Experimental Research on Mechanical Properties of Concrete-filled Double-skin Square Steel Tubular (Oblique Inside and Straight Outside) Columns Under Axial Compression

To investigate the mechanical properties of concrete-filled double-skin square steel tubular (oblique inside & straight outside) columns under axial compression, 8 composite column specimens and 1 ordinary concrete-filled square steel tubular columns specimen SC1 were prepared for the experiment. The parameters include the side length and thickness of inner square steel tube, the thickness of outer square steel tube, the slenderness ratio, and the core area proportion. In this study, the failure process and load-displacement curves were obtained. The failure modes, axial compressive bearing capacity, ductility, energy dissipation, stifness degradation and the effects of variable parameters on the mechanical properties of specimens were analyzed. The results showed that there were two failure modes:local buckling failure and bending failure in plastic plase. The inner tube showed continuous folds under the combined action of core concrete and sandwich concrete, while the outer tube heaved outward caused by the squeezing action of concrete. The steel tube bulged out in different degrees in the debonding area, and debonding area with large area and close distance had obvious influence on the final failure mode of the specimens. Compared with specimen SC1, the bearing capacity of composite column specimen with little difference in steel ratio had been increased by 35%. The load-displacement curves had no descending section and showed excellent plastic hardening characteristics. The initial stiffness and peak load of the specimen increased with the increasing of the width-thickness ratio of the inner steel tube. Affected by the uneven mixing of concrete, increasing the steel tube thickness (inside or outside) could effectively improve the mechanical properties of the specimen, but the peak load remained unchanged. Increasing the area ratio of core column could give full play to the advantages of steel tube and concrete when the steel content was not different. The calcuation method considering the effects of constratints of the superposition of double-skin square steel tube concrete on the inner was used to calculate the bearing capacity of specimens, which is in good agreement with the measured results.