Blast Vulnerability of Drop Ceilings in Roadway Tunnels

Tunnels are critical links within modern transportation networks and are susceptible to accidental explosion 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 $100\mathrm{years}$ have typically consisted of a road surface, tunnel liner, and drop ceiling. The tunnel liner was the main structural component that resists any over-tunnel loads as well as preventing substrate intrusion and cave-ins. 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 conduits, utilities, fire alarms, and fire suppression systems. The typical design of a tunnel drop ceiling only considers gravity loading and forces exerted by pressure within the ventilation plenum—severe impulsive uplift loading from a blast on the roadway below therefore presents significant risk of ceiling panel damage or collapse. This study evaluates the likelihood of significant damage and collapse of tunnel drop ceilings under blast loading. Six scenarios consisting of three trinitrotoluene (TNT) equivalent 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 panel elements. Dynamic single-degree-of-freedom (SDOF) analyses were verified against finite-element (FE) solutions and used to evaluate the damage for a wide range of blast demands. The results indicate that modestly sized explosive hazards can induce significant damage to the drop ceiling, ranging from permanent deformation up to widespread collapse. Hardening options include flexural enhancement with fiber-reinforced polymer (FRP) as well as retrofit of the hangers. As an alternative, removing the ceiling and reconfiguring the ventilation system with modern jet fans can be a more cost-effective option for blast mitigation depending on the tunnel owner’s operational and/or financial constraints.