident that: (1) Tie bars could increase the axial load-carrying capacity (Maximum 16%; average, 5%) and decrease the strength degradation rate of CFST columns. (2) Tie bars were not effective in improving the elastic stiffness of CFST columns. (3) Tie bars were effective to limit the lateral deformation of the core concrete and the steel tube at the location of tie caps. However, the confining pressure decreased between tie caps at both horizontal and vertical planes.