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Resilience Assessment of Tunnel Drop Ceilings Exposed to Blast and Fire
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2022-09-01
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[PDF-7.25 MB]
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Edition:Final Report (Sept. 2016 to Oct. 2018)
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Abstract:Tunnels are critical links within modern transportation networks and are susceptible to accidental explosion and fire from vehicle fuel and cargo. They also present a target for terrorism due to unscreened public access. Roadway tunnels constructed with circular, oval, or horseshoe cross-sections in the United States within the last 100 years have typically consisted of a road surface, tunnel liner, and drop ceiling. The drop ceiling is hung from the liner and creates a plenum above the roadway for ventilation of vehicle exhaust as well as throughput for electrical conduit, utilities, fire alarms, and fire suppression systems. Drop ceiling panels are lightweight relative to the much thicker tunnel liner and can experience significant damage due to a fire or blast event on the roadway below. Two studies are conducted on prototype drop ceiling systems examining bot the resistance to blast and fire events. The studies examine the likelihood of significant damage and collapse of tunnel drop ceilings. Six blast scenarios consisting of three Trinitrotoluene (TNT) charge sizes in two representative tunnels were analyzed using computational fluid dynamics (CFD) to generate blast demands on the drop ceiling, followed by a blast vulnerability assessment of the drop ceiling panels. The results illustrate that modestly sized explosive hazards can induce significant damage ranging from permanent deformation up to widespread collapse. Retrofit options include hardening via flexural enhancement with fiber reinforced polymer (FRP) as well as retrofit of the hangers; however, removal of the ceiling and reconfiguring the ventilation system with modern jet fans may be a more cost-effective option for blast mitigation. The systems are also evaluated to assess performance under fire events. Standard fire demands per the Rijkswaterstaat (RWS) and ASTM E1529 fire curves are uniformly applied to the ceiling panels, while the heat exposure contours for typical vehicle fires with heat release rates of 30 MW, 100 MW, and 200 MW are generated from the software Consolidated Fire and Smoke Transport (CFAST). The analysis results indicate that drop ceiling panels are highly vulnerable to fire-induced damage and potential collapse. Fire-induced damage can be mitigated by reducing the fire hazard, removing the drop ceiling, or enhancing the fire resistance of the panels via the application of passive protection or structural hardening.
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