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