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Fire Resilience of a Steel-Concrete Composite Floor System: Full Scale Experimental Evaluation for Influence of Slab Reinforcement and Unprotected Secondary Beam (Test #3)

Published

Author(s)

Selvarajah Ramesh, Lisa Choe, Matthew Hoehler, Matthew Bundy, Rodney Bryant, Giovanni Di Cristina Torres, Brian Story, Anthony R. Chakalis, Artur A. Chernovsky, Philip Deardorff, Michael Selepak

Abstract

The National Fire Research Laboratory at the National Institute of Standards and Technology conducted a series of large compartment fire tests to investigate the behavior and fire-induced failure mechanisms of full-scale composite floor assemblies with a two-story steel gravity frame, two bays by three bays in plan. A total of three 9.1 m × 6.1 m composite floor specimens with varying slab reinforcement and fire protection schemes for the secondary beam were tested under combined mechanical loads and compartment fire exposure. This report presents the experimental design and results from the third composite floor fire experiment (Test #3). The first experiment (Test #1) was designed to achieve a 2-hour fire resistance rating per current U.S. practice and create baseline data for the behavior of the building. Test #1 specimen exhibited mid-panel slab integrity failure at 70 mins of fire exposure. Test #2 was conducted to study the effect of enhanced slab reinforcement with larger area and ductility on the fire resilience of the composite floor systems. Test #2 showed that the use of enhanced slab reinforcement maintained the structural integrity of the tested slab for more than two hours. Test #3 was conducted to study the effect of enhanced slab reinforcement as well as unprotected secondary beams on the fire resilience of the composite floor systems. Similar to Test #2, the floor slab in the test bay was reinforced with 9.5 mm diameter deformed bars with a center-to-center spacing of 30 cm (230 mm2/m). Unlike in Test #1 and Test #2, the secondary beam and its end connections in the test bay were left unprotected in Test #3. The test floor was mechanically loaded to 2.7 kPa to mimic the code-prescribed gravity loads for fire conditions. The compartment test fire created upper-layer gas temperatures like those in standard fire resistance tests. A total of four natural gas burners distributed over the compartment floor created a peak gas temperature exceeding 1100 °C below the test floor. A wide transverse crack with flame leak above the floor developed at 132 min in the mid-panel region. The mid-panel vertical displacement reached 535 mm (equivalent to the ratio of L/17 where the span length L = 9.1 m) It reached a peak value of 655 mm (L/14) at 140 min, when the actuator loading was removed. The Test #3 showed that the use of deformed steel bars (230 mm2/m) for the slab reinforcement maintained the structural integrity of the tested slab longer than the specified rating period with an unprotected secondary beam. 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.
Citation
Technical Note (NIST TN) - 2267
Report Number
2267

Keywords

Compartment fire experiments, Composite floors, Fire resistance, Slab reinforcement, Unprotected secondary beam

Citation

Ramesh, S. , Choe, L. , Hoehler, M. , Bundy, M. , Bryant, R. , Di Cristina Torres, G. , Story, B. , Chakalis, A. , Chernovsky, A. , Deardorff, P. and Selepak, M. (2023), Fire Resilience of a Steel-Concrete Composite Floor System: Full Scale Experimental Evaluation for Influence of Slab Reinforcement and Unprotected Secondary Beam (Test #3), Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2267, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=956628 (Accessed April 23, 2024)
Created October 2, 2023