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　(T-u).　Torsion　tests　were　also　conducted　on　circular　CFSTs　(CCFSTs),　on　hollow　tubes　and　on　plain　concrete,　as　control　specimens.　It　was　shown　that　T-u　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　T-u.　However,　a　further　increase　from　0.54　to　1.0　resulted　in　an　additional　31%　increase　in　T-u.　The　effect　of　(h/D)　on　T-u　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　T-u　of　CCFSTs　and　DCFSTs　and　resulted　in　an　average　predicted-to-experimental　T-u　of　0.98.