This　paper　investigates　the　behavior　of　X-joints　with　square　hollow　section　(SHS)　chord　and　circular　hollow　section　(CHS)　braces　(XSCCB)　under　brace　axial　loading.　A　total　of　twenty-nine　specimens　were　designed,　including　twenty-three　empty　specimens,　three　specimens　filled　with　concrete　only　in　the　square　chord　and　three　specimens　filled　with　concrete　only　in　the　circular　braces.　The　effect　of　four　influential　factors　was　studied,　including　brace　diameter　to　chord　width　ratio　(beta　=　d/b),　chord　width　to　double　wall　thickness　ratio　(gamma　=　b/2　T),　brace　wall　thickness　to　chord　wall　thickness　ratio　(tau　=　t/T)　and　partial　concrete　filling　on　the　behavior　of　XSCCB　under　brace　axial　loading.　Initial　stiffness　and　ultimate　bearing　capacity　of　empty　XSCCB　increase　as　the　beta　increases　and　the　gamma　decreases.　The　ductility　coefficient　of　empty　XSCCB　increases　as　beta　increases,　while　gamma　has　little　effect　on　their　ductility　coefficient.　Also　tau　has　a　minor　effect　on　the　ultimate　bearing　capacity,　initial　stiffness　and　ductility　coefficient　of　empty　XSCCB.　However,　for　XSCCB　filled　with　concrete　only　in　the　chord　member,　the　initial　stiffness,　ultimate　bearing　capacity　and　ductility　coefficient　increase　as　tau　increases.　For　the　XSCCB　filled　with　concrete　only　in　the　braces,　tau　has　little　effect　on　the　ultimate　bearing　capacity,　initial　stiffness　and　ductility　coefficient　Chinese　Code　and　Eurocode　3　are　generally　conservative　for　designing　empty　XSCCB　under　brace　axial　loading.　Hence,　the　corresponding　formulas　are　proposed　for　calculating　the　ultimate　bearing　capacity　of　empty　XSCCB　under　brace　axial　loading.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.