Steel　frame　structures　repaired　after　fire　exposure　exhibit　markedly　different　collapse　behavior,　compared　to　their　performance　under　ambient　conditions,　when　subjected　to　extreme　loads.　This　study　investigates　the　progressive　collapse　resistance　of　steel　frame　structures　with　reduced　beam　section　(RBS)　connections　in　post-fire　conditions,　using　ten　beam-column　substructures:　one　tested　at　room　temperature　and　nine　exposed　to　various　fire　conditions.　Results　show　that　fire　exposure　shifts　the　failure　from　the　RBS　to　the　beam-column　connection,　significantly　impairing　the　RBS＇s　ability　to　relocate　the　plastic　hinge,　especially　at　higher　fire　temperatures.　Fire　temperature　significantly　affects　collapse　resistance,　especially　above　600　°C,　whereas　fire　duration　has　a　comparatively　smaller　influence　on　deformation　capacity,　particularly　at　800　°C.　Elevated　temperatures　weaken　tensile　catenary　action　(TCA),　with　substructures　exposed　to　800　°C　for　90　min　failing　to　transition　to　the　TCA-dominated　stage.　Numerical　simulations　show　that　for　substructures　exposed　to　400　°C　and　600　°C,　collapse　resistance　increases　with　greater　flange　reduction　length,　while　the　relationship　between　collapse　resistance　and　starting　reduction　distance　follows　a　rise-and-fall　pattern.　At　800　°C,　collapse　resistance　remains　relatively　consistent　across　different　starting　reduction　distances,　but　increasing　the　reduction　length　initially　enhances　and　then　reduces　resistance.　Increasing　the　reduction　depth　to　30　mm　significantly　reduces　both　the　flexural　and　tensile　capacities　of　the　RBS　region,　shifting　the　failure　mode　from　the　beam-column　connection　to　the　RBS　region.　©　2024　Elsevier　Ltd