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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.
