In　reinforced　concrete　(RC)　frames,　horizontal　progressive　collapse　(H-PC)　may　be　triggered　due　to　insufficient　lateral　stiffness　of　the　structural　systems.　Current　research　studies　have　been　mainly　focused　on　vertical　progressive　collapse　(V-PC)　rather　than　H-PC.　To　analyze　the　effect　of　lateral　stiffness　and　capacity　on　the　behavior　of　progressive　collapse　(PC)　of　RC　frames,　a　static　collapse　test　of　two　asymmetric　one-story　beam-column　substructure　specimens　with　flange　slabs　was　conducted.　The　two　specimens　S1　and　S4　were　subjected　to　an　interior　column　removal　scenario,　in　which　S1　has　one　edge　column　and　S4　has　an　edge　and　a　penultimate　column.　The　test　results　show　that　under　the　compressive　arch　action　(CAA),　S1　developed　a　smaller　peak　resistance　and　a　larger　vertical　displacement　as　compared　to　S4　due　to　insufficient　horizontal　restraint　of　the　columns.　In　addition,　the　contribution　of　the　CAA　within　the　beams　and　slabs　in　S1　was　significantly　reduced.　Under　the　catenary　action　(CA),　the　collapse　resistance　of　S4　developed　rapidly　and　eventually　exceeded　the　peak　resistance　of　CAA;　while　the　resistance　of　S1　remained　almost　constant,　caused　by　the　continuously　weaken　horizontal　constraint　provided　by　the　edge　column.　Additionally,　the　final　failure　mode　for　both　specimens　was　found　to　be　in　form　of　the　compression-bending　failure　of　the　edge　and　penultimate　columns.　©　2018　American　Society　of　Civil　Engineers.