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Design Fires in Road Tunnels (2011)

Chapter: Chapter Three - Tenable Environment Literature Review

« Previous: Chapter Two - Tunnel Safety Projects Literature Review
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Suggested Citation:"Chapter Three - Tenable Environment Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 17
Suggested Citation:"Chapter Three - Tenable Environment Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 17
Page 18
Suggested Citation:"Chapter Three - Tenable Environment Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
×
Page 18
Page 19
Suggested Citation:"Chapter Three - Tenable Environment Literature Review." National Academies of Sciences, Engineering, and Medicine. 2011. Design Fires in Road Tunnels. Washington, DC: The National Academies Press. doi: 10.17226/14562.
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Page 19

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17 To understand and interpret the objectives of fire regulations it is necessary to have basic knowledge in the physics of fire, tenable limits for escaping civilians and firemen, and damage criteria for tunnel construction and equipment. Fire produces high temperatures, heat radiation, a low con- centration of oxygen, low visibility, and different lethal toxic and/or corrosive gases. All of these physical phenomena, some of which can be calculated with some accuracy, can be dangerous to people, construction, equipment, and vehicles. The tenable environment is an environment that supports human life for a specific period of time. The goal of fire life safety systems is to provide a tenable environment for evacuation. The current technology is capable of analyzing and evalu- ating each of the unique conditions of each path to provide proper ventilation for pre-identified emergency conditions. The same ventilating devices may or may not serve both normal operating conditions and pre-identified emergency require- ments. The goals of the ventilation system, in addition to addressing fire and smoke emergencies, are to assist in the containment and purging of hazardous gases and aerosols, such as those that could result from a chemical or biological release. Some information, especially on heat effects, was taken from the annex material of NFPA 502 and is summarized here. HEAT EFFECTS Exposure to heat can threaten life in three basic ways (NFPA 502 Standard for Road Tunnels, Bridges, and Other Limited Access Highways): 1. Hyperthermia, 2. Body surface burns, and 3. Respiratory tract burns. The following are used in the modeling of life threat owing to heat exposure in fires: • The threshold of burning of the skin, and • The exposure at which hyperthermia is sufficient to cause mental deterioration and thereby threaten survival. Note that thermal burns to the respiratory tract from the inhalation of air containing less than 10% water vapor by vol- ume do not occur in the absence of burns to the skin (the face); therefore, tenability limits with regard to skin burns normally are lower than for burns to the respiratory tract. However, ther- mal burns to the respiratory tract can occur upon inhalation of air with a temperature above 60°C (140°F) that is saturated with water vapor. The tenability limit for the exposure of skin to radiant heat is approximately 2.5 kW/m2 (800 Btu/hr/ft2). Below this inci- dent heat flux level exposure can be tolerated for 30 min or longer without significantly affecting the time available for escape. Above this threshold value the time to burn skin result- ing from radiant heat decreases rapidly according to Eq. 1. where: tIrad = time to burning of skin resulting from radiant heat (minutes); and q = radiant heat flux (kW/m2 or Btu/hr/ft2). As with toxic gases, an exposed individual can be consid- ered to have accumulated a dose of radiant heat over a set period of time. The fraction equivalent dose (FED) of radiant heat accumulated per minute is the reciprocal of tIrad. Radiation is created by temperature. The level of radia- tion depends on the temperature and the emissivity of the smoke. When the temperature within the smoke layer is not constant integration is necessary to calculate the radiation level. The radiation is produced by the fire itself and by the hot smoke layer. Radiant heat tends to be directional, producing localized heating of particular areas of skin even though the air tempera- ture in contact with other parts of the body might be rela- tively low. Skin temperature depends on the balance between the rate of heat applied to the skin surface and the removal of heat subcutaneously by the blood. Thus, there is a threshold radiant flux below which significant heating of the skin is prevented but above which rapid heating occurs. Based on the preceding information, it is estimated that the uncertainty associated with the use of Eq. 1 is ±25%. More- over, an irradiance of 2.5 kW/m2 (800 Btu/hr/ft2) would cor- respond to a source surface temperature of approximately 200°C (392°F), which most likely would be exceeded near t qIrad = −4 11 36. ( ) CHAPTER THREE TENABLE ENVIRONMENT—LITERATURE REVIEW

the fire where conditions are changing rapidly. Near the fire the radiation is created by the fire itself, as well as the hot smoke. Farther from the fire it is only the smoke temperature that creates a dangerous condition. To make evacuation possible, the radiation level must be under the limit that causes severe pain on bare skin for an exposure time of several minutes: the threshold value is roughly 2 to 2.5 kW/m2 (635 to 800 Btu/ hr/ft2). Firefighters can normally withstand a radiation level of 5 kW/m2 (1600 Btu/hr/ft2) for at least seven minutes because of protective clothing. Their operation time is a function of a self-contained breathing apparatus and is typically not longer than 30 min. For a firefighter to withstand a stay of 20 min, the radiation level cannot exceed 2 kW/m2 (20). The amount of time to incapacitation, when exposed to con- vective heat from air containing less than 10% water vapor by volume can be made by using either Eq. 2 or Eq. 3. As with toxic gases, an exposed occupant can be consid- ered to accumulate a dose of convected heat over a period of time. The FED of convective heat accumulated per minute is the reciprocal of tIconv. Convective heat accumulated per minute depends on the extent to which an exposed occupant is clothed and the nature of the clothing. For fully clothed subjects, Eq. 2 is suggested: where: tIconv = time (minutes); and T = temperature (°C). For unclothed or lightly clothed subjects, it might be more appropriate to use Eq. 3: where: tIconv = time (minutes); and T = temperature (°C). Eqs. 2 and 3 are empirical and can be used for humans. It is estimated that the uncertainty associated with these equations is ±25%. Thermal tolerance data for unprotected human skin suggest a limit of about 120°C (248°F) for convective heat. Within minutes of exposure above this temperature there will be an onset of considerable pain and the production of burns. Depending on the length of exposure, convective heat below this temperature can also cause hyperthermia. The body of an exposed individual can be regarded as acquiring a “dose” of heat over a period of time. Generally, a short exposure to a high radiant heat flux or temperature is t TIconv = ×( ) −5 0 10 37 3 4. ( ). t TIconv = ×( ) −4 1 10 28 3 61. ( ). 18 less tolerable than a longer exposure to a lower temperature or heat flux. A methodology based on additive FEDs, similar to that used with toxic gases, can be applied. Providing that the temperature in the fire is stable or increasing, the total fractional effective dose of heat acquired during an exposure can be calculated using Eq. 4. Note 1: In areas within occupancy where the radiant flux to the skin is under 2.5 kW/m2 (800 Btu/hr/ft2) the first term in Eq. 4 is to be set at zero. Note 2: The uncertainty associated with the use of Eq. 4 would depend on the uncertainties associated with the use of the three previous equations. The time at which the FED accumulated sum exceeds an incapacitating threshold value of 0.3 represents the time avail- able for escape for the chosen radiant and convective heat expo- sures. Consider an example with the following characteristics: 1. Evacuees are lightly clothed. 2. There is zero radiant heat flux. 3. The time to FED is reduced by 25% to allow for uncer- tainties in Eqs. 2 and 3. 4. The exposure temperature is constant. 5. The FED is not to exceed 0.3. Eqs. 3 and 4 can be manipulated to provide the following equation: where: texp = time of exposure to reach a FED of 0.3 (minutes). This gives the results shown in Table 2. AIR CARBON MONOXIDE CONTENT Air CO tenable environment content is as follows: • Maximum of 2,000 ppm for a few seconds. • Averaging 1,150 ppm or less for the first 6 min of the exposure. • Averaging 450 ppm or less for the first 15 min of the exposure. • Averaging 225 ppm or less for the first 30 min of the exposure. • Averaging 50 ppm or less for the remainder of the exposure. These values need to be adjusted for altitudes above 984 m (3,000 ft). t Texp .. ( )= ×( ) −1 125 10 57 3 4 FED t t t Irad Iconv t t = + ⎛ ⎝⎜ ⎞ ⎠⎟∑ 1 1 412Δ ( )

19 TOXICITY The toxicity of fire smoke is determined primarily by a small number of gases, which may act additively, synergically, or antagonistically (21). For example, the addition of the influence of CO and hydrogen cyanide (HCN) may be represented by: where: [ ] indicates the actual concentration; LC50CO30 = 4,600 ppm (concentration level at which 50% of all individuals will die solely from CO after 30 min); and LC50HCN30 = 160 ppm (concentration level at which 50% of all individuals will die solely by HCN after 30 min). If A = 1, approximately 50% of the victims will die. This relation has been shown to hold for concentrations of CO and HCN equal to 25%, 50%, and 75% of their respec- tive 30-min LC50 values. Eq. 6 has been termed the fractional summation approach. An easier approach considers only the maximum allowable concentration for a certain fire. Klote and Milke (22, 23) have presented comprehensive lethal levels for 5 min and 30 min exposure, although it is evident that different authors propose different values. SMOKE OBSCURATION LEVELS, VISIBILITY Smoke obscuration levels need to be continuously maintained below the point at which a sign internally illuminated at 80 lx (7.5 fc) is discernible at 30 m (100 ft), and doors and walls are discernible at 10 m (33 ft). The properties of smoke are commonly expressed in terms of transmittance, as well as either optical density (OD) or atten- uation coefficient (also called the extinction coefficient) (21). A CO LC CO HCN LC HCN = ⎡⎣ ⎤⎦ ( ) + ⎡⎣ ⎤⎦ ( )50 30 50 30 6( ) The transmittance T of smoke is defined as: where: Io is the intensity of light at the beginning of the path; and Ix is the intensity of light remaining after it has passed through the path length. The OD per unit distance δ is related to the transmittance by the following equation: where: x is the distance travelled by light (the path length). The attenuation (or extinction) coefficient per unit dis- tance K is defined in the same way as the OD, but using Neperian logarithms: Sometimes the percentage obscuration λ is used and is defined as: Eq. 8 can then be replaced by The visibility distance V(m) can be estimated using the extinction (or attenuation) coefficient K(m−1) of the air– smoke mix: where: A is a constant between 2 and 6 depending on the signs to be seen (reflecting or illuminated). AIR VELOCITIES Air velocities in the enclosed tunnel need to be greater than or equal to 0.76 m/s (150 fpm) and less than or equal to 11.0 m/s (2,200 fpm). The maximum limit is set based on the ability of people to walk in a high air speed environment (NFPA 502 Standard). V A K= ( )13 δ λ = −( )log ( )10 1 100 12 x λ = −( )100 1 11T ( ) K = 2 303 10. ( )δ K T X e = − ( )log ( )9 δ = −( )log ( )10 8T x T I Ix o= ( )7 Exposure Temperature Maximum Exposure Time Without Incapacitation (min) °C °F 80 176 3.8 75 167 4.7 70 158 6.0 65 149 7.7 60 140 10.1 55 131 13.6 50 122 18.8 45 113 26.9 40 104 40.2 Source: NFPA 502 Standard for Road Tunnels, Bridges, and Other Limited Access Highways. TABLE 2 EXPOSURE TIME AND INCAPACITATION

NOISE LEVELS Noise levels need to be a maximum of 115 dBA for a few seconds and a maximum of 92 dBA for the remainder of the exposure (NFPA 502 Standard). GEOMETRIC CONSIDERATIONS Some factors that require consideration in establishing a ten- able environment in evacuation paths are as follows: • The evacuation path requires a height clear of smoke of at least 2.0 m (6.56 ft). The current precision of modeling methods is within 25%. Therefore, in modeling methods a height of at least 2.5 m (8.2 ft) needs to be maintained above any point along the surface of the evacuation pathway. • The application of tenability criteria at the perimeter of a fire is impractical. The zone of tenability needs to be defined to apply outside a boundary away from the perimeter of the fire. This distance will depend on the FHRR and radiation, and could be as much as 30 m (100 ft) (Figure 2). TIME CONSIDERATIONS The project is supposed to develop a time-of-tenability criterion for evacuation paths with the approval of the authority having jurisdiction. Some factors to be considered in establishing this criterion are the time for: • The fire to ignite and become established. • The fire to be noticed and reported. • The entity receiving the fire report to confirm the exis- tence of fire and time to initiate response. • All people who can self-rescue to evacuate to a point of safety. • Emergency personnel to arrive at the station platform. 20 • Emergency personnel to search for, locate, and evacuate all those who cannot self-rescue. • Fire fighters to begin to suppress the fire. If a project does not establish a time-of-tenability criterion, the system is designed to maintain the tenable conditions for at least 1 h. SUMMARY A tenable environment, an environment that supports human life for a specific period of time, is an important criterion for the design and operation of fire life safety systems. It is well-defined by NFPA 502 and similar in most national and international standards. This prescriptive tenable environ- ment requirement as the function of time is the basis for performance-based design of the fire life safety systems and risk analysis evaluations. For road tunnels tenable environ- ment as a function of time is defined for: • Heat effects in terms of temperature, humidity, and radiation; • Concentration of CO and other gases; • Toxicity of gases; • Smoke obscuration level and visibility; • Air velocities; • Noise levels; • Geometric considerations; and • Time consideration. Gaps in tenable environment are not addressed in this chapter. However, it is well known, for example, that visi- bility is one of the tenable environment limitations. This limitation does not allow using fixed fire suppression sys- tems in many tunnels. Although the visibility criteria were developed for smoke and toxic gases, the question arises as to whether the same criteria should be applied to water or water mist. FIGURE 2 Cross passage spacing.

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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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