Stochastic Thermal Demand and Resulting Capacity Loss of Concrete Tunnel Liners Subjected to Vehicle Fires

Abstract

Fire resistant design of both structural and non-structural components in road tunnels is predicated on the determination of fire demand intensity. Current practice typically uses a conservative, deterministic fire curve that does not necessarily provide a representative evaluation of the spatial and temporal distribution of thermal demands in tunnels that are caused by large vehicle fires. This paper proposes a tunnel-specific probabilistic framework for evaluating vehicle fire frequency and intensity based on tunnel geometry and traffic information. The framework leverages a fast-running computational tool that has been previously developed by the authors for calculating fire-induced heat flux exposure on tunnel liners because of enclosed vehicle fires. The likelihood of a vehicular fire and the associated fire size distribution are used to generate probabilistic distributions of total fire exposure for the reinforced concrete tunnel liner. Critical heat flux values according to these probabilistic distributions are then used to assess reductions in concrete material strength and resulting losses in the structural performance of the system. A case study of the Fort Pitt Tunnel in Pittsburgh, PA, is included for demonstration. The proposed framework enables decision making regarding design and renovation of tunnels for fire resistance as well as post-fire inspection by quantifying the risk of capacity reduction in the concrete liner because of a realistic range of fire hazard intensities.

Vulnerability of Drop Ceilings in Roadway Tunnels to Fire-Induced Damage

“…This ''shutdown'' of the RWS curve is used to allow a component that has survived the 2-h exposure (thus meeting the NFPA 502 fire resistance requirement) to undergo subsequent cooling. A similar approach to implementing the RWS curve was used by the authors to evaluate the residual state of the tunnel liner for a range of fire sizes (5). Also plotted in Figure 3 is the ASTM E1529 standard curve (10), which represents exposure to a large hydrocarbon fire by prescribing a rapid increase to 1,095°C in 5 min followed by an indefinite period of constant exposure at that temperature until the failure criterion is reached.…”

“…According to data from the National Highway Traffic Safety Administration, the number of annual car crashes in the United States is trending upward, from 5.4 million crashes reported in 2010 to 6.4 million crashes reported in 2017 (4). With greater crash risk comes a higher likelihood of a subsequent fire, the severity of which will depend on the vehicles involved (1,5). Roadway tunnels have an extensive history of fire events that have resulted in damage severities ranging from limited non-structural effects to severe structural damage, including individual element collapse in some of the worst cases (6).…”

“…Roadway tunnels have an extensive history of fire events that have resulted in damage severities ranging from limited non-structural effects to severe structural damage, including individual element collapse in some of the worst cases (6). The constant presence of moving vehicles carrying combustible and/or hazardous materials (both vehicle fuel and cargo) through an enclosed environment makes tunnels susceptible to severe fire events (5). As such, tunnels are often more vulnerable to fire than other highway structures such as bridges, which are typically subjected to open-air fire hazards (7).…”

“…In this paper, the performance of conventional drop ceilings is examined numerically using standard fire exposure curves as well as thermal demands from simulated vehicle fire events, which illustrate the effect of realistic heat distribution on the structural behavior of the panel. The sizes of the realistic fire events are selected based on a recently completed stochastic study of thermal demands from vehicle fires, which summarizes the cumulative frequency distribution for potential heat release rates (HRRs) in the Fort Pitt Tunnel near Pittsburgh, Pennsylvania (5). As per NFPA 502, the HRR of fire events from heavy goods vehicles (HGV) can range from 20 to 200 MW (1).…”