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97 CHAPTER ELEVEN DESIGN FIRE SCENARIO FOR FIRE MODELING The preceding chapter summarized information about fire Collision incidents (a collision of two to three passen- dynamics and the release of heat and toxic gases based on ger cars, of a passenger car with a truck or bus, or of a the literature review. Design fire scenarios are discussed in bus with a truck). chapter nine. A fire scenario is designed to provide an opti- mum fire life safety strategy for road tunnels. Design fire The consequences of fire incidents in the following traffic scenario discussion found in the literature is summarized in situations are investigated according to the characteristics of this chapter. the tunnel, such as an urban tunnel: Fire scenarios are used for the following: Congested traffic (e.g., rush hours) Traffic jam (e.g., as a result of another accident) Design of emergency exits, Flowing dense traffic (e.g., increased probability of Choice of a fire-detection system, multiple vehicle incidents). Choice of ventilation and fire suppression systems, Tunnel structural engineering, The worst conditions may not be considered in the design Specification requirements for tunnel structures and or may not be correctly identified in design. For example, an equipment, assumption is usually made based on one incident at a time. Operation of the tunnel, and In rear situation collisions, one incident may lead to another, Training of operators and first responders dealing with such as when a blackout leads to a collision and then a fire tunnel fires. event. Fire scenarios usually include: TIMETEMPERATURE AND TIME-OF-TENABILITY CURVES Governing standards and guidelines; Description of the scenario; TimeTemperature Curve Thorough definition of the fire parameters (e.g., HRR/ temperature versus time); If the specific fire scenario is known, such as with a truck Traffic scenario operation during fire emergency and with a specific load, it is recommended that a predetermined tunnel ventilation operation; timetemperature curve be used when designing the tunnel Guidelines for structural protection; and structure and equipment. Specifications for materials, equipment, and structure. Ideally, for a given fire scenario such as a single burning In general, a broad spectrum of design fire scenarios is pos- car, fire curves are used together with different exposure times. sible regarding their different goals (tunnel construction, equip- There are a number of known timetemperature curves used ment, tunnel operation). Therefore, the intention is to select worldwide and these are presented in Figure 27. the most important design fires and to prepare a short descrip- tion of the fire scenarios, as shown in Table 30. The Dutch RWStemperature curve includes the most strin- gent temperature requirements and is referenced in NFPA 502 The fire HRR of a vehicle is one of the most important for structural design, as shown in Figure 28. The RWS curve parameters. It is a main parameter in the calculation of criti- was developed by the Rijkswaterstaat, the Dutch Ministry of cal velocity required to prevent backlayering of smoke and Transport, Public Works, and Water Management and applies heat resulting from a fire, which in turn determines the air- to tunnels that are open to the transport of hazardous sub- flow required to be delivered by a longitudinal system of ven- stances. This curve is based on the assumption that, in a worst tilation. Among the possible fire loads the following vehicle case scenario, a 50 m3 (1,765 ft3) fuel, oil, or gasoline tanker fires are considered: fire with a fire load of 300 MW (1024 MBtu/hr) occurs, lasting up to 120 min (65). The RWS curve was based on the results Incidents with one vehicle (car, bus, truck, or gasoline of testing carried out by TNO (the Netherlands Organization tanker), and for Applied Scientific Research) in 1979. Recently, the accu-

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98 TABLE 30 EXAMPLES OF DESIGN FIRE SCENARIOS BASED ON INTERNATIONAL STANDARDS Fire Scenarios Important Requirements Description of Examples of No. Purpose That Have to Be Met the Design Fire Related Standards 1 Test of construction - Temperature at the interface - Time dependence of Dutch material for of heat insulation panels and the temperature in K.I.V.I. and immersed reinforced the concrete of the tunnel the test oven Rijkswater- concrete tunnel structure may not exceed according to the staat structures, when 380C (716F). RWS curve. guidelines passing of - Temperature at the steel - Maximum dangerous goods reinforcement of the tunnel temperature 1350C such as gasoline structure may not exceed (2462F)--duration of tankers is allowed 250C (482F). the test burning: 2 h. 2 Test of construction Temperature at the steel - Time dependence of ZTV-Tunnel, material for reinforcement of the tunnel the temperature in Germany reinforced concrete structure may not exceed 300C the test oven tunnel structures (572F). according to the ZTV when: Tunnel. - Dangerous good are - Maximum allowed and temperature 1200C - An immediate (2192F)--duration of tunnel collapse or the test burning: 1 h water intake is not 50 min (decline anticipated phase included). 3 Test of jet fans for The jet fans and their related The test equipment RABT 1994, longitudinal equipment for the electrical must be able to deliver Germany ventilation systems power supply must work at least hot soot-enriched air at 90 min, when hot air and smoke a temperature of 250C (temperature 250C or 482F) is (482F) for at least 90 flowing through them and min. surrounding them. 4 Designing of a - Enough power to push the - Fire data: see no. 2 RABT 1994, longitudinal smoke in one direction of - Smoke generation: Germany ventilation system the tunnel (e.g., account for approx. 60 m3/s with jet fans capable thrust loss of fans in hot air). (2,119 ft3/s) at a of controling a truck - Choice of fan distribution reference fire event with a along the tunnel for retaining temperature of calorific heat output enough fans for smoke 300C or 572F. of approximately control when some fans are 20 MW (68 MBtu/hr) damaged by the fire. - Availability of a fan operation mode which keeps emergency paths free from smoke. Source: PIARC (21). racy of the RWS fire curve as a design fire curve for road tun- tunnel with a single vehicle fire, the tunnel lining is exposed nels was reconfirmed in the full-scale tests in the Runehamar locally to heat fluxes from the flame volume and the hot Tunnel in Norway. The RWS curve and the temperature devel- smoky gases. opment table of the RWS fire curve is presented in the Annex (Explanatory material to Protection of Structural Elements) In a tunnel accident with multiple vehicles, the fire spreads of NFPA 502 (see Figure 28). from one vehicle to the next resulting in different heat expo- sures to the tunnel lining depending on time, location, fuel The RWS curve is based on the level of temperature found load, and oxygen available. The fire moves within the tunnel when a fire occurs in an enclosed area, such as a tunnel, in a dynamic manner and the heat fluxes to the linings vary where there is little or no chance of heat dissipating into the depending on the origin of the fire, the ventilation rate, the type surroundings. The RWS curve simulates the initial rapid and amount of fuel (HRR), and the size of the cross section. growth of a fire using a fuel tanker as the source and the grad- ual drop in temperatures to be expected as the fuel load is The gas temperature, the surrounding wall temperatures, burned off. the emissivity of the hot gases in the vicinity of the fire, and the surface temperature of the linings govern the net heat flux at In reality, the construction may not be exposed to these the surface of the linings. The net heat flux to the linings will timetemperature curves over the entire tunnel length. In a in turn govern the temperature rise inside the lining material.

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99 FIGURE 27 Time temperature curves (65). to the lining can be estimated by the The net heat flux q s The incident thermal radiation from the fire to the tunnel following equation: lining is highly dependent on the geometry of the flame vol- ume and its smokiness. The flame volume and its geometry = g Tg4 + (1 - g ) Twall 4 - Tlin 4 + hs ( Tg - Tlin ) qs (27) are dependent on the HRR and ventilation conditions within the tunnel. The fraction of the flame radiant heat flux of the where: total heat release varies for most fuels and is between 0.25 and 0.4. For large tunnel fires, the tunnel linings in the vicinity q s is the net heat flux to the linings, of the fire are primarily affected by this incident flame radiant g is the emissivity of the hot gas, heat flux. hs is the convective heat transfer coefficient, Tg is the gas temperature, The project shall develop a time-of-tenability criteria based Twall is the surrounding wall and floor temperatures, and on the design maximum HRR. This maximum HRR may dif- Tlin is the lining temperature where q s is determined. fer from 300 MW (1024 MBtu/hr) and gasoline tankers may FIGURE 28 RWS curve (65).