Effectivness of interior stiffeners on axially loaded concrete-filled square tubular steel columns

In this study, a stiffening scheme for square concrete-filled steel tubes (CFTs) under axial load is proposed. This scheme, called tie-bar stiffening, applies several sets of stiffeners, each of which consists of four tie bars, welded at certain spacing along the tube axis. Such stiffened square steel tubes are supposed to provide better confinement to the concrete cores under axial compression. A total of 19 specimens, unstiffened or tie-bar stiffened with various spacings and diameters, have been tested for verifying the feasibility of this scheme. Experimental results indicate that this tie-bar stiffening scheme can substantially delay the occurrence of local buckling of the tube. As a result, the strength capacity and ductility of a stiffened square CFT are improved. The concrete core in the square steel tube is found well confined through the maintenance of cross-sectional shape of the steel tube due to the presence of these stiffening tie bars. Effects of different arrangements of stiffeners by changing their spacing and diameter of tie bars on the performance of square CFTs, in terms of their strength, stiffness and ductility, are also discussed.