National Academies Press: OpenBook

Design Fires in Road Tunnels (2011)

Chapter: Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review

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Suggested Citation:"Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Suggested Citation:"Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 113
Suggested Citation:"Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 113
Page 114
Suggested Citation:"Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 114
Page 115
Suggested Citation:"Chapter Thirteen - Effects of Various Ventilation Conditions, Tunnel Geometry, and Structural and Nonstructural Tunnel Components on Design Fire Characteristics Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 115

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112 INFLUENCE OF VENTILATION ON FIRE HEAT RELEASE RATE Prior publications reported that mechanically ventilating a fire could result in a more rapid fire development (35). The Second Benelux Tunnel fire test addressed the influence of ventilation on the FHRR (74). The fire development rate, with a ventila- tion air velocity of 4 to 6 m/s (787 to 1,181 fpm), appeared to be two times faster than development without ventilation. The peak heat output was about 1.5 times higher. The increase in HRR and fire growth rate, a result of increased velocity, is the result of more effective heat transfer from the flames to the fuel surface. In some cases, it results in a more effective transport of oxygen into the fuel bed, which enhances the mixing of oxy- gen and fuel. Theoretically, a fire on densely packed wood cribs may be locally underventilated; however, in forced ven- tilation flow, the transport of oxygen to the underventilated region enhances the combustion rate. In most road tunnels, such as the Benelux Tunnel (74), there is a large amount of oxygen already available and the availabil- ity of oxygen is not increased by the ventilation. At the same time, the ventilation has a cooling effect on the fire environ- ment, whereby the heat can be easily let out to the environment. Ventilation has an influence on the fire development that does not always conform to expectations (75): • Owing to increased ventilation, the fire development for a car can be slowed if the fire is ignited at the front of the car. This is in contrast to the accepted view of supposed accelerated development resulting from ventilation. • The influence of increased ventilation on the observed fire behavior depends on the ignition location. Note that 95% of fires begin in the engine compartment (i.e., at the front). • Under the influence of a high-ventilation velocity, the fire development accelerates for a covered load at a rate 2 to 3 times faster, and not by a factor 20 as predicted by some authors. The fire size was 20% to 50% higher as a result of a high-ventilation speed. The results from model fire tests indicated that if the wood cribs are densely packed the increase in peak HRR by venti- lation can be up to a factor of 1.5. If not densely packed, there was little change in the peak HRRs. However, the Runehamar fire tests showed no significant HRR changes resulting from ventilation [up to air velocities of 2 to 2.5 m/s (394 to 492 fpm)]. Earlier discussions about a stronger dependence were not confirmed by the Runehamar experiments. Ventilation is applied during a fire to keep escape routes free from smoke and to assist the fire department and others in reaching the accident site. In most cases, mechanical ven- tilation will lead the fire to burn fully. Thus, the total duration of the fire will be limited and the structure will not be sub- jected to a high thermal load concentration. It is understood that there could be a negative effect on ventilation as forced ventilation may cause significant flame deflection, which leads to the chance that the fire might spread to other vehicles and threaten the integrity of the tun- nel structure on a larger surface, assuming the ventilation cooling effect and reduction in radiation at the source are insignificant. As reported, when a powerful ventilation system is sud- denly activated during an underventilated fire situation, the effects may be dramatic; the flames may suddenly increase in size and length and the fire may easily spread forward because of the preheated vehicles downstream of the fire. However, this phenomenon cannot be defined as flashover. This situation may become very hazardous for firefighters and those who are still trapped inside the tunnel. Starting a ventilation system when the fire has been going for some time in a tunnel with high vehicle density is always very risky. However, as venti- lation cannot immediately reach its full operating mode, the risk is not that significant. Literature observations were made from the Benelux and Runehamar fire tests on the influence of ventilation rate on fire growth rate and are presented in Table 34. Tests have indicated that it may be that the fastest fire growth occurs at about 3 m/s airflow velocity. Both higher and lower ventilation rates may result in slower growth fires. These observations were made on the basis of only a few experiments; more research is needed to confirm (or other- wise) the validity of these conclusions (72). CHAPTER THIRTEEN EFFECTS OF VARIOUS VENTILATION CONDITIONS,TUNNEL GEOMETRY, AND STRUCTURAL AND NONSTRUCTURAL TUNNEL COMPONENTS ON DESIGN FIRE CHARACTERISTICS—LITERATURE REVIEW

113 INFLUENCE OF TUNNEL GEOMETRY ON FIRE HEAT RELEASE RATE A tunnel is a confined space and presents one of the “worst case” geometrical shapes for fire development. The low ceiling and small cross section provide conditions that are conducive to high thermal loads to the tunnel structure. The Runehamar fire tests reached 200 MW (682 MBtu/hr) using mockups of HGVs with high combustible loads in a tun- nel with a relatively small cross-section area and under longi- tudinal airflow. These test results may not be directly applied to a tunnel with conventional cross-sections dimensions. Beard and Carvel (35) have developed the approach to evaluate the impact of geometry on the FHRR. The research showed that for a given combustible load the FHRR of a fire will vary depend- ing primarily on the relative width of the tunnel and the fire source (35). They concluded that fires that are small relative to the size of a tunnel will not be significantly influenced by the tunnel geometry. Fires up to about half of the width of a tunnel will be enhanced by the tunnel geometry, whereas fires with dimensions close to the width of the tunnel will be reduced. When compared, fires within narrow tunnels will generate a larger HRR for the same fuel load than within wider tunnels, assuming sufficient air is available for burning in both cases. (Tunnel height in those studies was much less a factor.) The equation that best describes the relationship between fire HRR and the tunnel width Wtunnel and fire width Wfire is: where: (The equation is valid for “enhancing regime” as identified by Beard and Carvel. The relationship between tunnel geom- etry and fire size has yet to be established in the “diminishing regime.”) This equation allows one to estimate the design FHRR against the values obtained in the Runehamar tests, consider- ing that QRunehamar = 203; WfireR = 2.9 m WtunnelR = 7.3 m, or in any other tests. Estimates show that for a 15 m (49.2 ft) tunnel, the design FHRR is about 100 MW (341 MBtu/hr). This method- B W W= ( ) +24 1 293fire tunnel ( ) HRR B B HRRtunnel tunnel Runehamar Runehamar= ( ) (28) ology was verified by the observation of numerous large-scale tests and by CFD results (75). The slope of the tunnel has an important influence on the dispersion of the flue gases. In general it can be said that owing to the chimney effect, the dispersion velocity of the flue gases increases with the increase in tunnel slope. The longitudinal air velocity’s increase will lead to changes of FHRR and fire growth rate, as was discussed in the previous chapter. INFLUENCE OF STRUCTURAL AND NONSTRUCTURAL COMPONENTS ON FIRE HEAT RELEASE RATE A tunnel will have a “fixed” and a “variable” fire load. The fire load resulting from fixed tunnel components, such as wall linings, and contents, such as cables, track, power sup- ply network, signaling system, lighting system, and radio transmission equipment, can be assessed based on a statistical survey of typical tunnels. In a road tunnel, the variable fire load consists of road vehicles and is more difficult to define because the density of the vehicles present in the tunnel is variable and a tunnel fire would not be expected to involve all of the vehi- cles in the tunnel. In a tunnel fire, it is unlikely that the fire will involve all of the available fuel. In the growth stages, road vehicles are of most interest. Later, elements of the tunnel, such as linings, might become involved. The size of the initiating fire and type of fuel is important because a relatively small fire source may not be capable of igniting the material contents or the compartment lining mate- rials of the vehicle. Somewhat larger sources may be capable of igniting certain material contents, but not lead to flashover. Larger or critical ignition sources result in flashover within the vehicle. Increasing the size of the initiating fire will increase the heat flux produced by the initiating fires. Increasing the HRR of the fire may also increase the flame height, exposing larger areas of material to high heat fluxes. Materials exposed to higher levels of heat will ignite more readily, release more heat, and potentially lead to the greater spread of flame. The location of the initiating fire will also affect the heat fluxes produced by the fire. The tunnel structure generally consists of a concrete lining. The primary function of the tunnel lining is to bear the loads acting on the structure, especially in the event of fire. Different types of concrete are used in tunnels. Depending on the type of tunnel, generally normal strength or high strength concrete are used. Different kinds of concrete will react differently to fires. The goal is to have cost-effective, durable concrete that will have sustained load-bearing capacity during fire and eventu- ally without structural damage. TABLE 34 INFLUENCE OF VENTILATION RATE ON FIRE GROWTH RATE Ventilation Rate Growth Rate Less than 1 m/s About 5 MW/min About 3 m/s About 15 MW/min About 6 m/s About 10 MW/min Source: Ko and Hadjisophocleous (31).

114 Concrete tunnel structures can lose their load-bearing capacity through several failure mechanisms. The main mech- anisms relevant for tunnel linings are (65): • Bending • Buckling • Shear • Spalling. A loss of resistance against these mechanisms is caused by a loss of strength of both concrete and reinforcement. Buckling, shear, and spalling can also be the result of addi- tional internal stresses that arise during a fire. When concrete is heated, the temperature increase will result in a loss of (compressive and tensile) strength. Although this effect is dependent on the composition of the concrete, as in the particular type of aggregate material, the best way to prevent strength loss of the concrete is by reduc- ing the heat penetration. This can be obtained by applying a heat-isolating layer on the concrete surface. A realistic structural load can be determined by advanced nonlinear finite element simulations. For concrete construc- tion, a main failure mechanism is spalling. When concrete is heated, spalling can lead to extensive damage to the construc- tion. Therefore it is of vital importance to consider the risk of spalling. It is important to note that spalling of concrete is not directly dependent on the strength of the concrete, but more on compression, concrete mix design, permeability, and moisture in the concrete. Spalling is a highly relevant failure mechanism in concrete tunnel structures, which is driven by the tempera- ture increase rate and thermal gradient over the structure than by temperature alone. As a result, a conservative upper bound for the fire curve cannot be obtained just by modification of the fire load. It includes other parameters as well, such as ventila- tion conditions and wall properties. Spalling is an umbrella term covering different damage phenomena that may occur to a concrete structure during fire. These phenomena are caused by different mechanisms: • Pore pressure rises as a result of evaporating water when the temperature rises. • Compression of the heated surface resulting from a thermal gradient in the cross section. • Internal cracking resulting from differences in thermal expansion between aggregate and cement paste. • Cracking resulting from differences in thermal expansion/ deformation between concrete and reinforcement bars. • Strength loss owing to chemical transitions during heating. In different combinations of these mechanisms, possible spalling phenomena include: • Violent spalling • Progressive gradual spalling • Corner spalling • Explosive spalling • Post-cooling spalling. It is important to understand the post-cooling spalling mech- anism because it leads to a better understanding of a fixed fire suppression system application for structural fire protection. Post-cooling spalling occurs after the fire is out, after cool- ing down, or maybe even during extinguishing. This type of spalling was observed with concrete types containing calcare- ous aggregate. An explanation is the rehydration of CaO to Ca(OH)2 after cooling, with an expansion of more than 40%. This occurs after cooling down, when moisture is again pres- ent on the concrete surface (65). The expansions resulting from rehydration cause severe internal cracking on the meso-level and, thus, complete strength loss of the concrete. Pieces of concrete will keep falling as long as there is water to rehydrate the CaO in the dehydrated zone. It appears that the application of a fixed fire suppression system on the very early stage of a fire development can actu- ally help cool down the fire and surface and protect the struc- ture, whereas a delay can initiate a post-cooling spalling. This leads to an understanding of the importance of a reliable fire- detection system and activation of the fixed fire suppression system at the very early stage of fire development to cool down the tunnel’s walls. By limiting the development of a fire its duration can be limited, resulting in the tunnel structure endur- ing less harsh conditions. Thequestionthatneeds additional studies is: it is well known that protecting tunnel structures with heat-resistant coatings or materials will reject the heat generated by the fire back into the tunnel environment. In other words, it will not allow for heat to dissipate through the walls. This could potentially increase the tunnel heat in the range of 30% or more. This requires addi- tional studies to answer the question: What is happening to the tunnel environment and tunnel heat by protecting the tunnel walls? Will the fire life safety systems, including tunnel fans withstand that additional heat component? The cooling down of the tunnel’s walls could be accom- plished by using sprinklers on a very early stage of fire devel- opment. By limiting the development of the fire the duration of the fire can be limited and the structure of the tunnel will be subjected to less harsh conditions. SUMMARY Ventilation has an influence on the fire development: • Owing to increased ventilation, the fire development for a car can be slowed if the fire is ignited at the front

115 of the car, or can increase if the fire is ignited in the back of the car. • The influence of increased ventilation on the observed fire behavior depends on the ignition location. • Under the influence of a high-ventilation velocity, the fire development accelerates for a covered load at a rate 2 to 3 times faster. The fire size is expected to be 20% to 50% higher owing to a high-ventilation speed. • If the wood cribs are densely packed, the increase in peak HRR by ventilation can be up to a factor of 1.5. • No significant HRR changes owing to ventilation [up to air velocities of 2 to 2.5 m/s (394 to 492 fpm)]. • In most cases, mechanical ventilation will lead the fire to burn fully. Thus, the total duration of the fire will be limited and the structure will not be subjected to a high thermal load concentration. • There could be a negative effect of ventilation because forced ventilation may cause significant flame deflec- tion, which leads to the chance that the fire might spread to other vehicles and threaten the integrity of the tunnel structure on a larger surface, assuming the ventilation cooling effect and reduction in radiation at the source are insignificant. • The fastest fire growth occurs at about 3 m/s airflow velocities. Both higher and lower ventilation rates may result in slower growth fires. Tunnel geometry may have a significant impact on fire HRR: • For a given combustible load, the FHRR of a fire will vary depending primarily on the relative width of the tunnel and the fire source: – Fires that are small relative to the size of a tunnel will not be significantly influenced by the tunnel geometry; – Fires up to about one-half of the width of a tunnel will be enhanced by the tunnel geometry; and – Fires with dimensions close to the width of the tun- nel will be reduced. • When compared, fires within narrow tunnels will gen- erate larger HRR for the same fuel load than within wider tunnels, assuming sufficient air is available for burning in both cases. • The slope of the tunnel has an important influence on the dispersion of the flue gases. In general it can be said that owing to the chimney effect, the dispersion velocity of the flue gases increases with the increase in tunnel slope. It is unlikely that the fire will immediately involve all of the available fuel. In the growth stages, road vehicles are of most interest. Later, elements of the tunnel, such as linings, might become involved. • Materials exposed to higher levels of heat will ignite more readily, release more heat, and potentially lead to more flame spread. • The best way to prevent strength loss of the concrete is by reducing the heat penetration. • Spalling is a highly relevant failure mechanism in con- crete tunnel structures, which is driven by the tempera- ture increase rate and thermal gradient over the structure than by temperature alone. • It is very important to understand the post-cooling spalling mechanism because it leads to a better under- standing of a fixed fire suppression system application for structural fire protection. • A fixed fire suppression system application on a very early stage of a fire development can actually help to cool down the fire and surface and protect the structure, whereas delay with its activation can initiate a post- cooling spalling. EXAMPLE OF DESIGN FIRE SIZE ESTIMATE Every tunnel is unique and this example is for illustration purposes only. Each project has to establish the design fire size accepted by the stakeholders and the Authority Having Jurisdiction. Consider that a tunnel is twice the width of the Runehamar Tunnel and the designer is using the most conservative test result—the maximum FHRR from the HGV fire of 200 MW. This example illustrates the impact of tunnel geometry, tun- nel exit design, reliable rapid fire detection, and benefits of fast activation of the fire suppression system capable of con- trolling the fire (Table 35). Table 36 represents the resultant fire curve modified by the fixed fire suppression system rapid activation. Rapid fire detec- tion, early start of self-rescue, and fast application of a suffi- cient fixed fire suppression system could reduce the design fire size 10 times or more. An insufficient fixed fire suppression system design will not control fire, which will keep growing. Proper fixed fire suppression system design will either keep the fire at the starting rate (10 MW in this example) or reduce the fire up to extinguishing. (In reality the process is more compli- cated and fire may keep growing for a short period of time after fixed fire suppression system activation, and then get reduced.) Early deactivation of the fixed fire suppression sys- tem may lead to explosion and unmanaged fire. This exam- ple is not applicable to the liquid fuel fires or alternative fuel vehicles fires.

116 TABLE 35 EXAMPLE OF DESIGN FIRE TABLE 36 EXAMPLE OF DESIGN FIRE CURVE Design Fire Scenario for Self-rescue Design Fire Scenario for Structural Protection Design Fire Scenario for Central Mechanical Equipment Fire Rating Maximum fire HRR for HGV with cargo similar to the Runehamar tests 200 MW (see “Full-Scale Tests” in chapter six) 200 MW; RWS curve (see “Time-temperature . . .” in chapter eleven) (1350ºC; 2462ºF) 200 MW With geometry correction (no tunnel grade correction made) 100 MW (see example in “Influence of Tunnel Geometry . . .” in chapter thirteen) 100 MW RWS curve (see “Time–Temperature . . .” in chapter eleven) (1350ºC; 2462ºF) 100 MW Consider self-evacuation to the nearest exit is 10 min 80 MW using ultra- fast fire growth curve (see “Combined Curve for Evacuation . . .” in chapter eleven) No correction No correction Consider fast fire detection and sufficient wet FFSS system activation within 4 min before ventilation in full effect 10 MW (see revised fire curve due to rapid FFSS activation illustrated in Table 36) 10 MW, but with a faster temperature growth rate (additional structural protection may not be required. Computational analysis needed) 10 MW, but design temperature not less than 250ºC (482ºF, see chapter nine) Correction for ventilation N/A as FFSS is activated before ventilation. Correction for tunnel drainage N/A as HGV was used in the example assuming no liquid fuel spillage. For illustration purposes only. N/A = not available. noitavitcAsmetsySSLFeucseR-fleS A. Make a decision to evacuate B. Disembark the bus C. Walk away from the fire-effected zone D. Reach cross passage 1. Detection time 2. Operator reaction time (alarm) 3. FFSS activation 4. All fans activated 5. Ventilation mode in full operation For illustration purposes only.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 415: Design Fires in Road Tunnels information on the state of the practice of design fires in road tunnels, focusing on tunnel fire dynamics and the means of fire management for design guidance.

Note: On September 20, 2011, the following errata was released related to NCHRP Synthesis 415. The electronic version of the publicaiton was changed to reflect the corrections.

On pages 106 and 107, an incorrect reference was cited. In the final paragraph on page 106, the last sentence should read: One study came to the conclusion that, although some minimum water application rates would achieve a certain objective, a marginally higher rate would not necessarily improve the situation (79). The figure caption for Figure 35 at the bottom of page 107 should read: FIGURE 35 NFPA 13, NFPA 15, and other International Water Application Rates (79).

The added reference is as follows:

79. Harris, K., “Water Application Rates for Fixed Fire Fighting Systems in Road Tunnels,” Proceedings from the Fourth International Symposium on Tunnel Safety and Security, A. Lönnermark and H. Ingason, Eds., Frankfurt am Main, Germany, Mar. 17–19, 2010.

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