COMPOSITE EFFECT OF STUB SQUARE STEEL TUBED COLUMNS UNDER AXIAL COMPRESSION

The in-filled concrete of square tube confined concrete columns (tubed columns for short) is non-uniformly confined and the effectiveness of its confinement is reduced, leading to a complex composite effect. This paper discusses key parameters, including friction, width-to-thickness ratio and chamfered corner radius of steel tubes, which would affect the confinement of square tubes on in-filled concrete. Eighteen specimens with large width-to-thickness ratio and six RC counterparts were tested under axial compression. Four main system parameters were considered in the tests: 1) width-to-thickness ratio (60-160); 2) types of steel tubes (galvanized and ordinary); 3) interface between tube and concrete (reduced friction and not); and 4) with and without reinforcement cage. It was found that the square tubed RC and plain RC columns were characterized by the shear failure mode, but the ductility performance of the tubed columns was much better than the RC specimens. The axial load-carrying capacities of the specimens with smaller friction were slightly lower than those with larger friction. A finite element analysis (FEA) model was developed to analyze the influence of friction and corner radius on of square tubed columns. An effective section confining model considering effective utilization index of square steel tubes was developed to predict the axial load resistances. The results are satisfactory when comparing the predictions to the experimental and nonlinear finite element analysis results.