Tenability criteria in unique situations and atypical buildings

Tenability criteria for smoke control systems in public buildings has become relatively standard in the fire protection industry. Prisons and enclosed amusement rides are unique occupancies which have increasingly been provided with smoke control systems designed to maintain tenability. These occupancies are unique spaces with characteristics different to many other buildings. This should be considered during an engineering analysis and could affect the design of a smoke control system.

Active smoke control systems can be required in atriums, prisons, underground buildings, smoke-proof enclosures and many other applications. One method of smoke control is a smoke management system, designed to maintain tenable conditions inside the zone of fire origin to facilitate occupant evacuation from the zone.

Computational Fluid Dynamics (CFD) models can be used to analyze tenability in a building environment where smoke is present. An effort to standardize tenability criteria in typical environments has been made in recent years and is typically focused on visibility, temperature and toxicity levels. However, standardized tenability criteria should only be used for the environment to which they are applicable. Atypical environments have unique characteristics that can affect visibility, temperature and toxicity thresholds for egress.

Visibility

Reduced visibility is not the cause of death in fires, but it is a crucial tenability criteria. Evacuees can become trapped in the early stages of a fire by smoke that makes them unable to safely locate an exit. The threshold of fire smoke density and visibility limits for safe evacuation has been examined by multiple sources. This has occurred through interviewing evacuees, analyzing questionnaires and performing experimental research.

Design visibility thresholds are based on an occupant’s ability to wayfind and egress during a fire scenario. One of the standards which outlines specific visibility tenability criteria is NFPA (National Fire Protection Association) Standard 130. It indicates smoke obscuration levels should be maintained below the point at which a sign internally illuminated is discernible at 30 metres and doors and walls are discernible at 10 meters.

The SFPE Handbook provides additional visibility criteria data, some of which are derived from an experiment conducted to monitor a subject’s emotional state when exposed to smoke from a fire. These experiments indicate that subjects who are unfamiliar with their location and egress route should have visibility of 13 metres for egress while subjects who are familiar only need visibility of 4 metres.

Visibility is also dependent on the geometry of the space being analyzed, specifically whether the space is a small or large enclosure. The SFPE Handbook references two studies, one in the United States of America and one in the United Kingdom. Based on these studies, a visibility tenability criteria to consider for small enclosures is 5 metres and large enclosures is 10 metres. Determining the appropriate criteria for visibility can be influenced, at least in part, by the geometry of the building (large vs small) and an occupant’s familiarity with the building.

Temperature

Exposure to heat can lead to incapacitation or death in fire victims through hyperthermia, body surface burns and respiratory tract burns. Hyperthermia occurs during prolonged exposure (approximately 15 minutes or more) to heated environments with temperatures too low to cause burns. During these conditions, when air temperature is less than approximately 120˚C for dry air or 80˚C for saturated air, the main effect is a gradual increase in the body’s core temperature. Above this temperature, onset of considerable pain followed by the production of body burns is of a greater concern. NFPA 130 recommends a tenability criteria for temperature utilizing the following equation:

texp = (1.125 x 107)T-3.4

Where:

texp = time to exposure (min) to reach a FED of 0.3

T = Temperature (Celsius)

The NFPA 130 equation assumes that: evacuees are lightly clothed, fractional effective dose (FED) is reduced by 25% to allow for uncertainty, exposure temperature is constant, FED does not exceed 0.3 and zero radiant heat flux. These assumptions should be reviewed for specific applications. Consider zero radiant heat flux, for example. Heat flux is based on proximity to the heat source. If occupants have free egress, they will be able to move away from the heat source and reduce a radiant impact. However, if occupants are secured then they cannot move away from the source.

The SFPE Handbook also provides data on the tolerance time (of occupants) for exposure to convected heat. It includes tolerance time for dry and humid air. These sources show that there are elevated temperatures that occupants can be exposed to indefinitely without causing incapacitation.

Toxicity

It is known that most fire deaths are caused by inhalation of toxic gases, oxygen deprivation and similar effects by what is referred to as smoke inhalation. In many cases, fire deaths do not occur in the same compartment as a fire, but in remote areas where smoke has accumulated.

Products of combustion are likely to include carbon dioxide (CO₂) and carbon monoxide (CO) and the byproducts present are impacted by the materials involved in a fire. For example, a fire involving nitrogen-containing items, such as synthetic plastics, fire retardant materials or wool, would produce hydrogen cyanide (HCN) among other products. Furthermore, the yields vary based on the conditions of the fire. The impact fire effluents have on occupants is as diverse as the range of gases produced. The gases that have been the most researched for their impact on occupants are CO and HCN.

Fractional effective dose (FED) is used to address exposure to effluents in which the received dose of a gas is divided by the dose causing the effects. This incapacitating threshold value must be determined based on the type of occupant present. NFPA 130 provides tenability criteria for CO levels for FED of 0.5, FED of 0.3 and AEGL-2 (Acute Exposure Guideline Level).

A value for the FED threshold limit of 0.5 is suggested for healthy adult population, 0.3 is suggested for more sensitive populations, and the AEGL-2 limits are intended to protect the general populations, including susceptible individuals, from irreversible or other serious long-lasting health effects.

Prisons

Prisons are unique compared to atriums and other large spaces to which the smoke management method is typically applied with regards to fire, building and occupant characteristics. Fire events in prisons differ from other spaces because the fuel load is often much more controlled by prison operations. The building characteristics are different because cell blocks in prisons are typically significantly smaller than atriums, which could be multiple stories tall.

Lastly, the occupants are noticeably different. Prisons are primarily inhabited by two groups of people, inmates and guards, with distinctively different movement profiles. In the event of a fire, the prisoners are unable to freely egress. This is in contrast to the other occupants (guards), who will not immediately be egressing; instead they will be trying to organize the inmates for egress. Guards and inmates are extremely familiar with their surroundings and the spaces are generally small enclosures with short travel distances.

These building and occupant characteristics are different to those of large public spaces and often lead to lower visibility limits than would be applicable to atriums or similar large public spaces. A critical visibility distance of 4 to 5 metres could be applicable for small prison spaces compared to the 10 metres often used in the analysis of large public spaces.

The types and quantities of fire byproducts are broad and can vary based on the combustible materials as well as conditions around the fire. However, it is nearly impossible to account for all different scenarios in a fire modelling analysis. As such, assumptions or limitations must be determined.

Increased amounts of CO₂ can cause hyperventilation, which increases uptake of asphyxiate gases.

In a number of models of severe-case fire scenarios, it was found that the average volume per cent of CO₂ did not exceed 0.7 when visibility was maintained above 4 metres. This quantity would slightly increase the uptake of asphyxiates and is likely higher than scenarios in large spaces, but would still have minimal impact on occupants.

NFPA 130 considers only carbon monoxide as an asphyxiate gas. However, NFPA 130 includes an uncertainty factor and uses a FED of 0.5 and 0.3 to account for healthy and at risk occupants, respectively. Only considering the asphyxiate effects of carbon dioxide and using an FED of 0.3 is an assumption which could be applied to prisons if other conservative measures, similar to those identified in NFPA 130, were utilized.

Enclosed amusement rides

Another unique scenario where smoke management has been provided is in enclosed amusement rides. These spaces also have unique building, fire and occupant characteristics when compared to atria and similar large spaces. The most prolific difference is that occupants are restrained during ride operation, something that is not addressed by building codes. For amusement park rides, occupants are restrained during ride operation, and therefore at risk of being unable to egress immediately from a malfunctioning ride in the event of a fire.

In scenarios when the occupant is restrained in a ride during a fire, an occupant’s familiarity with the building and the geometry of the building (small vs large travel distances) has little to do with evaluating applicable visibility tenability criteria. While maintaining visibility for restrained occupants to wayfind and egress is not a major concern, first responders will need visibility in order to free such occupants. First responders, such as firefighters, often train extensively for rescue operations at theme parks and could be considered familiar with the inside of the building. A tenability criteria of 4 to 5 metres could be applicable in such scenarios.

A reduced visibility tenability criteria should be used only for those occupants restrained on a ride and are expected to have knowledgeable assistance during egress. Occupants who are able to egress freely must be subject to more stringent visibility tenability criteria.

While visibility for restrained occupants has not been studied in depth, temperature and toxicity tenability criteria has. An FDS model was created of a scenario with a 5,000kW fire located in a large indoor ride that was equipped with a smoke management system. In this scenario, the occupants have the possibility to be restrained at the top of the ride (i.e. ceiling level). As such, the tenability levels are measured at the ceiling. It is important to maintain a level of visibility for first responders, who are looking for restrained occupants.

Figure 1 shows a comparison of visibility and temperature for the duration of the modelled fire exposure. The temperature remains around 30˚C, even when visibility decreases to 4 metres.

Figure 2 shows a comparison of visibility and the CO Concentration. The CO Concentration remains below 70 ppm, even when visibility decreases to 4 metres.

The FDS analysis considers a condition in which occupants are trapped for 45 minutes and subjected to visibility levels of 4 metres. The CO concentration and temperature levels are maintained below tenability limits at an exposure time of 45 minutes. This FDS analysis showed that visibility can be reduced to 4 metres, while still keeping toxicity and temperature levels below the more stringent values for extended exposure times.

Conclusion

As performance-based design is utilized more frequently for smoke control systems, quantifiable tenability criteria is becoming more abundant. Some standards are in the process of evaluating the standardization of tenability criteria for typical spaces such as atriums. While the standardization of these tenability criteria can be considered a step towards consistent modelling evaluations of smoke control systems, their application must always be reviewed for specific occupancies.

Prisons and amusement parks, specifically enclosed rides, are very different from one another, but they share some common if unusual traits which must be considered in determining tenability criteria. These characteristics could lead to less stringent visibility criteria. However, the possibility of restrained occupants could adversely affect temperature and toxicity criteria.

For more information, go to www.jensenhughes.com