5.3.2 Tenability criteria for means of escape
Where system performance is being assessed deterministically (and not compared to an
ADB compliant one) it will generally be necessary to set acceptance limits for one or more
performance criteria based on tenability. It is not appropriate to give definitive values here
as they need to be established on a case by case basis as part of the overall fire strategy.
However, published information is available (see, for example, BS 7974:2001 and
associated PD 7974 series Application of fire safety engineering principles to the design of
buildings, BS 7899-2:1999 Guidance on methods for the quantification of hazards to life
and health and estimation of time to incapacitation and death in fires, the SFPE Handbook
of Fire Protection Engineering and the ASHRAE Handbook of Smoke Control Engineering
and CIBSE Guide E Fire Engineering). Some recommendations that might be considered
are provided below. The appropriate choice for an individual system should take into
account the specific design details such as travel distances, occupancy characteristics etc.
Exposure gas temperature and thermal radiation flux (irradiance) limits of 60°C and
2.5kW/m
2
represent typical acceptance limits in respect to tenability for means of escape.
Visibility distance and toxicity levels may also be important performance criteria in respect
to means of escape. Both are functions of the smoke concentration, with visibility being
approximately inversely proportional to the density of soot particulate. Care and
engineering judgement is required as the calculated values will be strongly dependent on
the choice of soot and toxic yields (generally an input parameter in a zone or CFD model)
and also the ventilation conditions. Visibility distance is a widely used performance
criterion for smoke control design, and in addition to allowing an estimate of how far a
person could see provides a measure of the toxicity associated with the smoke. It is
generally accepted that if visibility is acceptable then the toxicity condition is likely also to
be acceptable, at least for the exposure times during the escape. It should be noted,
however, that visibility distance is a working engineering parameter rather than a precise
measure of how people will respond in a real emergency.
A commonly adopted conservative visibility distance limit is 10m (approx. 0.1m
-1
optical
density) as measured to a light reflective surface, representing an approximate value
through which persons unfamiliar with a building would be prepared to travel (see, for
example, ref: T. Jin,
“
Studies on Human Behavior and Tenability in Fire Smoke,
”
Proceedings, 5th International Symposium on Fire Safety Science, pp. 3
–
22, 1997). A
lower value of 5m might be acceptable where the persons escaping are familiar with the
building and the travel distance is relatively short. At a visibility distance of 5m (approx.
0.2m
-1
optical density) conditions will remain tenable with respect to asphyxiant gases for
at least 30 minutes (see PD 7974-6) for the majority of fires.
An alternative, and potentially more rigorous, approach is to determine whether or not the
stair door is visible from the apartment entrances. This involves calculating the spatially
averaged visibility distance along the line of sight from the apartment to the stair door,
which could be seen on the proviso that the average visibility distance exceeds the travel
distance. This approach is explored further, for example, in the ASHRAE Handbook of
Smoke Control Engineering.
It is often difficult to maintain a minimum visibility distance when the apartment door to the
corridor remains open; this is because the corridor fills with smoke generated by the
apartment fire. BRE Report 213179 found that it was difficult under most fire scenarios to
Page 15 of 68