Structural Fire Behaviour and Indicators

 

Reference: Adapted from CFBT–US (Ed Hartin)

Building Factors, Smoke, Air Track, Heat, and Flame (B-SAHF) are critical fire behavior indicators. Understanding the indicators is important, but more important is the ability to integrate these factors in the process of reading the fire as part of size-up and dynamic risk assessment.

Compartment Fire Development

Part of the process of reading the fire involves recognising the stages of fire development that are involved. Recognising the stages of fire development and likely progression through this process allows firefighters to predict what will happen next (if action is not taken), potential changes due to unplanned ventilation (such as failure of a window), and the likely effect of tactical action.

Compartment fire development can be described as being comprised of four stages:

  • Incipient,
  • Growth,
  • Fully developed and
  • Decay.

Remember that fire conditions can vary considerably throughout the building with one compartment containing a fully developed fire, an adjacent compartment in the growth stage, and still other compartments yet uninvolved.

Fire Behavior Indicators

Firefighters can easily observe some of the B-SAHF indicators. However, fire behavior indicators encompass a wide range of factors that firefighters may see, hear, or feel. Some factors are relatively unchanging (i.e. building construction) and others are quite dynamic, changing as the fire develops (i.e. smoke conditions and flames).

Building:

Unlike the other fire behavior factors, the building and its contents are present prior to ignition and can be examined during the pre-planning process. While many common types of buildings and occupancies (such as single family dwellings) are not the focus of a formal pre-plan program, firefighters can examine common features and their influence on fire behavior. This pre-fire assessment of likely fire development and spread should be compared with actual fire behavior encountered during emergency incidents to improve skill in reading building factors.

Smoke and Air Track:

Smoke conditions and the pattern of smoke and air movement are two of the most important indicators of fire behavior. The location and appearance of smoke can provide valuable cues related to the location of the fire, its burning regime (fuel or ventilation controlled) and the stage of fire in various areas of the building. It is critical that firefighters begin their assessment of smoke and air track indicators from outside the building, but continue this process on an ongoing basis from both the interior and exterior of the structure.

Heat:

While heat cannot be observed directly, observation of the effect of heat on air track (i.e., velocity of smoke discharge), the building or exposures, and sensation of changes in temperature can be significant fire behavior indicators. It is important to remember that our personal protective equipment provides significant insulation and slows the transfer of heat and resulting sensation of changes in temperature.

Flame:

Flaming combustion is often the most obvious or visible indicator observed by firefighters. However, do not get so focused on visible flames that you miss more important, but subtle building, smoke, air track, and heat indicators. Flame indicators such as location, volume, colour, etc. are important, but need to be integrated into the B-SAHF framework to provide a more complete picture.

Incipient Stage

Going back to the basics of fire behavior, ignition requires heat, fuel, and oxygen. Once combustion begins, development of an incipient fire is largely dependent on the characteristics and configuration of the fuel involved (fuel controlled fire). Air in the compartment provides adequate oxygen to continue fire development. During this initial phase of fire development, radiant heat warms adjacent fuel and continues the process of pyrolysis. A plume of hot gases and flame rises from the fire and mixes with the cooler air within the room. This transfer of energy begins to increase the overall temperature in the room. As this plume reaches the ceiling, hot gases begin to spread horizontally across the ceiling. Transition beyond the incipient stage is difficult to define in precise terms. However, as flames near the ceiling, the layer of hot gases becomes more clearly defined and increases in volume, the fire has moved beyond its incipient phase and (given adequate oxygen) will continue to grow more quickly.

Incipient Stage indicators

Building Size, contents, ventilation profile, and fire protection systems all have a significant influence on potential fire development and should be considered regardless of the stage of fire development.
Building factors (such as size and ventilation profile) influence how other fire behavior indicators will present. The building and its contents will also influence how quickly a fire will transition from incipient to growth stage.
Smoke Smoke will be limited and there will not be a well defined layer of hot gases in the upper area of the compartment. If smoke is visible from the exterior volume will generally be light in colour and have limited buoyancy.
Air Track Air track is generally not a major factor in recognition of incipient stage fires. However some light smoke discharge and inward air movement may be observed from openings close to the fire location.
Heat Low (near ambient) temperature within the compartment, condensation may be visible on windows in or near the fire compartment.
Flame Fire confined to a small area (i.e., the object of origin) and flames lower than ceiling height.

Growth Stage

If there is adequate oxygen within the compartment, additional fuel will become involved and the heat release rate from the fire will increase. While considerably more complex, gas temperatures within the compartment may be described as existing in two layers: 

  • A hot layer extending down from the ceiling and
  • A cooler layer down towards the floor.

Convection resulting from plume and ceiling jet along with radiant heat from the fire and hot particulates in the smoke increases the temperature of the compartment linings and other items in the compartment.

As gases within the compartment are heated they expand and when confined by the compartment increase in pressure. (This is often referred to as the "over–pressure" zone). Higher pressure in this layer causes it to push down within the compartment and out through openings. The pressure of the cool gas layer is lower, resulting in inward movement of air from outside the compartment. At the plane where these two layers meet, as the hot gases exit through an opening, the pressure is neutral. The interface of the hot and cool gas layers at an opening is commonly referred to as the neutral plane

The fire can continue to grow through flame spread or by ignition of other fuel within the compartment. As flames in the plume reach the ceiling they will bend and begin to extend horizontally. Pyrolysis products and flammable byproducts of incomplete combustion in the hot gas layer will ignite and continue this horizontal extension across the ceiling. As the fire moves further into the growth stage, the dominant heat transfer mechanism within the fire compartment shifts from convection to radiation. Radiant heat transfer increases heat flux (transfer of thermal energy) at floor level.

Growth Stage indicators

Building Size, construction, fire load and ventilation profile influence ongoing fire development.
Smoke A well defined layer of hot smoke is likely to exist in the upper level of the compartment. If smoke is not confined to the compartment it will be spreading into adjacent compartments.
Smoke may be visible from the exterior (see air track indicators)
Air Track Air track is dependent on the ventilation profile. If the compartment has a single opening (such as a door), there will be a bi-directional air track (smoke out the top and air in the bottom). As the fire grows, air track velocity of smoke discharge and air intake will increase. Velocity is likely to be greater at openings close to the fire. However, air track at exterior openings is significantly influenced by wind, and remember to consider the influence of ambient weather conditions.
Heat Temperature inside the fire compartment and adjacent spaces will be above ambient, but will be lower in compartments located further away from the fire.
Condensation disappears from windows in or near the fire compartment. Brownish staining on window glazing from pyrolysis products may become visible, heat indicators may be visible from the exterior of the compartment, particularly cracking window glass or heat at the upper level of doors, and Increasing overall temperature within the compartment.
Flame Fire extending beyond the object of origin and flames reaching ceiling height, bending and beginning to travel horizontally across the ceiling or through the hot gas layer. If there is an opening to the exterior in the fire compartment, flame may also be visible from the exterior.
Later in the growth stage, isolated flames may be observed in the hot gas layer away from the immediate fire area (one indicator of ventilation controlled conditions).

Flashover–Transition to a Fully Developed Fire

Flashover is the sudden transition from a growth stage to fully developed fire. When flashover occurs, there is a rapid transition to a state of total surface involvement of all combustible material within the compartment. Conditions for flashover are defined in a variety of different ways. In general, ceiling temperature in the compartment must reach 500°C-600°C. When flashover occurs, burning gases will push out openings in the compartment (such as a door leading to another room) at a substantial velocity.

Recognising flashover and understanding the mechanisms that cause this extreme fire behavior phenomenon is important. However, the ability to recognise key indicators and predict the probability of flashover is even more important. Indicators of potential or impending flashover are listed below.

Flashover Stage indicators

Building Flashover can occur in all types of buildings. Building factors can influence how quickly a fire will reach flashover (i.e., fire load, ventilation profile, thermal properties) and should be considered an integral part of ongoing risk assessment.
Smoke Smoke indicators may or may not be visible from the exterior of the structure. However, smoke conditions indicating a developing fire are a warning sign of potential flashover conditions.
After making entry, the presence of hot gases overhead and lowering of the hot gas layer are key indicators. Darkening smoke can be a flashover indicator, but do not depend on smoke color alone
Air Track A strong bi-directional (air in and smoke out) air track can be a significant indicator of flashover that will move in the direction of the opening. However, any air track that shows air movement in to the fire can result in flashover. Increasing velocity of the air track when combined with other indicators can be a strong flashover indicator.
Heat Outside the fire compartment, perception of increasing temperature may not provide reasonable warning of impending flashover. However, perception of increasing temperature and observation of heat indictors such as pyrolysis of fuel packages some distance from the fire should be considered as a strong indicator of worsening fire conditions and potential for flashover.
Flame Isolated flames traveling in the hot gas layer (ghosting) or more substantially through the gas layer or across the ceiling (rollover). It is important to note that these flames may or may not be visible.
A later (potentially too late) indicator of impending flashover is rollover moving along the ceiling of the fire compartment and into adjacent spaces.

Fully Developed Stage

At this post-flashover stage, energy release is at its greatest, but is generally limited by ventilation (Extreme fire). Unburned gases accumulate at the ceiling level and frequently burn as they leave the compartment, resulting in flames showing from doors or windows. The average gas temperature within a compartment during a fully developed fire ranges from 700°C – 1200°C.

Remember that the compartment where the fire started may reach the fully developed stage while other compartments have not yet become involved. Hot gases and flames extending from the involved compartment transfer heat to other fuel packages (e.g., contents, compartment linings, and structural materials) resulting in fire spread. Conditions can vary widely with a fully developed fire in one compartment, a growth stage fire in another, and an incipient fire in yet another.

Fully Developed Stage indicators

Building As with the growth stage, size, construction, and fire load influence fire development. Fire effects on the building can change the ventilation profile
Smoke Smoke will darken to darker gray, brown, or black. Smoke colour influenced to a substantial extent by what is burning and color may vary. Volume, optical density, and volume of smoke will increase. The height of the hot gas layer and neutral plane at openings is influenced by the ventilation profile, but if the compartment is not well ventilated, the hot gas layer will drop close to the floor as the fire progresses through this stage.
Air Track Air track is dependent on the ventilation profile. However, given a single opening such as a door, smoke will exit out the top while air moves in the bottom. A fully developed fire will generally develop a well-defined and strong air track. The velocity of smoke and air movement will commonly be quite high and smoke discharge will be turbulent.
Heat In this stage of fire development, the fire is producing substantial heat. There are likely to be visual indicators of high temperature such as blackend windows, crazing window glazing. Hot surfaces (i.e., doors) may be detected using a fire stream or thermal imager. In addition high temperature can be felt, even when wearing structural firefighting clothing.
Flame Flames may be visible from the exterior, with extent indicating the area and extent of involvement to some degree. Fire will involve the entire compartment in this post flashover stage of fire development. Flames may be readily visible, but also may be obscured by smoke as the fire becomes ventilation controlled.

Decay Stage

A compartment fire may enter the decay stage as the available fuel is consumed or due to limited oxygen. As discussed in relation to flashover, a fuel package that does not contain sufficient energy or does not have a sufficient heat release rate to bring a compartment to flashover, will pass through each of the stages of fire development (but may not extend to other fuel packages).

On a larger scale, without intervention an entire structure may reach full involvement and as fuel is consumed move into the decay stage. However, there is another, more problematic way for the fire to move into the decay stage. Heat release rate decreases as oxygen concentration drops, however, temperature may continue to rise for some time. This presents a significant threat as the involved compartment(s) may contain a high concentration of hot, pyrolised fuel, and flammable gaseous products of combustion.