The　behaviors　of　Concrete-filled　steel　tubular　(CFST)　directly-welded　K-Joints　with　studs　on　the　inner　wall　of　the　main　steel　tube　(inner　stud)　were　tested,　as　well　as　a　specimen　without　stud　for　comparison.　During　the　test,　the　main　tube　was　under　axial　pressure,　while　the　two　web　members　were　equivalently　and　simultaneously　under　a　pressure　and　a　tension,　respectively.　The　results　show　that　for　the　buckling　failure　of　web　member,　the　load-displacement　curve　of　compressive　web　member　consists　of　a　linear　elastic　stage　and　an　elastic-plasic　stage.　With　or　without　studs,　the　strains　of　the　main　tube　in　welding　area　are　larger　than　those　in　other　areas,　whose　load-strain　curves　include　linear　elastic　stage　and　elastic-plasic　stage,　while　the　strains　in　the　other　areas　are　in　elastic　state.　The　stain　distribution　of　stud　does　not　change,　but　the　stress　magnitude　is　reduced,　indicating　that　the　studs　are　able　to　transfer　force　from　steel　tube　to　core　concrete,　especially　at　the　strain　concentration　area.　In　the　axial　direction　of　main　tube,　strain　concentration　are　observed　at　the　two　sides　of　the　center　section　of　joint,　with　a　range　of　2.7　times　of　the　side　tube　diameter.　In　the　circular　direction,　strains　mainly　concentrate　in　an　area　between　directions　-60°　and　60°.　It　is　suggested　that　the　inner　studs　should　be　arranged　mainly　on　these　longitudinal　and　transverse　areas,　and　sparsely　or　not　arranged　outside.　©　2017,　Engineering　Mechanics　Press.　All　right　reserved.