Behaviour of uni-axially loaded CFST columns confined by tie bars

Abstract

Concrete-filled-steel-tube (CFST) columns have been widely adopted for column construction of tall buildings 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, which reduces the elastic strength and stiffness. To resolve the problem, confinement in the form of tie bars is proposed in this study to restrict the elastic lateral dilation of concrete. For verification, 30 CFST columns of various dimensions cast with normal- (NSC) or high-strength concrete (HSC) and installed with tie bars were tested under uni-axial compression in this study. From the results, it was evident that: (1) tie-confined CFST columns had uni-axial strength larger than those of unconfined CFST columns. (2) Tie bars can increase slightly the elastic stiffness of the CFST columns. (3) Tie bars can restrict effectively the lateral dilation of CFST columns. (4) The failure modes of tie-confined NSCFST and HSCFST columns were the local buckling at the ends and formation of longitudinal steel cracks initiated at bolt holes. Lastly, based on the test results, analytical model taking into account the confining effects of steel tube have been developed to predict the uni-axial strength of tie-confined CFST columns.