Fire Behavior Indicators and Fire Development - Part 1

A fire can continue to grow through flame spread or by ignition of other fuel within the compartment.


Previous articles have addressed building factors, smoke, air track, heat, and flame (B-SAHF) as 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. This is the first of two articles focused on fire behavior indicators and the stages of fire development.

Compartment Fire Development
Part of the process of reading the fire involves recognizing the stages of fire development that are involved. 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. Recognizing 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 (see Figure 1). Flashover is not a stage of development, but simply a rapid transition between the growth and fully developed stages. (see Figures 1 and 2.)

Review of 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 preplanning process. While many common types of buildings and occupancies (such as single family dwellings) are not the focus of a formal preplan 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 skills 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, color, etc. are important, but need to be integrated into the B-SAHF framework to provide a more complete picture.

Figure 3 integrates the concept maps presented in previous articles and outlines some of the major interrelationships between the B-SHAF fire behavior indicators. (Click Figure 3 for a larger view of the concept map.)

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