Abstract
Previous studies have shown that concrete-filled double-skin circular tube (CFDSCT) columns can offer equal or better performance than the concrete-filled circular steel tube (CFCST) columns; however, there is little knowledge about the contribution of the internal steel tube to the compressive resistance of the columns. Since this element provides the distinction between CFDSCT and CFCST columns, it is important to fully understand its contribution. To achieve this, an experimental study was performed to examine the effect of the internal steel tube on the behaviour of CFDSCT columns. A total of 26 stub columns were tested to failure, with two diameter-to-thickness ratios (d/t) for the internal tube of 25.4 and 38.1 and diameter-to-thickness ratios (d/t) of the external tubes of 47–65. Two concrete grades (Grades 30 and 50) of self-compacting concrete (SCC) were targeted; however, the large specimens (S193.7 and S219.1) were tested later than the rest of the specimens, which increased the strength of the concrete to Grade 38 and 58, respectively, at the time of testing. Most of the external tubes in the CFDSCT columns were deliberately slender to examine the effects of local buckling. The failure modes of the tested specimens were outward local buckling of the external steel tube, inward local buckling of the internal steel tube, and crushing of the sandwiched concrete in the buckled regions. The results of the experimental investigation revealed that there was confinement of the concrete core and significant ductility in some of the CFDSCT columns. Based on the experimental results, a model was developed to predict the compressive strengths of the specimens. The proposed formula was applied to more specimens collected from the literature, and the results show that the formula can estimate the capacity of short CFDSCT columns to a reasonable degree of accuracy.