After　steel　frame　structures　experience　a　fire　and　are　subsequently　subjected　to　extreme　loads,　the　mechanisms　of　progressive　collapse　resistance　and　the　weak　points　in　the　load　transfer　path　of　the　remaining　structure　may　differ　from　those　observed　under　ambient　conditions.　This　study　investigates　the　progressive　collapse　performance　of　steel　frame　structures　with　reduced　beam　section　(RBS)　connections　post-fire,　using　ten　beam-column　substructures—one　at　ambient　temperature　and　nine　exposed　to　varying　fire　conditions.　Results　indicate　that　fire　temperature　more　significantly　impacts　collapse　resistance　and　deformation　capacity　than　fire　duration.　Post-fire,　the　failure　mode　shifts　from　the　reduced　section　to　the　joint　weld　connection,　compromising　the　RBS＇s　ability　to　relocate　the　plastic　hinge.　Numerical　simulations　show　that　reinforcing　the　beam-column　weld　delays　failure　but　does　not　substantially　improve　collapse　resistance.　However,　flexible　reinforcement　with　V-shaped　stiffening　plates　markedly　enhances　both　collapse　resistance　and　deformation　capacity,　with　ultimate　load　improvement　being　approximately　twice　that　of　ultimate　displacement.　Determining　the　appropriate　corrugation　height　is　crucial;　insufficient　height　impedes　deformation,　while　excessive　height　becomes　effective　only　after　substantial　damage　to　the　RBS.　This　study　underscores　the　significance　of　selecting　a　corrugation　height　of　0.15　times　the　beam　depth,　which　optimally　balances　energy　dissipation　and　plastic　deformation　capacity　with　the　post-fire　progressive　collapse　resistance　of　the　substructure,　offering　critical　guidance　for　the　design　of　reinforcement　in　steel　frame　structures.　©　2025