Fire Behavior Indicators and Fire Development - Part 2

It is essential to recognize when the fire is, or may be ventilation controlled and the influence of planned and unplanned changes in ventilation profile.


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 second of two articles focused on fire behavior indicators and the stages of fire development.

Part 1 of FBI and Fire Development examined fire development and related fire behavior indicators from ignition through the growth stage. Part 2 examines continued fire progression into the fully developed and decay stages with a strong look at ventilation controlled decay and ventilation induced extreme fire behavior.

Fully Developed Stage
At this post-flashover stage, energy release is at its greatest, but is generally limited by ventilation (more on this in a bit). 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 to 1200 degrees C (1292 to 2192 degrees F).

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. It is important to note that while a fire in an adjacent compartment may be incipient, conditions within the structure are immediately dangerous to life and health (IDLH). See Table 4.

If the fire in our residential scenario has progressed to the fully developed stage (in the compartment of origin) what fire behavior indicators might be observed? Use the B-SAHF model to help you frame your answers.

Scenario 1:
You have responded to a fire in a one-story single-family dwelling of wood-frame construction. A fire which started in a bedroom on the Alpha/Bravo corner of the structure has reached the fully developed stage and now involves the contents of the room and interior finish of this compartment.

  1. What conditions would you expect to see from the exterior of the structure?
  2. What indicators may be visible from the front door as you make entry?

Remember that fire conditions will vary throughout the building. While the fire is fully developed in the bedroom, conditions may be different in other compartments within the building.

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. When the ventilation profile of the compartment or building does not provide sufficient oxygen, the fire may 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, pyrolized fuel, and flammable gaseous products of combustion.

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