Behavior of octagonal concrete-filled double-skin steel tube stub columns under axial compression

This paper presents an experimental investigation on the behavior of stiffened octagonal concrete-filled double-skin tube (CFDST) stub columns subjected to axial compression. A total of eight specimens were prepared and tested under axial compression aiming to study the effects of longitudinal stiffeners and hollow ratio on the elastic-plastic local buckling and ultimate load-carrying capacity of the test columns. The specimens consist of two control specimens without stiffeners (unstiffened columns) and two specimens stiffened with outer-inner tubes welded stiffeners (OI stiffeners) in four sides of the octagonal steel section. In addition, four specimens were stiffened with two types of stiffeners: OI stiffeners and outer tube welded stiffeners (O stiffeners) in eight sides of the outer steel tube section. Furthermore, two hollow ratios were employed. Based on the test results, the ultimate load-carrying capacity, load-displacement curves, load-strain response, ductility, and failure modes were discussed and clarified. The test results indicated that the strength and ductility of the specimens were improved greatly by the presence of the stiffeners. The stiffeners significantly enhanced the confinement and prevented the occurrence of elastic-plastic local buckling of the outer tube. Besides, current design equations were used to predict the ultimate load-carrying capacity of octagonal CFDST stub columns. The test results were then compared with the predicted ultimate capacities of existing design equations. These predictions showed good agreement with the test results.