This　study　presents　an　experimental　and　numerical　investigation　of　the　hysteretic　behavior　of　concrete　filled　thin-walled　steel　tubular　K-joints　with　spherical-cap　gaps.　Twelve　concrete-filled　steel　tubular　(CFST)　bottom　chord-steel　tubular　web　K-joint　specimens　were　tested　under　cyclic　loading　applied　to　the　thin-walled　CFST　chord.　The　influence　of　spherical-cap　gap　ratios,　web-to-chord　diameter　ratios,　and　chord　diameter-to-thickness　ratios　on　the　failure　modes,　overall　structural　behavior,　load-carrying　capacity,　stiffness,　and　ductility　was　examined.　Numerical　analyses　were　conducted　to　validate　the　experimental　results　and　perform　parametric　studies　to　further　investigate　the　impact　of　gap　ratios,　geometry,　and　material　properties　on　the　structural　behavior.　Additionally,　an　in-depth　analysis　of　the　stress　distribution　between　the　web　and　CFST　chord　at　the　K-joint　was　performed　to　reveal　the　influence　of　spherical-cap　gap　on　the　load　transfer　mechanism.　Existing　design　formulae　were　calibrated　using　proposed　strength　reduction　factors　to　calculate　the　load-carrying　capacity　of　these　K-joints　based　on　different　failure　modes.　The　results　demonstrate　that　the　presence　of　spherical-cap　gaps　can　have　a　detrimental　effect　on　the　overall　structural　integrity.　The　findings　of　this　study　provide　valuable　insights　for　the　design　and　safety　assessment　of　thin-walled　CFST　K-joint　connections　in　buildings　and　bridges.　©　2025