Calculating tenability conditions when smoke drops below 2m

A fire experiment in a building with a high roof showed that smoke would take considerable time to drop below 2 m therefore it is not recommended to rely on exposure conditions to determine the tenability conditions in buildings with high roofs(ceilings).

While it is always recommended to design fire safety systems to maintain the smoke layer 2m above the occupiable level, in many cases, this is difficult to achieve, especially in compartments with a low ceiling (or roof). Unlike calculating tenability conditions when smoke is above 2m, which, according to most references, is when the temperature reaches 200˚C, calculating tenability conditions when the smoke layer drops below 2m is not straightforward and most designers do not use the appropriate parameters and consequently authorities tend to be reluctant to accept proposed available safety evacuation times (ASET).

The aim of this article is to explain how to calculate tenability conditions for occupants when smoke drops below 2m. It is not intended to summarise the acceptable thresholds for each parameter as they are already summarised in most fire engineering guidelines.

The BCA and the International Fire Engineering Guidelines (IFEG)² do not specify the tenable thresholds nor the acceptance criteria for any of the parameters. The parameters are considered via a performance-based approach. For example, performance requirement EP2.2 of the BCA requires conditions in any evacuation route to be maintained for the period of time occupants take to evacuate the part of the building so that:

the temperature will not endanger human life; and
the level of visibility will enable the evacuation route to be determined; and
the level of toxicity will not endanger human life.

It does not quantify or specify any of the parameters.

Figure 1. A snapshot from a CFD model for a compartment. Temperature in the areas near the fire exceeds the maximum acceptable threshould (60˚C). However the temperature in the other areas is less than 60˚C — Figure 1. A snapshot from a CFD model for a compartment. Temperature in the areas near the fire exceeds the maximum acceptable threshold (60˚C). However, the temperature in the other areas is less than 60˚C

Parameters

If any of the parameters exceed the maximum permitted value then the conditions are considered untenable. The time when the first parameter exceeds the permitted value is considered the ASET and can be compared with the required safe evacuation time (RSET) to calculate the factor of safety.

Temperature

If the temperature of the smoke is higher than 60˚C3, noting that some references tolerate a temperature up to 100˚C, then the conditions are considered untenable.

Note: temperature directly above the fire is permitted to exceed the maximum threshold, as illustrated in Figure 1, as occupants are not expected to be directly near the fire when it grows. The high fire temperature is necessary to maintain the burning and the growth of the fire.

Visibility

Visibility does not represent a life hazard by itself, but it affects occupant speed, which needs to be considered when calculating the required safe evacuation time (RSET).

Most references require visibility at 2m to be greater than 5m or 10m (5m for small enclosures and 10m for all other cases)³. If visibility is less than the required 5 or 10m, then occupants speed is expected to be reduced dramatically to a low value, as low as 0.3m/s, and therefore the movement time and consequently the required safe evacuation time (RSET), need to be recalculated based on this speed. Most fire engineers use the same RSET for all tenability conditions (i.e. whether smoke is above 2m or below 2m) and they use a speed between 1-1.2m/s with no adjustment to consider the effect of visibility.

Parameters that affect visibility like soot yield, visibility factor and the mass extinction coefficient should be specified and justified by the designer and should not be left as the default values embedded in the software.

Note: visibility can be calculated from the extinction coefficient using equations listed in most smoke management handbooks.

Toxicity

As the smoke is below 2m, occupants are assumed to be walking through the smoke and toxicity should be always considered. There are many toxicity models that can be used to determine the acceptable limit of toxicity based on gases produced from burning materials, which depend on the chemical composition of the assumed burning materials as explained in most smoke management handbooks. The most used, widely accepted and supported by most software models are based on Carbon Monoxide (CO) concentration level and fracture effective dose (FED). This is based on studies that showed that the vast majority of fire deaths are attributable to carbon monoxide poisoning without requiring the effect of additional toxicants⁵.

The CO yield, similar to the soot yield, should be specified and justified by the designer and should not be left as the default value embedded in the software. CO is normally produced by inefficient, under-ventilated combustion. Figure 3 suggests values for CO yield. The CO yield will determine the concentration of produced CO from the fire which is the main parameter for determining toxicity.

The values in the figure are supported by the IFEG which states that: “The production of carbon monoxide is especially dependent on the ventilation conditions. In post-flashover fires, studies have shown that the yield of CO can generally be approximated as being 0.2 g CO produced/g fuel lost, irrespective of the test results for the material under well-ventilated conditions”. Unless it can be demonstrated that the gases are unlikely to cause post exposure fatality, tenability criterion should be based on prevention of both incapacitation and fatality.

Figure 2. The relationship between walking speed and the extinction coefficient4.

Figure 3. Approximate values of CO yield for room fire.

Figure 4. Toxicity should be always considered when smoke drops below 2m as there might be cases where the temperature of the smoke is low (less than 60oC) but it is toxic.

Conclusion and recommendations

While the article explains how to calculate ASET for occupants when smoke drops below 2m, it does not ignore the fact that smoke management systems should be designed to maintain smoke above 2m in the first instance. However, there are some cases where this might not be achievable due to difficulty in providing the required extraction rate and the associated make up air. Cases when it is tolerable for the smoke to drop below 2m include:

When the ceiling or roof is lower than 4.5m from the occupiable level. Note: the 4.5m height should not be considered as a definite number as the floor area of the compartment should be considered as well.
When performing sensitivity analysis, fire engineers will allow one fire safety system to fail to demonstrate that occupants are still able to evacuate even in the worst case scenarios.
It is not recommended to rely on exposure tenability conditions when smoke drops below 2m for more than 30 minutes, to allow for any uncertainty, as most of the toxicity models are based on experiments performed on animals and not on humans.

For more information, go to www.i-fire.com.au

References

Building Code of Australia 2016-Volume 1 – Amendment 1, Australian Building Codes Board
International Fire Engineering Guidelines Edition 2005, Australian Building Codes Board
Fire Engineering Design Guide, third edition, M. Spearpoint
Principles of Smoke Management, John H. Kolte and James A. Milke
Carbon Monoxide and Human Lethality: Fire and Non-Fire Studies, Hirschler et al, Elsevier, 1993.

firasshawash

Firas is a chartered professional and registered engineer in the field of fire safety and mechanical engineering. He has extensive experience in providing cost-effective performance solutions under the Building Code of Australia for different classes of buildings with different issues.