Fire resilience of a steel-concrete composite floor system :

Abstract

The National Institute of Standards and Technology is currently conducting a series of large compartment fire tests to investigate the behavior and fire-induced failure mechanisms of the full-scale composite floor assemblies with the two-story steel gravity frame, two bays by three bays in plan. This report presents the experimental design and results from the second fire experiment (Test #2) conducted at the National Fire Research Laboratory. The Test #2 was aimed to investigate the influence of the slab reinforcement on the structural integrity of the 9.1 m 6.1 m steel-concrete composite floor subjected to combined mechanical loads and compartment fire exposure. The fire test bay was situated on the ground floor in the middle edge bay of the two-story test building. The floor slab in the test bay was reinforced with the No.3 deformed bars placed 30 cm on center (230 mm2/m). The test floor was hydraulically loaded to 2.7 kPa to mimic the code-prescribed gravity loads for fire conditions. The natural gas burners created a peak gas temperature exceeding 1100 C below the test floor. The test fire lasted about 131 min, but the hydraulic loading was not removed until the test floor cooled down over 2 hours. This experimental study confirmed that the steel reinforcement played a vital role in maintaining the integrity of the composite floor under prolonged compartment fire exposure. The mid-panel vertical displacement increased at a rate less than 1 mm/ C as the protected steel beams were heated to 850 C on average. The peak vertical displacement of the test slab was recorded 475 mm surpassing the displacement limit prescribed in the standard fire test. Although the test slab developed extensive surface cracks, it successfully contained the test fire underneath while sustaining the imposed loads simultaneously. The test floor retained the post-fire flexural strength exceeding 90 % of the ambient design strength of the composite secondary beam prior to fire exposure. The experimental results presented in this report can be used for validation of predictive models to perform parametric studies incorporating the variability in the steel reinforcement scheme (area, spacing, and material) for safer and cost-effective composite floor construction for fire safety.