In　addition　to　strengthening　or　weakening　the　connections,　castellated　steel　beams　can　realize　plastic　hinge　rotation,　which　is　a　common　strategy　for　improving　the　resistance　of　steel　structures　against　progressive　collapse.　This　paper　presents　experimental　and　numerical　investigations　of　a　substructure　with　castellated　steel　beams　under　a　column　removal　scenario　considering　the　diameter　D　of　the　openings,　first　opening　position　A,　and　spacing　B　between　openings.　The　deformation　capacity,　vertical　force-deformation　response,　failure　mode,　and　contributions　of　the　flexural　and　catenary　action　were　studied　as　having　different　parameters　for　the　openings.　The　test　results　demonstrate　that　the　first　web　opening　underwent　the　greatest　deformation,　changing　from　a　circular　to　shuttle　shape　at　the　cracking　moment,　whereas　the　second　and　subsequent　openings　exhibited　only　small　deformations　in　all　specimens.　The　vertical　force　vs.　deformation　response　can　be　divided　into　four　states:　elastic,　plastic,　catenary　transition,　and　catenary　states.　The　final　resistance　against　progressive　collapse　was　dependent　on　the　catenary　action　caused　by　excessive　axial　tension.　Moreover,　the　bearing　capacity　and　deformations　in　the　specimens　having　a　small　diameter　of　openings,　D　＜　0.6　h,　decreased　with　an　increase　in　the　opening　diameter,　whereas　the　respective　properties　in　the　specimens　having　a　large　diameter　of　openings,　D　＞　0.6　h,　increased　with　an　increase　in　D.　The　recommended　distance　from　the　edge　of　the　first　opening　to　the　surface　of　the　column　is　equal　to　the　depth　h　of　the　beam　section.　A　large　value　of　spacing　B　between　the　openings　has　a　minor　influence　on　the　bearing　resistance　in　the　collapse　analysis.