Fourteen　specimens　of　concrete　filled　double　skin　steel　tubular　(CFDST)　stub　columns　were　tested.　Both　outer　and　inner　tubes　were　circular　hollow　sections　(CHS).　The　main　experimental　parameters　were　diameter-to-thickness　ratio　and　hollowness　ratio.　A　mechanics　model　is　developed　in　this　paper　for　the　CFDST　stub　columns.　A　unified　theory　is　described　where　a　confinement　factor　(xi)　is　introduced　to　describe　the　composite　action　between　the　steel　tube　and　filled　concrete.　The　predicted　load　versus　axial　strain　relationship　is　in　good　agreement　with　stub　column　test　results.　Simplified　models　are　derived　to　predict　the　load-carrying　capacities　of　the　composite　sections.