Analysis of Temperature and Deflection Behavior of a Steel-Deck Plate Composite Slab Under Real Fire for Assessing Structural Instability During Firefighter Operations

This study investigated the thermal and deflection behavior of a steel–deck plate composite slab under real-fire conditions to provide fundamental information for assessing structural instability during firefighter operations. A steel-framed test specimen with a 0.15 m composite slab was subjected to a gravity load representative of the design service condition and exposed to an outdoor compartment fire with a single opening. Temperatures of the beams, columns, and deck-plate surfaces were recorded using thermocouples, while structural deformation was assessed using conventional video, thermal imaging, and three-dimensional laser scanning before and after the test. Non-uniform heating occurred due to the configuration of the fire source and opening. Downward deformation of the rear beam was first detected approximately 20 min following ignition, followed by a noticeable increase in visible downward deformation at approximately 35 min after ignition. Valid upper-floor surface temperatures remained below approximately 50 °C, whereas locally high temperatures were recorded on the fire-exposed deck-plate surface. After cooling, an approximate residual downward deformation of 0.2 m was identified near the rear beam, corresponding to a deformation scale of approximately L/25 for a 5 m span. The findings indicate that structural instability should be assessed by considering the duration of high-temperature exposure, observed deformation, upper-floor surface temperature increase, and residual deformation collectively, rather than relying on a single critical temperature or time threshold. Because continuous displacement and heat release rate were not measured, the results provide event-based indicators rather than formal criteria for collapse or fire resistance.