A　series　of　tests　on　concrete　filled　double　skin　steel　tubular　(CFDST)　stub　columns　(14)　and　beam-columns　(12)　were　carried　out.　Both　outer　and　inner　tubes　were　circular　hollow　sections　(CHS).　The　main　experimental　parameters　for　stub　columns　were　the　diameter-to-thickness　ratio　and　hollow　section　ratio,　while　those　for　beam-columns　were　slenderness　ratio　and　load　eccentricity.　A　theoretical　model　is　developed　in　this　paper　for　CFDST　stub　columns　and　beam-columns.　A　unified　theory　is　described　where　a　confinement　factor　(xi)　is　introduced　to　describe　the　composite　action　between　the　outer　steel　tube　and　the　sandwiched　concrete.　The　predicted　load　versus　deformation　relationships　are　in　good　agreement　with　stub　column　and　beam-column　test　results.　Simplified　models　are　derived　to　predict　the　load-carrying　capacities　of　the　composite　members.　(C)　2003　Elsevier　Ltd.　All　rights　reserved.