Extreme Fire Behavior: Flashover

Editors Note: To view more detailed images of the figures displayed on the right, please click on the links to each figure in the body of the article.

Previous articles examined fire development in a compartment and the combustibility of smoke. This is the first of three articles dealing with the extreme fire behavior phenomena, flashover, backdraft, and smoke explosion. Rapid fire progress presents a significant threat to firefighters during structural firefighting. Figure 1 illustrates post-flashover conditions encountered by Lemley Fire Rescue firefighters during live fire training in an acquired structure. (see Figure 1.)

If firefighters do not have a high level of situational awareness this hazard is increased. It is difficult to develop proficiency in recognizing fire behavior indictors and developing an understanding of fire dynamics from fireground experience or classroom study alone.

This article examines flashover; the sudden transition from a developing to fully developed fire. This phenomenon involves a rapid transition to a state of total surface involvement of all combustible material within the compartment. If flashover occurs, the rate of heat release in the compartment as well as the temperature in the compartment increases rapidly. Flashover may occur as the fire develops in a compartment or additional air is provided to a ventilation controlled fire (that has insufficient temperature to backdraft).

Case Study Method

What is a case study? Cases are not simply narratives for entertainment. They are stories with an educational message. Each of the case studies in this article is based on an actual incident where Firefighters were injured or killed by rapid fire development or other extreme fire behavior. The purpose of these cases is not to lay blame or simply identify the mistakes of others; it is to develop an improved understanding of structural fire behavior.

How should you approach learning through the use of case studies? Read the questions to be answered first, this provides you with a framework for understanding the information presented. Second, read the case to get an overall understanding of the incident. Last, examine the incident in detail to answer the questions posed at the start of the case.

One excellent source for case studies are reports prepared by the National Institute for Occupational Safety and Health (NOSH) on firefighter fatalities. Particularly when fire behavior was a significant factor, the National Institute for Standards and Technology (NIST) Building and Fire Research Laboratory also prepares reports including fire test and modeling data. When using case studies as an element of fire behavior training, the following questions serve as a good starting point for your analysis:

Was extreme fire behavior involved in this incident? If so, what type of event happened?

Look at the reported conditions and observations of individuals involved in the incident. Was this a flashover, backdraft, or smoke explosion? Recognize that it may be difficult to determine based on limited information. If available, NIST fire test and modeling data can shed a great deal of light on the nature of extreme fire behavior phenomena.

How did the fire develop and what factors influenced the occurrence of the extreme fire behavior phenomenon?

As with the question of what happened, this question is complex. Many factors influence fire development and extreme fire behavior phenomena. Think about building factors such as fuel type, fuel load, and ventilation profile, changes in ventilation profile (may be caused by the fire or human action) and actions of firefighting forces.

What cues were present that may have indicated potential for rapid fire development?

Frequently there is limited information on exactly what was observed (particularly by the individuals most impacted by the incident). However, in some cases critical fire behavior indicators are mentioned and/or photographs of incident conditions are included in the reports.

Compare and contrast these the case study with other cases or events in your own experience. What aspects of these incidents were similar? Which were different?

Building your knowledge base using case studies is enhanced by integrating this information with your existing knowledge of fire behavior. Often discussion of a case with others results in sharing of personal experience. Expand this discussion beyond simple "war stories" to consider commonalities and differences.

In addressing the first two questions, what happened and what were the contributing factors it might be useful to examine the fire development curve presented in Fire Development in a Compartment. What stage (incipient, growth, fully developed, or decay) and what burning regime (fuel controlled or ventilation controlled) is the fire in immediately prior to changes in fire behavior.

Case Study

This incident involved an early morning fire in a two-story, wood frame duplex that resulted in the deaths of three children and three firefighters. The fire occurred in the unit on Side Bravo of the structure (See Figure 2). This building was originally constructed in the 1870s as a single family dwelling and divided into two dwelling units in the 1970s. This case study will focus on fire behavior related aspects of this incident. However, this case provides an opportunity to learn a number of other important lessons (see the NIOSH and NIST reports for additional information on the incident).

Configuration: The unit involved in the fire had a kitchen, dining room, and living room on floor one and three bedrooms and a bathroom on floor two. Figures 3 and 4 show plan view of the first and second floor of the involved unit. A wall separated the first floor hallway and stairs to the second floor from the rooms on floor 1. There was a door leading from the floor one hallway into the living room (this door was open at the time of the fire). In addition, there was a door from the floor one hallway to the dining room (this door was closed at the time of the fire).

Fuel Profile: Contents were typical of a residential structure and included ordinary kitchen, dining, and living room furniture. Ceilings were covered with combustible wood fiber ceiling tile with the exception of the dining room, which had the original plaster and lath ceiling. Interior walls were covered with gypsum board. However, the walls of the first and second floor hallways, stairwell and bedrooms two and three (see Figure 4) were covered with wood paneling.

Ventilation Profile: At the time of ignition, there were no ventilation openings. The only air movement would have been due to normal building ventilation and leakage. Prior to the arrival of the fire department a building occupant opened a window in bedroom one on floor two (see figure 4). The front door was opened approximately two minutes after the fire department arrived on scene. The window in the kitchen was composed of small panes with wood framing and failed over a period of time (starting approximately when the front door was opened).

Fire Development: The fire originated in plastic materials on top of the stove in the kitchen that was located on floor one of the dwelling. The exact time of ignition and the speed with which the fire may have progressed from incipient to growth stages is unknown. The fire extended from the burning material on top of the stove to interior finish of the kitchen. Firefighters observed that there was smoke, but "no heat" at the first floor level shortly after beginning primary search. While smoke began to spread through the structure shortly after ignition, fire did not extend beyond the kitchen until eight minutes after flaming ignition (estimated to be six minutes after the arrival of the first company). The NIST fire model of this incident is consistent with this observation, showing near ambient temperature at floor level with temperatures between 570 degrees F and 840 degrees F at the ceiling in the living and dining rooms. Between six and eight minutes after the first company arrived on scene, conditions changed radically fire rapidly extending sequentially in the kitchen, dining room, living room, floor one hallway and stairway to floor two. Figure 5 illustrates the 12 critical minutes from the time the alarm was received by the dispatch center until 10 minutes after the arrival of the first company.

Initial Tactical Operations: Initial response to this incident was a quint (two personnel), engine (three personnel) and chief. On the arrival of the first company firefighters observed a woman and child trapped on the porch roof and received a report of three children still inside the involved unit. Initial tactical operations involved rescue of the woman and child from the roof (performed by a police officer), primary search, and deployment of a (dry) hoseline to the entry way of the involved unit. Firefighting operations were not initiated until approximately nine minutes after arrival due to the commitment of resources to rescue, primary search, and care of injured occupants. (see Figure 5.)

Fire Modeling

The NIST report includes output from computer modeling of this incident (this is also available from NIST on CD). This data provides another way to visualize fire conditions. Figure 6 illustrates temperature conditions at various levels within the structure at approximately 0832, six minutes after the arrival of the first company. Figure 7 illustrates conditions one minute later.

As illustrated by the NIST computer model, thermal conditions changed radically as the fire extended quickly from the kitchen through the dining and living rooms into the floor one hallway and stairwell to floor 2.

Study and Discussion Questions

Use the information presented in the case to answer the following questions.

  1. Was extreme fire behavior involved in this incident? If so, what type of event happened? Use the fire development curve illustrated in figure 8 to work out your answer.(see Figure 8)