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Design Fires in Road Tunnels (2011)

Chapter: Chapter Nine - Design for Tunnel Fires Literature Review

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Suggested Citation:"Chapter Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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 Nine - Design for Tunnel Fires 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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54 BACKGROUND Every tunnel is unique, making it difficult to generalize designs for road tunnel fires. As reported, design fires and design fire scenarios are essential inputs for a fire safety-engineered approach to fire safety design of new tunnels and any appraisal of fire safety protection measures in existing tunnels. An effective fire protection design for life safety and property protection in tunnels requires a systematic assessment of a number of component “sub-systems,” which contribute to the overall safety of the design. These sub-systems are: • The initiation and development of fire spread • Spread of smoke and toxic gases • Detection of fire and activation of active fire life safety systems • Tunnel users’ evacuation • Fire service intervention. Further complexity arises because the time scales for the response of active fire protection measures such as fire detection and safety systems activation are different from the response time of occupants during evacuation or the response time for structural integrity. The first priority identified in the literature for fire design of all tunnels is to ensure: 1. Prevention of critical events that may endanger human life, the environment, and the tunnel structure and installations. 2. Self-rescue of people present in the tunnel at time of the fire. 3. Effective action by the rescue forces. 4. Protection of the environment. 5. Limitation of the material and structural damage. Furthermore, part of the objective is to reduce the consequences and minimize the economic loss caused by fires. A 100% safety scenario against a tunnel fire is not possible; however, actions can be taken to reduce the risk to a reason- able minimum. Preventive measures are safety measures that reduce the probability of an unwanted event. Preventive safety measures in tunnels can be related to: • Organization and traffic management; • Structural or geometrical solutions; and • Safety equipment, such as heat detection of vehicles before entering the portals. Preventive measures on fire in a tunnel are related to • Removal of sources of ignition, • Reduction of the likelihood of a fire, and • Prevention of the development from the ignition to a severe fire. Gasoline tankers are prohibited from using many U.S. tunnels. Dangerous goods that travel through tunnels can be costly in terms of human lives, tunnel damage, transport dis- ruption, and the environment. Conversely, needlessly banning dangerous goods from tunnels may create unjustified economic costs and force transport operators to use more dangerous routes. The fire prevention measures presented in Table 11 can be implemented in tunnels to reduce either the probability or the consequences of an incident in a tunnel. The main engineering goals regarding the fire protection of road tunnels are listed here in order of priority, as identified in the literature: 1. Objectives related to life safety: – Minimize the risk of injury or death for tunnel users in the event of a fire. – Minimize the risk for people outside of the tunnel. – In densely populated areas, people outside the tun- nel may also be affected by the fire inside the tunnel (e.g., when buildings are present above the tunnel or when dense and toxic smoke may cause secondary incidents on roads adjacent to the tunnel). – Minimize the risk of injury or death for rescue teams and repair workers. 2. Objectives related to economic consequences and to the quality of life: – Avoid damage that threatens the tunnel construction. – Avoid the need to incur expensive repair work. – Avoid long-term interruption of service. The proactive measures comprise all of the general actions taken in the planning phase to improve tunnel safety— independently of a specific tunnel project. CHAPTER NINE DESIGN FOR TUNNEL FIRES—LITERATURE REVIEW

55 • Legislative initiatives and other actions highlight the awareness of the problem and contribute to an improve- ment in the standards for designing and operating tunnels. • Research projects and similar actions that develop and disseminate knowledge about tunnel fires contribute to future tunnel safety. • In addition, any proactive measures regarding user behav- ior such as an increase in awareness to safer driving and correct behavior in the event of an incident may signifi- cantly influence safety in the tunnel. Mitigation measures are safety measures that aim to limit the consequences once the ignition has taken place and developed into a fire. The mitigation measures may be related to: • Reduction of the fire development, • Reduction of the consequences to humans, and • Reduction of the consequences to structure and equipment. Reduction of Fire Development Structural Measures Flammable liquids may leak during or before a fire. A suitable drainage system reduces the quantity of flammable liquids from the source of the fire and thereby mitigates a serious fire development. Safety Equipment • The main function of ventilation during a fire is to control the smoke and, to some degree, influence the development of the fire. • Fixed fire suppression systems can prevent fires from developing into severe fires, but could reduce visibility in the tunnel. The best chances of successful fire fighting are in the ini- tial phase of a fire. Therefore, systems directed by operators or end users may be beneficial. Such installations are easy to use because the tunnel’s users will probably be unfamiliar with fire fighting and with the tunnel’s environment. Response to Fire The fire resistance of doors and walls reduce the probability of the development and spread of fires from one compart- ment to another. The fire resistance of the active fire life safety systems (e.g., ventilation and fire suppression) ensures that the development of the fire can be controlled. Reduction of Consequences to Humans Structural Measures The highest priority of tunnel design safety is to mitigate consequences to humans. The geometrical layout of a tunnel can contribute to the mitigation of a fire. For example, it is easier to ensure that the majority of tunnel users have smoke- free conditions if the tunnel is operated in one-way traffic. Also, the cross-sectional area influences the chances of creating smoke-free areas and providing conditions for escape from a fire. One of the most important mitigation measures for users exposed to a fire in a tunnel is the provision of escape routes. Safety will be influenced by the spacing and design of the emergency exits. In some cases, the rescue and evacuation of the injured and physically disabled will have to be assisted by the rescue forces, tunnel operators, etc. Emergency exits can serve as access routes for the rescue forces. Measures to Reduce the Probability of an Accident Related to tunnel design and maintenance Tunnel cross section and visual design Alignment Lighting (normal) Maintenance Road surface (friction) Related to traffic and vehicles Speed limit Prohibition to overtake Escort Distance between vehicles Vehicle checks Measures to Reduce the Consequences of an Accident Alarm, information, communication of operator, and rescue services Closed-circuit television Automatic incident detection Automatic fire detection Radio communication (services) Automatic vehicle identification Emergency telephone Communication with users Emergency telephones Radio communication (users) Alarm signs/signals Loudspeakers Evacuation or protection of users Emergency exits Smoke control Lighting (emergency) Fire-resistant equipment Failure management Reduction of accident importance Fire-fighting equipment Rescue teams Drainage Road surface (non-porous) Emergency action plan Escort Reduction of the consequences on the tunnel Fire-resistant structure Explosion-resistant structure Source: Safety in Tunnels (2001) (41). TABLE 11 RISK REDUCTION MEASURES CLASSIFIED ACCORDING TO THEIR MAIN PURPOSE

Safety Equipment The ventilation system is a crucial safety measure when a fire occurs, because it allows for smoke-free escape routes. The ventilation system is designed to control the smoke spread, and this can be achieved by blowing smoke in one direction, supporting smoke stratification and extracting smoke at the ceiling or near the ceiling, or blowing smoke in one direction and extracting it at a few places. Alarm systems including telephones, push buttons, pull boxes, detection, and surveillance are important to alert the operator and thereby activate the emergency procedures, ventilation systems, etc. Unfortunately, those systems are seldom utilized by tunnel users. In 46 tunnel fire incidents in Austria, the emergency telephone was used only 10 times (22%). Pull boxes/push buttons were used only four times (9%) (42). Communications systems influence the evacuation of the tunnel during a fire and thereby reduce the number of people at risk by exposure. The communications systems can be radio re-broadcast (audio warning) and message boards (visual notification). Once the evacuation has been initiated, it is important that the tunnel users reach the safe area as quickly as possible. Exit signs, exit route guidance, lighting, and markings are mitigation measures that can make the escape more efficient. All these elements are discussed in details in the NFPA 502. Response to Fire (Fire Resistance of Safety Systems) It is particularly important that the installations necessary during an emergency continue to function for a suitable dura- tion during a fire. Reduction of Consequences to Structure and Equipment Safety Equipment Fire suppression systems can potentially prevent severe damages to the structure and to the equipment once acti- vated at an early stage of fire development. It is possible to have fixed installations in the traffic space to suppress fires in vehicles or other sources. Often, the purpose of the fixed suppression installations is not to extinguish the fires but to control and limit fire development. (See Annex of NFPA 502 2008 edition for additional information.) Response to Fire: Structural Fire Resistance and Fire Protection One of the serious consequences of a fire is damage to the tunnel structure and its ultimate collapse. By suitable design of the tunnel structure and by passive fire and/or fixed fire suppression protection, the tunnel can withstand the rele- vant fire scenario and tunnel rehabilitation and repair costs can be reduced or eliminated. Fire Resistance of Equipment, Power Supply, and Cabling The installations are often the first part of the tunnel system to suffer damage in a fire. By suitable design and passive or active fire protection systems the damage to the installations can be reduced. The cabling and other installations are pro- 56 tected to resist fire damage. Safety critical equipment in road tunnels must be able to function in the event of fire. The physical location, such as a lower level, may also reduce fire damage. Different countries have different requirements for the equipment. For example, a minimum temperature requirement in the NFPA 502 for the tunnel ventilation fans, dampers, and sound attenuators is 250°C (482°F) for 1 h of exposure. A higher temperature is used depending on the results of the calculations. Many international standards have requirements of 250°C (482°F) for 90 min of exposure and a maximum of 400°C (752°F). The development of emergency response plans requires a consideration of activities before, during, and after the incident, and covers: • Prevention and training, • Accident management, and • Fire emergency operations. INTEGRATED APPROACH TO SAFETY IN TUNNELS Safety is a result of the integration of the infrastructural measures, the operation of the tunnel, and user behavior, as well as preparedness and incident management. The assess- ment of fire safety in tunnels is a complex issue, where broad multi-disciplinary knowledge and application of different physical models are necessary to explore the causes and devel- opment of fires to evaluate measures to prevent and reduce their consequences. In general, an overview of the entire sys- tem is necessary to determine the best possible actions (43). The systems to take into account comprise: • The occurrence and physics of fire development. • The tunnel systems; that is, – Infrastructure and – Operation. • Human behavior of users, operators, and emergency services. • Other factors influencing safety. Prescriptive Approach Traditionally fire safety standards for tunnels and other struc- tures have been prescriptive; they have contained minimum requirements that must be fulfilled. These requirements have been established over the years based on experience, tradition, and engineering/expert judgment. They apply in principle by absolute evaluation of safety: if the design is in accor- dance with the standard, the safety is acceptable; if not, it is unacceptable. The advantage of a prescriptive standard is that it is not difficult to use and it ensures a minimum level of requirements. On the other hand, prescriptive standards may not be applicable to unusual situations and in some cases may not be able to take into account the interaction between different parts of the tunnel structure, installations, and the local conditions.

57 Performance-based Approach In recent years, the national and international standards have tended to favor the performance-based approach. NFPA 502 is not an exception. By application of fire performance concepts, fire safety is achieved based on a scientific understanding of the fire phenomena, of the effects of fire, and of the reaction and behavior of people. Emphasis is given to the safety of life, whereas fire safety engineering can also be used to assess property loss, interruption of service, contamination of the environment, etc. Furthermore, risk of fire and its effects are quantified and the optimum safety measures are evaluated. By a performance-based approach, the regulatory require- ments are given on a more general level specifying the safety of the users, economic values, and so forth. This may result in different interpretations leading to undersized fire and underdesigned safety systems. Fire safety engineering will normally involve the following steps: • Qualitative design review: – Definition of objectives and safety criteria, with reference to performance-based standard require- ments and coordination with the authorities having jurisdictions; – Definition of the tunnel system; – Identification of fire hazards; – Selection and definition of fire scenarios; – Identification of methods of analysis; and – Identification of design options. • Quantitative analysis of design using the appropriate subsystems: – Fire ignition, development of heat and smoke; – Spread of fire, heat, and smoke; – Structural response to fire; – Detection, activation, and suppression; and – Behavior of tunnel users and influence of fire on life safety. • Assessment of the outcome of the analysis and evalua- tion against criteria. The objectives and the associated acceptance criteria used in a performance-based approach is clearly defined and established for the particular design. The acceptance criteria, which establish the adequacy of the design, can be according to the following approaches: • Deterministic (including, when appropriate, safety factors). • Probabilistic (risk-based used in European countries). • Comparative (comparison of performance with accepted codes of practice). The deterministic and the comparative approaches are to some extent similar to the prescriptive approach, but allow for more flexibility. The performance-based approach gen- erally requires more data and procedures resulting in a more complex and time-consuming design. For the designer, a prescriptive approach is an advantage with respect to liability. A design fire is an idealization of a real fire that might occur. A design fire scenario is the interaction of the design fire with its environment, which includes the impact of the fire on the geometrical features of the tunnel, the ventilation and other fire safety systems in the tunnel, occupants, and other factors. Nobody can precisely predict every fire scenario given the range of variables and human behavior. It is not known • What will cause the fire (collision, electrical problem, terrorism)? • What exactly will be burning (goods, furniture, car body, etc.), including the number of vehicles involved in the fire? • Where will (in which part of the tunnel) the fire occur? • When will the fire start (month, time of the day, etc.)? • What the outside environmental conditions will be at the time of incidence (winds, hurricane, earthquake)? • What will be the traffic conditions? • How will the tunnel users’ behave in an emergency? • How will the operators behave during an emergency? Therefore, the designer makes a number of assumptions to ensure that the design will save lives and retain structural integrity under most of the foreseeable fire scenarios. International standards on preventive fire protection are based on a risk approach. In a European study, it was found that risk estimates produced by different users differed by “several orders of magnitudes.” The estimates varied signi- ficantly from case to case. Serious concerns on risk analysis have recently been found in the Channel Tunnel design that has already experienced several large fire events since its opening. The PIARC report reiterates the need for a greater focus on the definition of appropriate fire scenarios dealing with specific aspects of tunnel fire safety. This can be achieved by accurate specification of the input and output characteristics of design fires. The main cause of death in a fire is related to inhalation of smoke and hot gases and not from the fire itself. Therefore, with respect to life safety, attention is given to the determi- nation and mitigation spread of (possibly toxic) hot gases and smoke. Some key design fire scenarios relevant to the fire safety in tunnels are listed here: 1. Design fire scenario for ventilation and other systems (e.g., fixed fire suppressions) design and assessment— Smoke ventilation in tunnels needs to be designed on

the basis of smoke flow rates (i.e., the volume flow in the fire plume) using a design fire and local gas tem- peratures downstream from the design fire because they determine the ventilation volume flow rates. The design fire scenario takes into account all the important issues such as a time factor, ambient conditions, wall proper- ties, and the efficient operation of detection and venti- lation systems, which can have a significant influence on the appropriate design fire characteristics. 2. Design fire scenario for egress analysis—Evacuation measures for tunnel users or emergency rescue services need to be within tenable environmental conditions, identified as breathable gas temperatures and concentra- tion of toxic gases at head height in the tunnel, as well as hot gases at higher levels in the tunnel that radiate down onto evacuees. A tenable environment is well-defined in NFPA 502. Time is a very important factor. The times for hazardous conditions to develop at particular loca- tions as discussed later in this chapter need to be com- pared with occupant egress times. These in turn need to take into account the time it takes for occupants to real- ize they are in danger and begin their escape. Evacuation time from buses also needs to be considered. 3. Design fire scenario for thermal action on structures— See time–temperature curve discussion. 4. Design fire scenario for the safety of tunnel fire equipment—Usually the critical fire life safety equip- ment is required to be designed for the expected envi- ronment during a fire emergency. 5. Design fires for work on tunnel construction, refur- bishment, repair, and maintenance—Fires related to, for example, tunnel boring machines and the refur- bishment of tunnels are considered out of the scope of this report. Based on the tunnel experience and tunnel fire tests, several observations can be made: • Each tunnel is unique • A tunnel is a risky environment. No tunnel is absolutely safe regardless of how it was designed. The designer’s 58 goal is to make it as safe as possible based on previous experience, on current knowledge, and on technical equipment development. Consequences of tunnel fires can be disastrous. • Tunnels are generally safe. Tunnel fires are rare events and happen less often than fires on open roads. Fewer than 150 people have been killed anywhere in the world in road tunnel incidents involving a fire—and that includes those killed by any preceding accident (44) (collisions). Fewer than 20 tunnels around the world have suffered substantial damage as the result of a fire emergency. • Road tunnel fires cannot be completely eliminated until vehicle fires are eliminated. DESIGN FIRE SIZE Design fire size is one of the most important parameters for tunnel fire engineering. The materials that burn in a fire mostly come from the vehicles involved, and they include elements of the vehicles, such as the seats, tires, plastic materials in the finishing or even in the body work itself; cargo; the fuel from the vehicle tanks, which amounts to hundreds of gallons for trucks; and the loading, especially for goods vehicles. The goods loadings vary and can lead to many different kinds of fires. Some examples of combustion energy outputs are given in Table 12. For design purposes it is necessary to choose fire charac- teristics corresponding to the traffic that uses a particular tunnel. Conditions, such as the allowance of transporting haz- ardous vehicles and materials, have to be taken into account. Tunnel fires differ from open fires in at least two impor- tant ways: 1. The heat feedback of the burning vehicles in a tunnel fire tends to be more effective than that in an open fire because of the confined enclosure. This effective heat feedback often causes vehicles that do not burn intensely in an open fire to burn vigorously in a tunnel fire. For Type of Vehicle Approximate Energy Content [MJ (MBtu)] Remarks dnalniFnistseterifrofdesU)7.3–8.2(009,3–000,3sraCetavirP stseterifAKERUErofdesU)7.5(000,6raCetavirP )6.6(000,7raCcitsalP )93(000,14suBcilbuP Heavy Goods Vehicle (HGV) 88,000 (83) ehtnistseterifehtnidesU)5.36(000,76VGHrofsdaoL Runehamar Tunnel 129,000 (122) 152,000 (144) 240,000 (227.5) Tanker with 50 m³ Gasoline 1,000,000 (948) Medium tanker 1,500,000 (1422) Dutch assumption for a ìlar ge” design fire Source: PIARC (21). TABLE 12 EXAMPLES OF COMBUSTION ENERGY OUTPUTS

59 example, Beard and Carvel (35) concluded that the HRR of a fire within a tunnel could increase by a factor of 4 compared with that of the same material burning in the open. Furthermore, the oxygen needed for com- bustion is not always as readily available in tunnels as in the open (depending on the tunnel geometry and fire size). The fire conditions may either develop to a: – Fuel-controlled fire where unreacted air bypasses the burning vehicles (typical tunnel fire controlled by limited fuel available), or a – Ventilation-controlled fire, giving rise to large amounts of toxic fumes and products of incomplete combustion. Essentially, all the oxygen is consumed within the combustion zone and fuel-rich gases leave the exit of the tunnel (e.g., extremely severe tunnel fires, such as the Mont Blanc fire where oxy- gen is limited). 2. As a fire develops in a tunnel, it interacts with the ven- tilation airflow and generates aerodynamic disturbances in the tunnel flow. This interaction and disturbance may lead to drastic changes in the ventilation flow pattern, such as throttling of airflow (buoyancy effects) and reverse flow of hot gases and smoke from the fire into the ventilation air stream (backlayering). Such effects on the ventilation not only complicate firefighting pro- cedures, but also present extreme hazards by propa- gating toxic fumes and gases far from the fire. Impact of ventilation on fire size is discussed in chapter thirteen. Design fires in tunnels are usually given as the peak fire HRR. There are various methods and techniques to calculate and estimate the fire HRR of a given vehicle; some could be provided by manufacturers (for cars and buses), others calculated; however, there is no common ground on how to calculate the HRR. One method is the weighting of the burning components of a vehicle, another is analytical. Some calculations incorporate burning efficiency, which means that the fire may not consume the entire heat load available. The leftover content is typically in the form of either a char residue or as soot and smoke particles displaced by the com- bustion gas stream (45). The magnitude and development of fire depends on: • Vehicle combustion load (often called the fuel load, which is usually greater than the potential fire size), • Source of ignition, • Intensity of ignition source, • Distribution of fuel load in the vehicle, • Fire propagation rate, • The tunnel and its environment (including available oxygen), and • Other factors that will be discussed in the following chapters. The fire power is measured in megawatts (MW) or MBtu/hr (1,000 Btu/hr), although it has become more common for engineers to combine the peak HRR with the fire growth rate. For example, full-scale tests of HGV loads in the Runehamar Tunnel showed that the HRR can exceed more than 100 MW (341 MBtu/hr) in less than 10 min. This means that the fire growth rate will be crucial in determining whether those caught in the fire can escape. Studies showed that the fire growth rate is more important than the peak HRR when investigating the safety of people in the tunnel. The peak HRR varies between 1.5 MW (5 MBtu/hr) and 202 MW (689 MBtu/hr) for road vehicles. The gas temperatures in the ceiling vary from 110°C (212°F) to 1365°C (2489°F). It must be emphasized that most of the test results are dependent on the test conditions. These include low air velocities during most of the tests and a cross section signi- ficantly smaller than usually found in road tunnels. This overestimates the heat radiation coming back from the walls and may underestimate the amount of oxygen available in the tunnel. The design fire size selected for design significantly affects the magnitude of the critical velocity needed to prevent back- layering. Table 13 provides general fire size data for a selection of road tunnel vehicles. It presents typical fire size data for passenger cars and multiple passenger cars, for buses, HGVs, and tankers; however, this does not allow for evaluation of Cause of Fire Peak Fire Heat Release Rate, 106 Btu/h (MW) )01ot5(43ot71raCregnessaP Multiple Passenger Cars (2 to 4 vehicles) 34 to 68 (10 to 20) Bus 68 to 102 (20 to 30) )002ot07(286ot932kcurTsdooGyvaeH Tanker3 682 to 1,023 (200 to 300) Source: NFPA Standard for Road Tunnels, Bridges, and Other Limited Access Highways (2008) (19). Notes: 1. The designer should consider the rate of fire development (peak HRRs may be reached within 10 min), number of vehicles that could be involved in a fire, and the potential for a fire to spread from one vehicle to another. 2. Temperatures directly above a fire can be expected to be as high as 1800°F to 2550°F (1000°C 3. Flammable and combustible liquids for tanker fire design could include adequate drainage to limit the area of pool fire and its duration (see Table 14). 4. HRR may be greater than listed if more than one vehicle is involved. to 1400°C). TABLE 13 TYPICAL FIRE SIZE DATA FOR ROAD VEHICLES

multiple HGV or bus accidents. Fire HRR, especially for vans and heavy goods trucks, depend on the size of cargo load, which is usually unknown. A risk analysis for the Oresund Tunnel (43) considers the possibility of fuel leakage from holes of 15, 35, and 50 mm (0.6, 1.4, and 1.9 in.) equivalent diameter. These represent the potential failure of small diameter fuel lines or a small amount of damage to a delivery hose flange. They do not rep- resent the complete destruction of a delivery hose that would give a hole diameter of 100 mm (3.9 in.). The leakage flow depends on the diameter of the hole and the fluid pressure at the hole. For the holes considered, the mass flows are 0.5, 2.7, and 5.6 kg/s (1.1, 6, and 12.3 lb/s), respectively. The drainage capacity of the drainage outlets is normally 10 times greater. However, it was assumed that in an accident this could obstruct and limit the amount of drainage. The calculations for the different fire scenarios gave calorific power outputs of between 22 MW (75 MBtu/hr) and 245 MW (836 MBtu/hr). Fire duration can be determined by the amount of available combustible material. The amount of fuel is different for each study based on the type of vehicles, loads, and traffic patterns. Tables 15 and 16 present several examples on design fire scenarios in the Netherlands and France. EXPLORING THE EMERGING ISSUES OF ALTERNATIVE FUEL VEHICLES ON DESIGN FIRES Environmental issues such as climate change and scarcity of resources have stimulated the development of new energy carriers for vehicles. This also means that there will be an increase in the number of vehicles running on these new energy carriers in tunnels and other confined spaces. New energy carriers do not necessarily imply higher risks; however, they do represent a new situation with inherent new risks, and such risks need to be considered and evaluated. The mixture of different energy carriers, such as flammable liquids, gases lighter than air, gases denser than air, batteries, and so forth, can also constitute a risk itself, because there are situations where different safety measures need to be implemented depending on the energy carrier used and the scenario in question. Some countries have restrictions on the use of some energy carriers in confined spaces. This 60 section explores the emerging issues of alternative fuel vehicles on design fires. Natural Gas and Liquid Gas Vehicles CNG and compressed biogas are primarily composed of methane, which is a gas lighter than air. Biogas can be clas- sified as a renewable natural gas. CNG is the more widely used of the two. CNG is usually stored in a fuel tank at a pres- sure of 200 to 250 bar (2900 to 3625 psi). The use of CNG is increasing around the world and in 2008 there were more than 9 million CNG vehicles and 13,000 refueling stations worldwide. The situation with a CNG engine is more complicated because the exhaust gas temperature from the CNG engine is much higher (∼750°C or 1382°F) than from the diesel engine (∼450°C or 842°F). Additional measures can be considered to reduce the risk of fire: • Reduce the high exhaust temperature in the engine compartment by installing a water-cooled system. The exhaust system must be made of noncorrosive special steel with no leaks. • Check the exhaust gas system for leaks and insulate as needed. • Provide a means of ventilation (additional louvers) in the engine compartment for heat dissipation. • Facilitate the removal of oil-contained contamination in the engine and gear compartments. • Install fire alarm sensors on busses. Hydrogen Hydrogen is a colorless, odorless, tasteless, nontoxic, non- corrosive gas approximately 14 times lighter than air. Much research and development is currently focused on hydrogen and its feasibility as a vehicle fuel; however, in most cases only demonstration models are available (46). Hydrogen can be used either for internal combustion engine (ICE) vehicles or fuel cell vehicles (FCVs). It is expected that after 2015, fuel cells will be more common. There are several hydrogen vehicle projects currently being tested. There is a Network of Excellence called HySafe, which aims to safely introduce hydrogen technologies and applications. This network has TABLE 14 EFFECT OF LEAKAGE DIAMETER AND DRAINAGE RATE ON THE FIRE SIZE OF FUEL TANKERS Equivalent Diameter of Leakage [mm (in.)] Leakage Mass Flow of Fuel [kg/s (lb/s)] Calorific Power [MW (MBtu/hr)] Drainage Mass Flow of Fuel 0 kg/s (0 lb/s) 1 kg/s (2.2 lb/s) 2 kg/s (4.4 lb/s) 5 kg/s (11 lb/s) 15 (0.6) 0.5 (1.1 lb/s) 22 (75) — — — 35 (1.4) 2.7 (6 lb/s) 120 (409.5) 76 (259) 33 (113) — 50 (1.9) 5.6 (12.3 lb/s) 245 (836) 201 (686) 158 (539) 27 (92) Source: PIARC (21).

61 Size Heat Release Rate (MW) [MBtu/hr] Scenario Remarks Small 6.1 [20.8] - A passenger car is completely burnt - Estimated duration of the fire: 25 min - Smoke temperature less than 150°C (302°F) at a few meters from the source of the fire - Ventilation speed 1.5 m/s (295 fpm) - Jet fans will only be impaired if they are right above the fire - Fire fighting is possible from within a few meters of the source of the fire - Limited damage to the tunnel interior - Limited amount of soot — Medium 100 [341] - A heavy goods vehicle loaded with wood is completely burnt - The temperature of the fumes is about 800°C (1472°F) at a distance of 50 m (164 ft) from the source - Ventilation speed 1.5 m/s (295 fpm) - Fire fighting is possible at a distance of 10 to 20 m (33 to 66 ft) from the source of the fire when protective clothing is worn - Damage to the tunnel interior, soot formation - Breakdown of jet fans at a distance of 150 to 300 m (492 to 984 ft) downstream of the fire is expected Scenario applicable to Dutch tunnels in urban areas or on secondary roads where the transport of dangerous goods is forbidden Large 300 [1,024] - A tanker loaded with 50 m³ of gasoline is completely burnt - Estimated duration of the fire: 2 h - Fire fighting is possible at a distance of from 10 to 20 m (33 to 66 ft) from the source when the ventilation speed is increased to 3 m/s (591 fpm) and protective clothing is worn - Use of water/foam should be considered - The temperature of the smoke will be about 1400°C (2552°F) at a distance of about 20 m downstream of the fire - All jet fans will be damaged over a distance of 300 to 500 m (984 to 1,640 ft) downstream of the fire - Considerable damage to the interior of the tunnel over a large distance downstream of the fire; distance is increased when the ventilation speed is increased Criterion for tunnels that are opened to the transport of dangerous goods (such as propane or other toxic substances) Source: Fire in Tunnels (9). Parameter Clearance of the Tunnel Height <2.7 m (8.9 ft) Height 2.7 m to 3.5 m (8.9 ft to 11.5 ft) Height >3.5 m (11.5 ft) (no dangerous goods allowed) Height >3.5 m (11.5 ft) (dangerous goods allowed) Typical Fire 2–3 cars 1 van 1 HGV 1 fuel tanker Heat Release Rate (MW) [MBtu/hr] 8 [27] 15 [51] 30 [102] 200 [682] Smoke Flow Rate (m3/s) [ft3/s] 30 [1,059] 50 [1,766] 80 [2,825] 3001 [10,594] Growth Time tg (min) 5 5 10 10 Peak Duration tmax (min) 20 30 60 60 Decline Time td (min) 20 20 30 30 Released Energy (GJ) [MBtu] 15 [14.2] 40 [37.9] 150 [142.1] 1000 [947.2] Source: Fire in Tunnels (9). 1In France, this smoke flow rate is generally not taken into account for the design of semi-transverse ventilation, even if the transport of dangerous goods is allowed. TABLE 15 DUTCH FIRE SCENARIOS FOR TUNNELS WITH LONGITUDINAL VENTILATION IN RELATION TO HEAT RELEASE RATES TABLE 16 FRENCH DESIGN FIRES WITH COMPLEMENTARY DATA FOR CFD CALCULATIONS

led to a number of projects, including HyTunnel and InsHyde. The goal of HyTunnel is to develop codes, standards, and regulations so that additional risks from the introduction of hydrogen vehicles into tunnels can be handled safely. During the test period, no major safety-related incidents occurred to the fuel cell buses. However, for the ICE buses there was one unexpected release of hydrogen when a check valve within the tank nozzle failed. The results indicated that owing to the nature of flame and fire development, tunnels with greater slopes and with horseshoe cross sections (compared with equivalent rectan- gular cross sections) present lower hazards. In InsHyde, many different aspects of hydrogen safety in confined spaces are evaluated and discussed, such as regulations, detection, ventilation, fire, and explosion. Both computer modeling and experiments were performed to study different parameters and effects. In that study, it was determined that among hydrogen incidents the ignition source could not be identified in 86% of the cases and was probably caused by spontaneous ignition. However, in another research project, Wu (47) showed that conditions of oxygen deficit could be reached for a higher release rate of hydrogen. This can lead to higher temperature ceiling flows and damage to tunnel structures. For hydrogen buses with internal combustion engines, these impacts also apply, and the installation of hydrogen sensors is advisable. Batteries Electric cars that use batteries as an energy source are seen as the single most promising future energy carrier, in partic- ular, for city traffic. One problem is the relatively short avail- able driving distance before recharging is needed. Therefore, hybrid solutions are currently of greatest interest. In most cases a hybrid vehicle has both a conventional internal com- bustion engine and an electric motor. There are also plug-in electric vehicles, with batteries that can be plugged in for charging, such as to house electricity, in addition to being charged while running. Presently, nickel-metal-hydride batteries are the most common used batteries in hybrid vehicles. These batteries are robust, but have a relatively high self-discharge rate. Therefore, for a variety of reasons, most interest is currently directed to lithium-ion batteries. Lithium-ion batteries have a high energy density and a high cell voltage. In addition, the maintenance need is low and there are no memory effects. However, to limit the peak voltage during charging for safe operation, a protection circuit is built into each battery pack. This also limits the discharge current. Other safety features are also studied for lithium-ion batteries. Two main types of risks can occur with vehicle batteries. One is that the battery (system) itself is the cause of the incident, such as with an electrical fault, which can be caused by a short 62 circuit or an overcharge, and could result in a fire. The other is that the battery is exposed to an external risk, either some mechanical force or a thermal attack, as with a fire. There have been instances of batteries exploding or releasing jet fires. There are some who believe that electric cars have been responsible for the larger number of fires when compared with nonelectric cars. This type of fire can also emit toxic fumes from hydrogen fluoride and oxides of carbon, aluminum, lithium, copper, and cobalt. The lithium salts used in the electrolyte contain fluorine or a chlorine compound, where hydrogen fluoride or hydrogen chloride can be produced during a fire. There are some restrictions and regulations concerning the use of alternative energy carriers, especially for compressed or liquefied gases. In relation to underground constructions, most restrictions concern underground garages; however, some also specifically address tunnels. Many of the restric- tions can be related to LPG, which is also considered to be an alternative fuel, together with liquefied natural gas (LNG), CNG, hydrogen, propane, methanol, ethanol, and biodiesel, in accordance with the U.S. Energy Policy Act of 2005. LPG vehicles run on liquefied gas, which is denser than air. The following are examples of tunnels where LPG and CNG are restricted (see Table 17): • In Maryland, LPG is forbidden in the Baltimore Harbor and Fort McHenry tunnels. • LPG is forbidden in the Summer, Callahan, Prudential, and Dewey Square tunnels in Massachusetts. • LPG is forbidden in the Holland, Lincoln, Brooklyn Battery, and Queens Midtown tunnels in New York and New Jersey. • In Virginia, an LPG ban covers the Chesapeake Bay Bridge tunnel. • In Italy, vehicles using LPG or gas are labeled before entering the Mont Blanc Tunnel or the Frejus Tunnel. • In France and the United Kingdom vehicles running on gas are prohibited in the Euro Tunnel. • In Austria, LPG and CNG are not permitted in the Tauern Tunnel. However, there are no restrictions on LPG vehicles in tunnels in Japan and many other countries. Some examples of LPG fire incidents include: • A car crash in a highway tunnel near Palermo, Italy, occurred on March 18, 1996. The accident involved a tank truck transporting LPG, which caused propane to be released, which formed a burning gas cloud resulting in critical burns to 25 people. The subsequent boiling liquid expanding vapor explosion (BLEVE) led to five fatalities. The cause of the accident was not strictly the result of a new energy carrier, but it did involve a vehicle transporting fuel for a new energy carrier.

63 Date Place Type of Premises No. of vehicles Fuel Ignition Consequences Jan. 31, 1999 Venissieux, France 1 LPG Arson Explosion; 6 fire fighters severely injured Sep. 2002 U.S. 1 CNG Car fire Rupture of gas cylinder Nov. 9, 2002 Seine-et- Marne, France Garage 1 LPG Unknown Explosion; building of origin collapsed; in total 39 buildings affected Aug. 28, 2005 Firenze, Italy San Donato tunnel LPG Engine fire Dense smoke June 2006 Collatino, Italy Parked on the street 1 LPG Arson Explosion, several cars, 2 garages, shops, fire spread to apartments March 2007 Seattle, WA U.S. Row of parked vehicles 12 One with CNG Arson 12 cars damaged or destroyed; CNG tank exploded when fire fighters were approaching; debris approx. 30 m away May 2007 Carson, CA, U.S. Refueling 1 CNG Driver killed Dec. 16, 2007 Salerno, Italy Underground garage LPG Leakage of gas from vehicle Explosion; one 3-storey building totally destroyed; 5 other buildings affected June 7, 2008 U.S. Running on the highway 1 Hybrid converted to plug-in Short circuit One burned- out car Sept. 19, 2008 Rovigno, Italy Underground garage LPG Fire spread to neighboring garage and threatened the building Oct. 2008 South Yorkshire, U.K. Running on the road 1 LPG Lighting of cigarette Explosion, burns, broken windows Nov. 8, 2008 Mallaca, Malaysia Filling station 1 LPG Explosion of vehicle; passengers severely injured Dec. 28, 2008 Sampford Peverell, U.K. Running on the highway 1 LPG Unknown One burned- out car Oct. 28, 2009 Marigliano, Italy Parking 6 One with LPG The cause of the initial fire unknown Large explosion damaged vehicles and buildings Source: Lönnermark (48). TABLE 17 SUMMARY OF KNOWN INCIDENTS INVOLVING CARS RUNNING ON LPG OR NEW ENERGY CARRIERS (NOT ALL IN TUNNELS)

• On the night of January 31, 1999, a vehicle fuelled with LPG was set on fire by an arsonist in Vennissieux out- side Lyon, France. The LPG system was not equipped with a safety valve. This led to an increase in pressure in the tank during the fire and the tank later exploded. Six firemen attempting to extinguish the fire were severely injured. This incident led to action that could help avoid this kind of incident in the future. Later, a requirement that such vehicles have safety valves was introduced. • On November 9, 2002, a vehicle fuelled with LPG began to leak in a garage in Seineet-Marne in France. The high density of the gas allowed it to spread over a large area and down into the basement. At 11 p.m. the gas ignited, an explosion occurred, and the building collapsed, burying several individuals, who were later saved. In total, the explosion affected 39 buildings within a radius of 200 m. The roof of the LPG vehicle was found 150 m from the place where the vehicle had been parked. • In June 2006, arsonists ignited an LPG-fuelled vehicle in Collatino, Italy. The car was parked with other vehicles on a street outside an apartment building. The fire started in the rear part of the vehicle, where the LPG tank was positioned. The subsequent explosion of the tank led to an intense fire, which ignited several other cars. The pressure wave destroyed two small garages and shops located in the apartment building. The fire damaged the façade and several balconies. • In March 2007, an arsonist set fire to a row of vehicles parked under a highway bridge in Seattle. The first responders were not aware that one of the cars was CNG-fuelled. When they were 15 to 20 m (49.2 to 65.6 ft) from the burning vehicles the CNG tank exploded. The fuel tank and other large pieces of debris landed about 30 m (98.4 ft) from the CNG vehicle. The fuel tank was equipped with a safety valve, but exploded before the valve could release the pressure. • In May 2007, a CNG tank in a vehicle in Carson, Cali- fornia, ruptured. The rupture occurred during refueling and killed the driver. A day earlier, the driver had col- lected the vehicle from a repair shop after a collision three weeks prior. • In June 2008, a fire in a hybrid car converted to a plug-in started while the car was running. The car used a lithium- ion battery, which was partly damaged during the fire, but still provided power. According to the investiga- tion, the most probable explanation of the incident was incorrect electrical wiring, which led to excessive heat generation. The heat destroyed some cells in the battery leading to a short circuit and the fire. • In October 2008, a car running on LPG suddenly exploded in South Yorkshire, United Kingdom. Remark- ably the driver survived and was able to describe the accident. He had recently refueled this car and was pro- ceeding slowly when he smelled gas. He had been told that this was normal after refueling. When he lit a ciga- rette the gas was ignited and filled the car with flames. 64 Owing to the increase in pressure, the windows broke and the bonnet and the trunk blew opened. The driver suffered minor burns to the face and body, but the seat absorbed most of the energy of the explosion, saving his life. The most likely explanation for the explosion was a leak in the tube between the filling valve and the tank. The car, which had been purchased second-hand three weeks earlier, had been checked and approved twice at workshops. • The most recent reported incident occurred on October 28, 2009, in Marigliano, Italy. A fire started in a parked car running on a traditional fuel. It developed quickly and spread to nearby vehicles. Six cars were ultimately involved, including one using LPG, which quickly exploded after catching on fire. The explosion damaged cars in the vicinity and a nearby building. Debris from the exploded car was found on the balcony of that build- ing and windows were broken up to the eighth floor. Stores at street level sustained severe damage. In addition to these car fires, some conclusions can be drawn from various bus fires. Three bus fires involving CNG tanks are analyzed here. The first responders were unable to extinguish these fires. The first conclusion was that the pressure relief devices (PRDs) do not always release. This can happen when there is local thermal exposure, such as from an imping- ing jet flame, which leads to insufficient heat for the PRD, or it could be a malfunctioning heat release device. Either way it is important to minimize or eliminate areas with weaker fire protection, such as sun roofs, which could lead to such localized fire exposure. Another important issue is the time necessary to completely empty the tank. In the incidents described, it would be preferable to have early PRD opening and fast emptying of the tank, although the situation could be completely different if the buses had been located in a confined environment such as a tunnel or underground garage. One main conclusion is that the safety of these types of vehicles does not rely only on component tests. For example, it is important to test the entire system, where the tanks and other components can be evaluated using relevant and realistic scenarios. The incidents summarized and described earlier are not meant to imply that all vehicles running on new energy carriers will explode when used or when exposed to fire. However, seeking the worst case scenarios is important when new energy carriers are developed. It is also important to realize that all risks are not eliminated by introducing PRDs. The outcome still depends on the design of these devices and on the fire scenario. Wu of Sheffield University performed a CFD analysis of hydrogen fires in tunnels. Hydrogen cars generate fast, high rising flames that quickly reach high temperatures (47). The body of the hydrogen car was not ignited and the flames lasted only a few minutes. It was concluded that a supercritical velocity in the tunnel can completely eliminate the smoke

65 backlayering with a normal hydrogen HRR or keep the back- layering under control with a high HRR. She concluded that with a high HRR the flame inside the tunnel may have encountered oxygen deficiency. This will result in the impinge- ment of hydrogen jet flames on the tunnel ceiling, which would produce high temperature ceiling flows reaching substantial distances and damage the tunnel infrastructure. The oxygen- deficient hydrogen fire also poses a risk of flashover inside the tunnel and ventilation ducts. In early 2004, fire tests of FCVs in the event of low pres- sures of 20 MPa (2900.8 psi) and high pressures of 35 MPa (5076.3 psi) were conducted in Japan in a simulated full-scale tunnel 80 m (262.5 ft) long with a cross-sectional area of 78 m2 (840 ft2). Tests were also performed with the natural gas cars (CNG) for comparison. CNG cars and FCVs generated a large quantity of heat compared with gasoline cars. The flame of the CNG cars and FCVs tended to rise faster when compared with gasoline cars. The highest air temperature was reached at 6 m (19.7 ft) above the roadbed at 319°C (606°F) for CNG cars, 243°C (469°F) for FCVs with high pressure, 228°C (442°F) for gasoline cars, and 166°C (331°F) for FCVs with low pressure. The maximum radiation heat for CNG cars was 5125 W/m2 (1625 Btu/hr/ft2); for gasoline cars, 4471 W/m2 (1417 Btu/hr/ft2); for FCVs with high pressure, 4141 W/m2 (1313 Btu/hr/ft2); and for FCVs with low pres- sure, 1774 W/m2 (562 Btu/hr/ft2). In all cases, the temperature rose to 1100°C (2012°F). In the case of FCVs with high pres- sure the temperature grew rapidly to 1435°C (2615°F) within 290 s. According to an inspection of the concrete above the fire, damage was limited, with little impact on its compres- sion strength. At its conclusion, the CNG and FCV cars caught fire rapidly and burned intensely. With air velocities of 2 m/s (394 fpm), stratification was observed; therefore, the tenable environment was maintained at 1.5 m (4.9 ft) from the roadbed. A concern was raised of possible gas deto- nation if tunnel air velocity reached close to 0. Additional research and modeling is needed. It is difficult to properly evaluate what are the emerging trends concerning use and what risk scenarios are possible or most likely with alternative fuel vehicles. This can be, for example, a problem for the rescue services, because they will be exposed to incidents involving different types of fuels and energy carriers. This means that they must have information concerning not only the situation itself but also the energy carriers involved. Some tunnels require drivers of vehicles running on CNG or LPG to report this before entering the tunnel and to correspondingly label their vehicles. It is impor- tant that an overall system be developed as the diversity of vehicles increases. There are a variety of views on how vehicles running on LPG, CNG, or similar fuels are treated and what safety mea- sures are needed. It is important that restrictions are premised on correct information based on additional systematic research on new energy carriers. It is important to provide correct and detailed information concerning safety issues and the behav- ior of these energy carriers where a fire can develop. Systems, not only components, need to be tested to within different scenarios and that models be developed for these scenarios. When the scenarios are described in a representa- tive way, technical safety solutions, mitigations systems, and rescue service tactics can be developed. It is also important to study how the different systems (detection, ventilation, and mitigation) interact, and how the models developed are altered depending on the scenario. The incidents analyzed show that when there is a fire new energy carriers can explode with catastrophic consequences. The outcome does, however, vary with different scenarios. It is important to learn from incidents that have occurred, and that experiments and relevant research be performed to maximize the understanding of the risks. Such incidents also show that safety systems do malfunction, especially in used vehicles. Such malfunctions can be the result of accidents, mistakes, conversions, or erroneous repairs, but the conse- quences of such malfunctions are always potentially serious. The field of new energy carriers is very diverse and con- stitutes many different areas of research. This makes a detailed review of all aspects of risks associated with new energy carriers and safety in tunnels beyond the scope of this study. On the other hand, this is exactly why this issue is so important. When new energy carriers are developed and used in vehicles traveling through tunnels, a variety of different safety aspects converge and need to be dealt with properly and promptly. Clearly, more research is needed concerning how safety in tunnels is affected by the introduction and development of new energy carriers. FIRE SMOKE AND SMOKE PRODUCTION— LITERATURE REVIEW Almost all fires generate smoke. Smoke is a mixture of gases, fumes, and particles. The generation of smoke is affected by the following factors: • Possible reduced supply of oxygen to the fire site, • Heat release, • Heat convection, • Longitudinal slope, • Type of ventilation, • Dimensions of the traffic space and possible obstructions, • Thrust caused by any moving vehicles, and • Meteorological influences (wind strength and direction). Smoke mixes with the surrounding air and dilutes in the plume. This process depends on the size of the source of fire, fire and air temperature, buoyancy, and height in the plume. With no obstructions and no longitudinal air move- ment, the plume of smoke and hot gases rises to the tunnel

ceiling directly above the source of the fire and spreads in both directions, fire forming a relatively dense smoke layer. A relatively low-density cold smoke layer sits below the hot layer. Basically, it can be said that as a result of the heat released around the fire site and thermal buoyancy, the smoke is lifted up to the ceiling near the fire site and spread in the upper area of the tunnel. The smoke continues its flow in one direction when the longitudinal velocity is high (with or without back- layering), but in both directions when the longitudinal velocity is low. Thus, there is a limited space above the road surface without any smoke gases, at least for a short period of time. Note that this may not be true for small fires with limited heat dissipation, because the smoke can be relatively cold. A smoke layer may be created in tunnels at the early stages of a fire with essentially no longitudinal ventilation. However, the smoke layer will gradually descend further from the fire. If the tunnel is very long, the smoke layer may descend to the tunnel surface at a specific distance from the fire depending on the fire size, tunnel type, and the perimeter and height of the tunnel cross section. When the longitudinal ventilation is gradually increased, the stratified layer will gradually dissolve. A backlayering of smoke is created on the upstream side of the fire. Downstream from the fire there is a degree of stratification of the smoke that is governed by the heat losses to the surrounding walls and by the turbulent mixing between the buoyant smoke layers and the normally opposite moving cold layer. The particular dimensionless group, which determines whether a gas will stratify above another, is the Richardson number (Ri) defined by Eq. 16. The Richardson number is similar to the inverse of the Froude number (Fr) defined by Eq. 15; however, the Richardson number is thought of as controlling a mass transfer between layers, whereas the Froude number gives the general shape of a layer in an air stream. The destratification downstream from the fire is a result of the mixing process between the cold air stream and the hot plume flow created by the fire. The phenomenon is 3D in the region close to the fire plume. The gravitational forces tend to suppress the turbulent mixing between the two different density flows. It becomes possible for cold unreacted air to bypass or pass beneath the fire plume without mixing, even though the flow is turbulent. The longitudinal aspect of the fuel involved in the fire, therefore, may play an important role in the mix- ing process between the longitudinal flow and the fuel vapors generated by the fire. There is a correlation between temperature stratification at a given location and the local mass concentration of chemical compounds. There is also a correlation between local smoke OD (or visibility), the local density (or temperature), and the oxygen concentration in tunnels. Therefore, it is reasonable to 66 assume that there is a correlation between the local tempera- ture stratification, the gaseous composition (CO, CO2, O2, etc.), and smoke stratification in tunnels. The temperature stratifi- cation is, however, not only related to the air velocity but also to the HRR and the height of the tunnel. These parameters can actually be related through the local Froude number (Fr) or Richardson number. Three distinct regions of temperature stratification are defined by the Froude number (Fr) or Richardson number. The first region (region I), when Fr < 0.9, results in severe stratification, in which hot combustion products travel along the ceiling. For region I, the gas temperature near the floor is essentially ambient. This region consists of buoyancy- dominated temperature stratification. Also, this region is next to the fire location and allows for the evacuation of motorists. The second region (region II), when 0.9 < Fr < 10, is dominated by strong interaction between imposed horizontal flow and buoyancy forces. Although not severely stratified or layered, it involves vertical temperature gradients and is mixture-controlled. In other words, there is significant inter- action between the ventilation velocity and the fire-induced buoyancy. The third region (region II1), when Fr > 10, has insignificant vertical temperature gradients and consequently insignificant stratification. Because a tunnel can be used by different types of vehicles, such as cars, buses, trucks, and special vehicles, which may have different loads (persons, nonflammable cargo, flammable cargo, explosives, toxic goods, etc.), it is possible that tunnel fires may differ in terms of quantity and quality. In most cases, car fires are relatively harmless for small tunnel fires with minor temperature and smoke development. However, it is very dangerous when there is a tanker fire with the result- ing high temperatures and enormous smoke production, plus the danger of explosion. Therefore, it is not possible to describe the temperature and smoke development for every possible kind of tunnel fire. The main design parameter is the smoke flow rate produced by the fire. For the smoke flow rates by fires of passenger cars, buses, and trucks, the PIARC assumptions in the Brussels’ report were confirmed by the EUREKA fire tests. German regulations (RABT from the year 1994) quote smoke produc- tion rates somewhat higher than those of PIARC. CFD calculations made in France by CETU (Centre d’Etudes des Tunnels) show a decrease in smoke volume flow with increased distance to the fire for HRRs above 60 MW (205 MBtu/hr) (49), as shown in Figure 14. For fires up to 60 MW (205 MBtu/hr), the volume flow does not depend on this distance. From at least 10 to 120 m (32.8 to 393.7 ft) from the fire, the smoke cools down; how-

67 ever, fresh air is entrained so that the volume flow does not change. For 100–150 MW (341–512 MBtu/hr) fires, the entrainment of fresh air does not compensate for the very strong reduction of smoke temperature 50 to 100 m (164 to 328 ft) from the fire. These calculations were performed with no longitudinal ventilation airflow. The smoke flow rate was calculated as the volume flow of gases that moved away from the fire in the upper part of two cross sections located at given distances at both sides of the fire. Also, according to the CFD results, the smoke flow rate varies nearly linearly with the HRR—from about 50 m3/s (1,765.7 ft3/s) at approximately 10 MW (34 MBtu/hr) to about 250 m3/s (8,828.7 ft3/s) at approximately 150 MW (512 MBtu/hr), as shown in Figure 15. Table 18 presents smoke production rates, CO, and CO2 as published in different literature sources (summarized exper- imental results and standards values). To convert the smoke masses produced to smoke volumes it is necessary to know the smoke temperatures. The theoretical stoichiometric com- bustion temperatures of regular gasoline are about 2000°C (3632°F). The real fire temperatures are usually much lower, primarily because the combustion is not stoichiometric or because the smoke mixes with air. The dangerous nature of smoke gases in tunnel facilities not only results from the visibility obscuring effect but also from FIGURE 14 Variation of smoke volume flow with (plume flow) distance to fire (CETU)(9). FIGURE 15 Smoke flow rate versus fire heat release rate (9, 50). Burning Vehicle Smoke Flow [m3/s (ft³/s)] CO2 Production (EUREKA tests) [kg/s (lb/s)] CO Production [kg/s (lb/s)] PIARC (1987) RABT (1994) EUREKA Tests CETU (1996) Passenger Car 20 (706) 20–40 (706– 1,412) — 20 (706) — — Passenger Van (plastic) — 30 (1,059.4) 30 (1,060) 0.4–0.9 (0.88–2) 0.020–0.046 (0.04–0.1) 2–3 Passenger Cars — — 30 (1,060) — — 1 van — — 50 (1,765) — — Bus/Truck Without Dangerous Goods 60 (2,120) 60–90 (2,120– 3,180) 50–60 (1,765– 2,120) 80 (2,825) 1.5–2.5 (3.3–5.5) 0.077–0.128 (0.17–0.28) Heavy Goods Vehicle — — 50–80 (1,765– 2,825) 6.0–14.0 (13.2–30.9) 0.306–0.714 (0.67–1.57) Gasoline Tanker 100–200 (3,531– 7,063) 150–300 (5,300– 10,600) — 300 (10,600) — — Sources: Fire in Tunnels (9) and PIARC (21). TABLE 18 SMOKE, CO2 AND CO PRODUCTION

the possible toxicity of gases including CO, carbon dioxide (CO2), and other gases depending on the burning materials, especially toxicity caused by cargo. To address these con- cerns, during the EUREKA and Runehamar fire tests, the CO and CO2 levels were monitored at several measuring points along the tunnel. During the EUREKA fire tests, the CO level was monitored at several measuring points along the tunnel. In the region from approximately 20 to 30 m (65.6 to 98.4 ft) downstream of the burning vehicles, the following peak CO concentrations were measured at head height: • Passenger van (plastic): 300 ppm • Public bus: 2,900 ppm • HGV: 6,500 ppm. CO concentrations of more than 500 ppm were exceeded from about 10 to 15 min from the start of the fire and lasted approximately 2 h during the bus fire and approximately 15 min during the HGV fire. During an experiment with a mixed fire load, CO concentrations of 500 ppm and more occurred not before about 80 min after the start of the fire and lasted for 90 min. The EUREKA results depend very much on the different ventilations of the test tunnel during the fire tests. Furthermore, they are related to the type of burning material. Therefore, the EUREKA results may not be transferred directly to other tunnels. However, the EUREKA results indicated that down- stream of the fires there is, at least for larger fires, a need for escape and rescue within about 10 to 15 min from the start of a fire. Harmful CO concentrations are also expected in the progressive stage of vehicle fires. The mass generation of CO2 can be estimated using a ratio of 0.1 kg/s per MW of HRR. A reasonable linear correlation between the production rates of CO2 and CO was found when analyzing the EUREKA test data. These results suggest an average ratio of 0.051, with a standard deviation of ±0.015. This average is used for the calculation of the CO production rates. As an order of mag- nitude, the volume concentration of CO is also approximately 5% of the concentration of CO2. The correlation of the smoke-dependent visibility mea- sured by the OD and the concentration of CO2 produces a lin- ear relation when a correction for the smoke gas temperature is made. The following formula can be used to estimate the OD from the CO2 volume concentration: where: T is the local temperature in Kelvin, T0 = 273 K; [CO2] is the concentration in percent of volume; and α is a coefficient which is: OD T T= ( ) ⎡⎣ ⎤⎦α g g0 2 18CO ( ) 68 • Approximately 1.3 for the plastic passenger van fire, • Approximately 0.5 for the bus fire, and • Approximately 0.8 for the HGV fire. Another approach is based on the mass OD. Visibility depends on: • Smoke density, • Tunnel lighting, • Shape and color of objects and signs, • Light absorption of smoke, and • Toxicity of smoke (eyes irritating). The visibility in smoke can be related to the extinction coefficient, K, by the following equation: where: OD is the optical density, and X is the path length of light through smoke. The optical density per unit optical path length can also be expressed as: where: ζ is the specific extinction coefficient of smoke or particle OD (m2/kg); Ys is the yield of smoke (g/g); mf denotes the mass flow of material vapors of the burning material; VT is the total local volumetric flow rate of the mixture of fire products at the actual location (measuring point) and air (m3/s); ζ Ys is defined as mass OD, Dmass (m2/g); Q is the HRR in kW at the actual location and H is the effective heat of combustion (kJ/kg) obtained from the tables for different materials (but not of the burning vehicle); and u (m/s) is a unified longitudinal ventilation velocity across the tunnel cross section A (m2). For objects such as walls, floors, and doors in an under- ground arcade or long corridor the relation between visi- bility and the extinction coefficient was defined earlier by Eq. 13. Thus, by combining the equations, a correlation between the visibility V and the HRR in a tunnel at an actual position OD X Y m V D Q uAHs f T = =ζ g g mass ( )20 K OD X = ( )ln ( )10 19

69 downstream of the fire with a ventilation air velocity of u (m/s) is: In Table 19 values of Dmass for different types of vehicles are given based on large-scale tests. These values may be used as an engineering tool for determining the visibility in fires depending on the fuel load. For CFD modeling, engineers use equations and tables of yields of CO, CO2, HCN, heat of combustion, production of soot, and mass OD for different types of materials, such as wood, polyurethane foam, polystyrene, and mineral oil. Such tables can be found in the SFPE Handbook for Fire Protection Engineering (51) and other literature. Surprisingly, the vehicles are assumed to be one material, which leads to inconsistency in the results, as there is no uniform agreement on the numbers to use and to the inaccuracy of the CFD results. The average mole fraction Xi,avg of CO2, CO, or HCN over the cross section of the tunnel and at a certain position down- V uAHQD= 0 87 21. ( )mass stream of the fire can be obtained from the following general equation: Assuming ma ∼ mg, where mg is the mass flow rate of combus- tion gases. Here Ma is the molecular weight of air, Mi is the molecular weight of chemical species i, and Yi is the mass yield of species i for well-ventilated fires. The value of Xi,avg can be converted into a percentage by multiplying it by 100. The yields of YCO2, YCO, and YHCN for well-ventilated conditions can be obtained for different fuels. Table 20 presents some values for different fuels for well- ventilated conditions. A lack of sufficient experimental data and test results requires designers to use values from this table. By using this table, the designer is making an assumption by replacing an actual vehicle fire with pseudo-fuel. Different designers use different fuels and different values to approxi- mate the actual fuel, which causes inconsistency in modeling and design results. The yield values are the mean values for different material types (polyurethane foam, polystyrene, mineral oil). However, there is a need to replace the simulated materials with design values for fires involving HGVs, buses, cars, and tankers. Additional testing results are needed. TEMPERATURE OF FIRE GASES AND TUNNEL WALLS Tunnel fires significantly increase the air temperature in the tunnel roadway and in the exhaust air duct. Therefore, both the tunnel structure and ventilation equipment are exposed to high smoke and gas temperature. The air temperatures, X Y M M Q T m Hi i a i a T, ( )avg = × × ( ) × 22Type of Vehicle Average Mass Optical Density Dmass (m2/kg or ft2/lb) Car (steel) 381 (1,860) Car (plastic) 330 (1,610) )199(302suB Truck 76–102 (371–498) Source: Fire in Tunnels (9). TABLE 19 MASS OPTICAL DENSITY FROM BURNING VEHICLES Type of Material YC0 kg/kg YCO kg/kg YHCN kg/kg Ys kg/kg Dmass m2/kg (ft2/lb) Hec MJ/kg (Btu/lb) 73510.0400.072.1dooW (181) 12.4 (5,331) Rigid Polyurethane Foam 1.50 0.027 0.01 0.131 304 (1,480) 16.4 (7,050) 533461.060.033.2enerytsyloP (1,640) 27 (11,610) 7.13790.0140.073.2liOlareniM (13,630) Swiss Fire Modeling Assumption on Average of Three Materials Above 2.07 0.043 0.01 0.13 Source: SFPE Handbook of Fire Protection Engineering (51). Ys = yield of smoke. Dmass = mass optical density and is proportional to yield of smoke. Hec = XHT – effective heat of combustion. Mass loss rate of the fuel, kg/s: mf = Q(T)/ Hec. Q(T) = fire heat release rate, HRR (kW). 2 TABLE 20 YIELDS OF CO2, CO, HCN, AND SMOKE AND EFFECTIVE HEAT OF COMBUSTION, FOR WELL-VENTILATED FIRES

shown in Table 21, provide guidance in selecting design expo- sure temperatures for ventilation equipment. British standards provided data on distances over which jet fans were assumed to be destroyed by the fire; this is reproduced in Table 22. BD 78/99 also specifies that heavy items, such as fans, subjected to temperatures of 450°C (842°F), are to be designed to not fall down during the fire- fighting phase (52). The French Inter-Ministry Circular (2000) specifies that jet fans must be capable of operating continuously in smoke- laden air at a temperature of 200°C (392°F) for at least 2 h. For transverse ventilation systems, a distinction must be made on the basis of whether the fans are or are not likely to be subjected to very high temperatures. In the general case, extraction fans, located at the end of a duct, must be capable of operating at a temperature of 200°C (392°F) for at least 120 min. However, under certain circumstances, it may be necessary for the fans to be capable of withstanding 400°C (752°F) for at least 120 min. Rather than providing informa- tion on the distances over which jet fans may be considered as destroyed, the French guidance provides smoke tempera- tures at various distances (CETU 2003). This is reproduced 70 in Table 23. This also refers to the need to ensure that equip- ment does not fall when exposed to a temperature of 450°C (842°F) for at least 120 min. Different fire characteristics are needed depending on whether the purpose is to design the tunnel structure or the ventilation facilities. • The design of structures for fire resistance is based on the temperature of the hot air (degrees centigrade or degrees Fahrenheit) and radiation heat versus time. • The design of a ventilation system is based on the HRR (thermal power in megawatts or million British thermal units per hour) or the smoke release rate (flow at the temperature of the hot smoke in cubic meters per second) versus time. The dependence on time is important for evaluating the conditions at the beginning of the fire, taking into account the self-rescue phase (time for the fire department to arrive and get organized). PIARC recommends the following maximum temper- atures at the tunnel wall or ceiling to be considered for Nominal FHRR, MW (MBtu/h ) Temperature at Central Fans, a °C (°F) Temperature at Jet Fans, b °C (°F) 20 (68) 107 (225) 232 (450 ) 50 (170) 124 (255) 371 (700) 100 (340) 163 (325) 677 (1250) Source: ASHRAE Handbook (22). FHRR = Fire heat release rate. aCentral fans located 700 ft (213 m) from fire site. bJet fans located 170 ft (52 m) downstream of fire site. TABLE 21 MAXIMUM AIR TEMPERATURES EXPERIENCED AT VENTILATION FANS DURING MEMORIAL TUNNEL FIRE VENTILATION TEST PROGRAM Fire Size, MW (MBtu/h) Distance Upstream of Fire, m (ft) Distance Downstream of Fire, m (ft) ——)71(5 20 (68) 10 (32.8) 40 (131.2) 50 (171) 20 (65.6) 80 (262.5) 100 (341) 30 (98.4) 120 (393.7) Source: Hall (52). TABLE 22 DISTANCES OVER WHICH JET FANS ARE ASSUMED TO BE DESTROYED BY FIRE (BD 78/99) Downstream Distance 10 m 100 m 200 m 400 m Light Vehicle 250°C 80°C 40°C 30°C Heavy Vehicle 700°C 250°C 120°C 60°C Tanker >1000°C 400°C 200°C 100°C Source: Hall (52). TABLE 23 SMOKE TEMPERATURES NEAR THE CEILING, WITH AIRFLOW CLOSE TO CRITICAL VELOCITY

71 tunnel structure and the cargo-traffic regulations for specific tunnels: • Passenger car—400°C (752°F)* • Bus/small truck—700°C (1292°F)* • HGV with burning goods (not gasoline or other danger- ous goods)—1000°C (1832°F) • Gasoline tanker (general case)—1200°C (2192°F) • Gasoline tanker (extreme cases: e.g., no benefits owing to tunnel drainage and limited leakage rate; large tanker; avoidance of the flooding of an immersed tunnel)— 1400°C (2552°F). These temperatures were estimated for a location 10 m (32.8 ft) downwind of the fire near the tunnel walls at the minimum air velocity to prevent backlayering. The EUREKA tests confirmed these maximum temperatures. The tests them- selves gave slightly higher results for the passenger cars [up to 500°C (932°F), depending on type] and the coach [800°C (1472°F)] because of the small cross-sectional area and the low air velocity used [0.3 m/s and 0.5 m/s (59.1 and 98.4 fpm)] in the test tunnel. The fire tests of EUREKA and Runehamar also showed that fires resulting from HGVs can produce maximum temperatures between 1000°C and 1350°C (1832°F and 2462°F) at the tunnel ceiling. For fully devel- oped fires of gasoline tankers, temperatures between 1200°C and 1400°C (2192°F and 2552°F) are studied. As can be seen by Figure 16 in the EUREKA tests, temper- atures of more than 300°C (572°F), which can be dangerous to the steel reinforcement of the concrete tunnel lining, were found as far as approximately 100 m (328 ft) downstream of the fire. In addition, because of backlayering, this tempera- ture can be reached about 30 m (98.4 ft) upstream of the fire. According to actual fires and to the Memorial Tunnel tests, the extension of this region can be quite different from these values owning to many factors, such as the ventilation, tunnel grade, surface roughness, and fire-resistant coatings. Many known real tunnel fires and also the EUREKA and Runehamar fires showed a very fast development during the first 5 to 10 (sometimes 15) min. The gradient of temperature is especially steep at the beginning of a full car fire, with a corresponding high emission of heat and smoke. Between 7 and 10 min after ignition a flashover needs to be taken into account (even sooner in the case of a passenger car). The temperature during the Runehamar fires followed the Rijkswaterstaat (RWS) curve. That test comprises the largest amount of combustible material of the four tests conducted. With the lowest calorific energy output the temperatures were recorded to be in the same magnitude, although for a shorter period of time. The duration of the hot phases of the EUREKA and Runehamar fires covered normally a time interval of about 30 min after the ignition stage. On the other hand, the Mont Blanc and Nihonzaka fires lasted significantly longer. The EUREKA and Runehamar tests showed a steep decline of temperatures just after the hot phase. FIRE DEVELOPMENT BASED ON LITERATURE REVIEW Combustible materials in a vehicle or tunnel are set on fire by an external ignition source. Energy is released and part of the solid matter of the fire material is converted into gases being part of the smoke. These gases mix with ambient tunnel air. The constant release of energy greatly heats up the mixture of combustion gases and air, forcing it upwards, the phenomena of buoyancy effect. There is also direct radiation from the flames. The heated gas–air mixture comes into contact with the ceiling and walls. The mixture conveys part of its heat to surrounding surfaces through radiation and thermal conduc- tion and continues spreading it through the tunnel as smoke, with the temperature progressively declining as it moves away from the fire source. The thickness of the smoke and its concentration are reduced as it mixes with the tunnel air. The ability to escape the smoky environment depends on the smoke’s concentration and the height of the smoke layer above the roadbed. The combustion process efficiency depends on sufficient oxygen availability. The air stream caused by the fire often creates a suction effect that assures oxygen supply from the portals or shafts. This results in continuous feeding of the fire with oxygen, which allows for continuous heating of the fire materials, and possible re-ignition. The process continues until either the combustible material is completely burned or the fire extinguishing measures interrupt the burning process. The growth and development of a fire will be influenced in its early stages by the ignition scenario and the fire performance of the materials. Fires can start developing inside vehicles or*Higher temperature if flames touch the walls. FIGURE 16 Maximum gas temperatures in the ceiling area of the tunnel during tests with road vehicles (21).

outside in the cargo container. As fires develop, heat builds up leading to elevated gas temperatures within the enclosure. The elevated temperatures will in turn have a significant impact on the growth rate of the fire. Elevated gas temperatures will pre-heat materials that have not been ignited and potentially accelerate flame spread. Gas temperatures in an enclosure can be affected by the size of the enclosure, the ventilation into the enclosure, and the FHRR (see Figure 17). Development of fires inside vehicles is dependent on a number of factors including: (1) the fire performance of interior materials and features, (2) the fire performance of vehicle cargo, (3) the size and location of the initiating fire event or ignition scenario, (4) the size of the enclosure where the fire is located, and (5) the ventilation into the enclosure. The specification of a design fire may include the following phases: 1. Incipient Phase, characterized by the initiating source such as smoldering or flaming fire. 72 2. Growth Phase, the period of propagation spread poten- tially leading to flashover or full fuel involvement. 3. Fully Developed Phase, the nominally steady ventilation or fuel-controlled burning. 4. Decay Phase, the period of declining fire severity. 5. Extinction Phase, the point at which no more heat energy is being released. Figure 18 represents all phases of fire development. A smoldering fire is caused by a combination of the fol- lowing (input) parameters: 1. Nature of the fuel, 2. Limitation of ventilation, and 3. Strength of the ignition source. The smoldering fire generally burns over a long period in limited ventilation conditions with insufficient oxygen to fully burn the fuel. It produces relatively low levels of heat, but considerable unburned combustibles and a higher con- centration of smoke. This creates a relatively low visibility with large toxic products of combustion such as CO and soot (e.g., the burning of rubber tires of those vehicles involved in the fire). The relatively low temperatures generated create less buoyancy in the combustion products, and thus decreases the likelihood of smoke stratification under the tunnel roof as with hotter fires. Therefore, the principal hazards posed by a smoldering fire are high concentrations of CO and low visi- bility conditions. The construction and combustible contents of a vehicle, such as electrical fault or overheating parts in the engine compartment, could be a potential source of a smol- dering fire in tunnels. Pre-flashover fires include the incipient and growth phases and are of primary interest in life safety analyses. The growth FIGURE 17 Maximum gas temperatures in the cross section of the tunnel during tests with road vehicles (21). FIGURE 18 Simplified phases of fire development. Note: Sprinkler activation is shown as a representative example and its impact on fire development depends on its activation time and sprinkler system characteristics discussed later (9).

73 of a fire is dependent on fuel and the availability of oxygen for combustion. Typically, as the fire grows in size, the rate of growth accelerates. The rate of fire growth may be modified owing to compartment effects, radiative feedback, activation of sprinklers or the application of other suppressants, avail- ability of fuel, and the availability of oxygen, among other factors. It is important to recognize that the total fuel load has little bearing on the rate of fire growth; however, the rate of fire growth is governed by the HRR of the individual fuel items burning. There are numerous methods available to mathematically represent a design fire curve in tunnels. These include different types of fire growth rates; for example, linear growth (a  t), quadratic growth (a  t2), or exponential growth (see Figure 19). An exponential or power-law is often used for characterizing the transient growth of the HRR. The most common is the “t2 fire,” where the HRR increases with the square of the time. These growth and decay functions can be combined with the maximum fire HRR to obtain the fire curve. Fire growth phase for: ;Q t at t t( ) = 2 10 < < Max HRR: Q(t) = Qmax at t = t1, where t1 equals time when fire reaches its maximum HRR The quadratic growth curve is defined in the NFPA stan- dards such as NFPA 204; they differ with: • Ultrafast growth rate • Fast growth rate • Medium growth rate • Slow growth rate. Figure 20 represents different fire growth quadratic growth curves. The ultrafast fire growth curve with the fire growth coef- ficient of 0.178 kW/s2 meets most of the Runehamar Tunnel fire tests. An example of a design fire curve is shown in Figure 21. No allowance in Figure 21 was made for the possible spread of fire between vehicles, nor for the possible effects of under- ventilation on HRR development. If necessary, these effects Decay phase: maxQ t Q e t tb t t( ) = − −( )1 1− > FIGURE 19 An example of fire development curves linear, quadratic, and exponential proposed as the result of UPTUN tests (28). FIGURE 20 Quadratic fire growth curves based on NFPA 204 (2007).

must be investigated separately. There are a limited number of studies found in the literature on fire spread between vehicles in tunnels. If the fire remains isolated to the first item ignited, it will likely become fuel-controlled and decay. However, if the fire spreads to other combustibles, this can lead to the onset of rapid transition from a localized fire to the combustion of all exposed surfaces within the vehicle. This phenomenon is referred to as flashover, which is a sudden transition from localized to generalized burning. The key characteristic of a fully developed fire is a signifi- cant steady-burning phase. Fully developed fires may refer to either fuel- or ventilation-limited fires. The transition from fuel- to ventilation-controlled burning occurs when where: mf and mox refer to the mass fraction of fuel and oxidant, respectively, and s refers to the stoichiometric oxidant to fuel ratio (8). The air-to-fuel equivalence ratio can be used to determine whether a fire is ventilation-controlled or fuel-controlled. Usual tunnel fires are fuel-controlled fires; however, in a severe fire such as the Mont Blanc fire, with multiple vehicles involved, the fire was a ventilation-controlled (oxygen-limited) fire. If the base of the fire source is completely surrounded by vitiated air it may self-extinguish. The vitiated air, which is a mixture of air and combustion products, is usually composed of about 13% oxygen when the fire self-extinguishes such that the flammability limits were exceeded. However, this value can be to some extent temperature-dependent. Increasing temperature tends to lower the flammability limits. If the air-to-fuel mass ratio is greater than or equal to the stoichiometric value, then the fire is assumed to be fuel- controlled and the HRR is directly proportional to the fuel mass loss rate. This can be exemplified by stating that the oxygen concentration in the gases flowing out of the compartment or the tunnel exit is greater than zero. The chemical HRR, Q (kW), m m sf ox= ( )23 74 which is directly proportional to the fuel mass loss rate, mf (kg/s), can then be calculated using the following equation: where: HT is the net heat of complete combustion (kJ/kg), and X is the ratio of the effective heat of combustion to net heat of complete combustion. If the air-to-fuel mass ratio is less than the stoichiometric value, then the fire is defined as ventilation-controlled and the HRR, Q, is directly proportional to the mass flow rate of air (i.e., proportional to the oxygen supply) available for com- bustion. The following equation assumes complete combus- tion and that all the air, ma, is consumed: Where r is the stoichiometric coefficient for complete combustion. A good indication of when a fire has become ventilation- controlled is when the ratio mco/mco2 begins to increase con- siderably where mco is the mass flow rate of CO and mco2 is the mass flow rate of carbon dioxide (CO2). Tests show that the ratio mco/mco2 increases exponentially as the fire becomes ventilation-controlled for diffusion flames of propane, propy- lene, and wood crib fires. Fires that grow sufficiently large can reach flashover, where all of the items inside a vehicle or compartment ignite. Usually this phenomenon occurs during a short period and results in a rapid increase of HRR, gas temperatures, and production of combustion products. The largest HRRs are expected just after flashover occurs (post-flashover) and are often the basis for tunnel smoke control system designs. During this period, the HRR is driven by the oxygen flow and the fire is therefore often considered to be “ventilation controlled.” However, the HRR history of a vehicle fire ought to include HRR informa- tion during all stages of the fire: the ignition or incipient phase, the growth phase, potentially the post-flashover phase, and the decay phase. Before undertaking any fire scenario analysis, it is essen- tial that the fundamental aspects of fire science and fire safety engineering, and limitations of the mathematical models used for hazard analysis are clearly understood. Design fires, which are the basis of the design fire scenario analysis, are described in terms of variables used for the quan- titative analysis. These variables typically include the HRR of the fire, yield of toxic species, and soot as functions of time. When the mathematical models are not able to predict the growth of the fire and it spreads to other objects within the tunnel traffic or any other part of the tunnel, such growth and spread needs to be specified by the analysis as part of the design fire or determined on experimental basis. A design fire Q m H ra T= ( )25 Q m XHf T= ( )24 FIGURE 21 Example of design fire with curve decaying phase.

75 scenario would typically define the ignition source and process, the growth of fire, the subsequent possible spread of fire, the interaction of the fire with its enclosure and environment, and its eventual decay and extinction. 1. Input characteristics Each design fire scenario is represented by a unique occurrence of events and is the result of a particular set of circumstances associated with active and passive fire protection measures. Accordingly, a design fire scenario represents a particular combination of events associated with factors such as: a. Type, size, and location of ignition source; b. Type of fuel; c. Fuel load density, fuel arrangement; d. Type of fire; e. Fire growth rate; f. Fire’s peak HRR; g. Tunnel ventilation system; h. External environment conditions; i. Fire suppression; j. Human intervention(s); and k. Tunnel geometry. Design fires are further characterized in terms of the fol- lowing variables as functions of time: a. Fire characteristics (flame length, air velocity, radiation, convection, temperatures). b. Critical velocity to prevent backlayering (only relevant in longitudinal ventilated tunnels). c. Toxic species (smoke) production rate. d. Time to key events such as fire spread from one vehicle to the next. Alternatively, design fires can be characterized by thermal actions on the tunnel structure and equipment, as well as in terms of time–temperature curves that depend on the emissivity of the fire, surface temperature, and emissivity of the walls. Table 24 and Figure 22 show the application of different design fire curves developed as the result of the UPTUN project. Many known actual tunnel fires and fire curves show a very fast development during the first 5 to 10 (sometimes 15) min. The gradient of temperature is rather steep and the emission of heat and smoke are important. Therefore, several tem- perature curves were presented that more closely correlate to important phases of a tunnel fire. NFPA 502 recognized the RWS curve. The standard reference curve for tunnel fires (the Rijkswaterstaat temperature–time curve) indicates temperatures exceeding 1,200°C (2,192°F) for a period of about 100 min and a maximum temperature of 1,350°C (2,462°F). The duration of the hot phase of a fire normally covers a time interval of about 30 to 60 min after ignition stage, unless there are unusual circumstances such as a big pool fire caused by a gasoline tanker or a situation similar to the Mont Blanc fire. For a big gasoline tanker, the Dutch regulations indicate a hot phase of about 2 h. If fire trucks arrive on the scene quickly (within minutes) and deal with the fire effectively, the duration of the hot phase will be shorter. However, it is realized that access to such a fire will be difficult. HRR MW Road, Examples Vehicles At the Fire Boundary R isk to Li fe 438OSIsrac2–15 438OSIsrac3–2,navllamS01 20 Big van, public bus, multiple vehicles ISO 834 438OSIVGHytpme,suB03 R isk to Co n st ru ct io n 438OSIkcurtnodaolselbitsubmoC05 70 HGV load with combustibles (approx. 4 tons) HC 100 CH)egareva(VGH 150 Loaded with easy comb. HGV (approx. 10 tons) RWS 200 or higher Limited by oxygen, petrol tanker, multiple HGVs RWS Source: Ingason (28). TABLE 24 FIRE SCENARIO RECOMMENDATION, UPTUN WP2 PROPOSAL BY INGASON FIGURE 22 Fire scenario recommendation, UPTUN WP2 proposal by Ingason (28).

After the hot phase, it takes time for the fire to decay if it is not extinguished. The German ZTV Tunnel assumes that it can take about 110 min of linear temperature decaying. The EUREKA tests confirmed the duration of fires, but show a steeper decline of temperatures just after the hot phase. On the other hand, the Nihonzaka fire lasted for four days. SUMMARY The assessment of fire safety in tunnels is a complex issue, where broad multi-disciplinary knowledge and application of different physical models are necessary to explore the causes and development of fires to evaluate measures to prevent and reduce its consequences. The systems to take into account comprise: • The occurrence and physics of fire development. • The tunnel systems; that is, – Infrastructure and – Operation. • Human behavior of users, operators, emergency services. • Other factors influencing safety. The first priority for fire design of all tunnels is to ensure: • Prevention of critical events that may endanger human life, the environment, and the tunnel structure and installations. • Self-rescue of people present in the tunnel at the time of the fire. • Effective action by the rescue forces. • Protection of the environment. • Limitation of the material and structural damage. Fire prevention measures reduce either the probability or the consequences of an incident in a tunnel. They are related to: • Tunnel design and maintenance; • Traffic and vehicles; and • Notification, communication, operator, and rescue services. Mitigation measures are conceived to limit the conse- quences once the ignition has taken place and developed into a fire. The mitigation measures may be related to: • Reduction of the fire development, • Reduction of the consequences to humans, and • Reduction of the consequences to structure and equipment. The fire safety engineering will normally involve the following steps: • Qualitative design review: – Definition of objectives and safety criteria, with reference to performance-based standard require- ments and coordination with the authorities having jurisdictions; 76 – Definition of the tunnel system; – Identification of fire hazards; – Selection and definition of fire scenarios; – Identification of methods of analysis; and – Identification of design options. • Quantitative analysis of design using the appropriate subsystems: – Fire ignition, development of heat and smoke; – Spread of fire, heat, and smoke; – Structural response to the fire; – Detection, activation, and suppression; and – Behavior of tunnel users and the influence of fire on life safety. • Assessment of the outcome of the analysis and evaluation against criteria. The acceptance criteria, which establish the adequacy of the design, can be developed according to the following approaches: • Deterministic (including, when appropriate, safety factors). • Probabilistic (risk-based used in European countries). • Comparative (comparison of performance with accepted codes of practice). A design fire is an idealization of a real fire that might occur. A design fire scenario is the interaction of the design fire with its environment, which includes the impact of the fire on the geometrical features of the tunnel, the ventilation and other fire safety systems in the tunnel, occupants, and other factors. Nobody can precisely predict every fire scenario given the range of variables and people behavior. Therefore, the designer makes a number of assumptions to make sure that the design will save lives and retain structural integrity under most of the foreseeable fire scenarios. For design purposes, it is necessary to choose fire charac- teristics corresponding to the traffic that uses a particular tunnel. Conditions, such as the allowance of transporting hazardous vehicles and materials, have to be taken into account. Design fires in tunnels are usually given as the peak fire HRR. There is no common ground on how to calculate the HRR. One possibility is weighting of the burning components of a vehicle, the other is the analytical method. Some calcu- lations incorporate burning efficiency, which means that the fire may not consume the entire heat load available. The left- over content is typically in the form of either a char residue or as soot and smoke particles displaced by the combustion gas stream. The magnitude and development of fire depends on: • Vehicle combustion load (often called the fuel load, which is usually greater than the potential fire size),

77 • Source of ignition, • Intensity of ignition source, • Distribution of fuel load in the vehicle, • Fire propagation rate, • Tunnel and its environment (including available oxygen), and • Other factors discussed in the next chapters. Studies showed that the fire growth rate is more important than the peak HRR when investigating the safety of people in the tunnel. Fire duration can be determined by the amount of avail- able combustible material. The amount of fuel is different for each study based on the type of vehicles, loads, and traffic patterns. The duration of the hot phase of a fire normally covers a time interval of about 30 to 60 min after ignition stage, unless there are unusual circumstances such as a big pool fire caused by a gasoline where a hot phase of about 2 h is considered. The specification of a design fire may include the following phases: 1. Incipient Phase—characterized by the initiating source, such as a smoldering or flaming fire. 2. Growth Phase—the period of propagation spread, potentially leading to flashover or full fuel involvement. 3. Fully Developed Phase—the nominally steady venti- lation or fuel-controlled burning. 4. Decay Phase—the period of declining fire severity. 5. Extinction Phase—the point at which no more heat energy is being released. A smoldering fire is caused by a combination of the fol- lowing (input) parameters: 1. Nature of fuel 2. Limitation of ventilation 3. Strength of the ignition source. The principal hazards posed by a smoldering fire are high concentrations of CO and low visibility conditions. Pre-flashover fires include the incipient and growth phases, and are of primary interest in life safety analyses. The growth of a fire is dependent on fuel and the availability of oxygen for combustion. Typically, as the fire grows in size, the rate of growth accelerates. The rate of fire growth may be modified owing to compartment effects, radiative feedback, activation of sprinklers or the application of other suppressants, avail- ability of fuel, and the availability of oxygen, among other factors. It is important to recognize that the total fuel load has little bearing on the rate of fire growth; however, the rate of fire growth is governed by the HRR of the individual fuel items burning. Quadratic growth curves are defined in the NFPA standards. They can be categorized as: • Ultrafast growth rate • Fast growth rate • Medium growth rate • Slow growth rate. The ultrafast fire growth curve with the fire growth coeffi- cient of 0.178 kW/s2 meets most of the Runehamar Tunnel fire tests. If the fire remains isolated to the first item ignited, the fire will likely become fuel-controlled and decay. However, if the fire spreads to other combustibles, this can lead to the onset of rapid transition from a localized fire to the combustion of all exposed surfaces within the vehicle. This phenomenon is referred to as flashover, which is a sudden transition from localized to generalized burning, where all of the items inside a vehicle or compartment ignite. Usually this phenomenon occurs during a short period and results in rapid increase of HRR, gas temperatures, and production of combustion products. The largest HRRs are expected just after flashover occurs (post-flashover) and are often the basis for tunnel smoke control system designs. During this period the HRR is driven by the oxygen flow and the fire is therefore often considered to be “ventilation controlled.” The key characteristic of fully developed fires is a signifi- cant steady-burning phase. Fully developed fires may refer to either fuel- or ventilation-limited fires. Usual tunnel fires are fuel-controlled fires; however, in a severe fire with multiple vehicles involved, the fire can be a ventilation-controlled (oxygen-limited) fire. There are a limited number of studies found in the literature on fire spread between vehicles in tunnels. Almost all fires generate smoke. Smoke is a mixture of gases, fumes, and particles. Its generation is affected by the following factors: • Possible reduced supply of oxygen to the fire site • Heat release • Heat convection • Longitudinal slope • Type of ventilation • Dimensions of the traffic space and possible obstructions • Thrust caused by any moving vehicles • Meteorological influences (wind strength and direction). The main design parameter is the smoke flow rate produced by the fire. The smoke flow rate varies nearly linearily with the HRR—from about 50 m3/s (1,765.7 ft3/s) at approximately 10 MW (34 MBtu/hr), to about 250 m3/s (8,828.7 ft3/s) at approximately 150 MW (512 MBtu/hr).

Smoke reduces visibility in tunnels. Visibility depends on: • Smoke density, • Tunnel lighting, • Shape and color of objects and signs, • Light absorption of smoke, and • Toxicity of smoke (eyes irritating). The dangerous nature of smoke gases in tunnel facilities not only results from the visibility obscuring effect but also from possible toxicity of gases including CO, CO2, and other gases depending on the burning materials, especially toxicity caused by cargo. The mass generation of CO2 can be estimated using a ratio of 0.1 kg/s per MW of HRR. A reasonable linear correlation between the production rates of CO2 and CO was found at an average ratio of 0.051 with a standard deviation of ±0.015. The correlation of the smoke-dependent visibility measured by the OD and the concentration of CO2 produces a linear relation when a correction for the smoke gas temper- ature is made. A lack of sufficient experimental data and test results requires designers to make an assumption by replacing an actual vehicle fire with a pseudo-fuel. Different designers use different fuels and different values to approximate the actual fuel, which causes inconsistency in modeling and design results. There is a need to use actual (or mutually agreed upon) design values for fires involving HGVs, buses, cars, and tankers. Additional testing results are needed. Tunnel fires significantly increase the air temperature in the tunnel roadway and in the exhaust air duct. Therefore, both the tunnel structure and ventilation equipment are exposed to the high smoke and gas temperatures. Different fire char- acteristics are needed depending on whether the purpose is to design the tunnel structure or the ventilation facilities. • The design of structures for fire resistance is based on the temperature of the hot air (centigrade or Fahrenheit) and radiation heat versus time. • The design of ventilation is based on the HRR (thermal power in megawatts or million British thermal units per hour) or the smoke release rate (flow at the temperature of the hot smoke in cubic meters per second) versus time. The dependence on time is very important for evaluating the conditions at the beginning of the fire, taking into account the self-rescue phase (time for the fire department to arrive and get organized). Memorial Tunnel Fire Tests, EUREKA tests, and Runehamar Tunnel fire tests provided ample data that allow for the estimating of a maximum temperature experienced by the tunnel ventilation equipment and by the tunnel structure. New energy carriers or vehicles transporting fuel for new energy carriers do not necessarily mean higher risks, but they 78 do represent a new situation and imply new risks. These risks need to be evaluated and considered. The incidents analyzed show that new energy carriers can lead to explosions with catastrophic consequences when there is a fire, although it does not mean that all vehicles running on new energy carriers will explode when used or when exposed to fire. However, seeking the worst case scenarios is important when new energy carriers are developed. It is also important to realize that all risks are not eliminated by introducing PRDs. Safety systems do malfunction, especially in used vehicles. The outcome still depends on the design of these devices and on the fire scenario. Hydrogen can be used either for ICE vehicles or fuel cell vehicles. Hydrogen cars generate fast, high rising flames that reach high temperatures and can lead to higher temperature ceiling flows and damage to tunnel structures. The oxygen- deficient hydrogen fire also poses the risk of flashover inside the tunnel and ventilation ducts. As a result of the nature of flame/fire development, tunnels with greater slopes and with horseshoe cross sections (compared with equivalent rectan- gular cross sections) present lower hazards. The aim of the HyTunnel European on-going project is to develop codes, standards, and regulations so that additional risks owing to the introduction of hydrogen vehicles into tunnels can be handled safely. Electric cars that use batteries as an energy source are seen as the single most promising future energy carrier, in partic- ular, for city traffic. Some countries have restrictions on the use of some energy carriers in confined spaces. Many of the restrictions can be related to LPG, which is also considered to be as an alternative fuel, together with LNG, CNG, hydro- gen, propane, methanol, ethanol, and biodiesel in accordance with the U.S. Energy Policy Act of 2005. LPG and CNG vehicles are restricted in tunnels in New York, New Jersey, Massachusetts, Maryland, and Virginia, as well as in Italy, France, and Austria. However, there are no restrictions on LPG vehicles in tunnels in Japan and many other countries. The issue of new energy carriers is very diverse and con- stitutes many different fields of research. There are a variety of views on how vehicles running on LPG, CNG, or similar fuels are treated and what safety measures are needed. It is important that restrictions are based on correct information that is based on additional systematic research on new energy carriers. It is also important to provide correct and detailed information concerning safety issues and the behavior of these energy carriers where a fire can develop. Unless this occurs in a timely manner, there is a risk that decisions will be based on too little or erroneous information. The concern was raised of possible gas detonation if tunnel air velocity is close to 0. Additional research and numerical modeling is needed to address the risk posed by alternative fuel carriers and structural protection against their fires or explosions. The risk to humans from explosions and from oxygen displacement may also be critical and needs to be studied.

Next: Chapter Ten - Compilation of Design Guidance, Standards, and Regulations »
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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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