Thin-walled　square　concrete-filled　double　steel　tubular　(CFDST)　columns　composed　of　the　inner　circular　tube　filled　with　concrete　can　be　used　to　carry　the　large　axial　loads　or　strengthen　existing　CFST　columns　in　composite　constructions.　This　paper　reports　an　experimental　program　carried　out　on　short　square　CFDST　columns　loaded　concentrically.　The　influences　of　important　column　parameters　on　the　post-buckling　performance　of　such　columns　are　investigated.　Test　results　exhibit　that　the　inner　circular　tube　significantly　improves　the　ultimate　loads　and　the　ductility　of　such　columns　compared　to　conventional　concrete-filled　steel　tubular　(CFST)　and　double-skin　CFST　(DCFST)　columns　with　an　inner　void.　A　mathematical　model　developed　is　used　to　simulate　the　ultimate　strengths　and　load-strain　curves　of　such　columns　loaded　axially.　Furthermore,　the　ultimate　strengths　of　such　columns　are　predicted　using　existing　codified　design　models　for　conventional　CFST　columns　as　well　as　the　formulas　proposed　by　previous　researchers　and　compared　against　a　large　database　comprising　500　CFDST　columns.　Lastly,　an　accurate　artificial　neural　network　model　is　developed　for　the　practical　applications　of　such　columns　under　axial　loading.