Torsional behavior of a new dumbbell-shaped concrete-filled steel tubes

This paper presents the first study of its kind on the torsional moment behavior of a new dumbbell-shape concrete-filled steel tube (DCFST) system. Experiments were conducted to investigate the effect of the dimensions of the web connecting the two circular CFSTs, in terms of its width-to-diameter (b/D) and its minimum depth-to-diameter (h/D) ratios, on torsional strength (Tu). Torsion tests were also conducted on circular CFSTs (CCFSTs), on hollow tubes and on plain concrete, as control specimens. It was shown that Tu of DCFSTs ranged from 1.3 to 2.2 that of the CCFST, depending on (b/D) and (h/D) ratios. Increasing (b/D) from 0.31 to 0.54 resulted in only 7% increase in Tu. However, a further increase from 0.54 to 1.0 resulted in an additional 31% increase in Tu. The effect of (h/D) on Tu was more pronounced at higher (b/D) ratios. In general, increasing the web width appears to be more effective than web height. A rigorous nonlinear finite element model was developed and a simple mechanics-based design equation have been proposed to predict Tu of CCFSTs and DCFSTs and resulted in an average predicted-to-experimental Tu of 0.98.