Fire Behavior Myths and Misconceptions


Fire indicators are the basis for determining the area of origin and the cause of a fire. However, there are many fire indicators that used to be valid but aren't any longer. This is due primarily to the changes in fuel packages found in modern structures. There are also fire indicators that were believed to be true, but have been disproved under controlled studies.

Due to the synthetic materials in modern furnishings, it is customary for a fire to spread rapidly throughout a bedroom-sized compartment. Post-flashover conditions at the time of extinguishment are commonplace. This was not the case in the past. Note that the behavior of fire, being a chemical reaction, remains unchanged. It is the materials being burned that are different. The following fire indicators or conditions illustrate how fire progression has changed or how time-dated fire indicators have been disproved.

Depth of char / alligatoring

A timeline used to be attributed to the depth of char found on wood framing. An investigator could estimate how long the fire burned by how deep the charring was. This has been proven to be false. Where the fire started and the intensity of the fire will dictate how much charring will occur, further influenced by fuel configuration and ventilation.

Simply put, if a match is placed against a piece of wood for one minute, it will cause less damage that a torch placed against the same piece of wood for one minute. It is simply a mater of time vs. intensity; a long duration low intensity event can present very similar indicators to a short duration high intensity event. Note that in both cases there will be more damage to vertical arrangements than to horizontal arrangements, again due to fire behavior.

Depth of char is a very important indicator and can be used to show trends of fire travel, that is, all things considered equal, the fire will tend to cause more damage where it burns the longest.

Alligatoring is the rounded char blisters that occur as wood burns. In the past, if the blisters were large and shiny, it was thought to be an indicator that an ignitable liquid was used. This is untrue. An examination of various wood members in a fire scene will reveal that different pieces of wood show different sizes of char blisters while exposed to the same fire. There is no significance to the appearance of the alligatoring.

Spalling

After a fire, sections of concrete have been seen to break down, exposing the aggregate below. This will sometimes occur in small areas, or may sometimes cover larger portions of the floor. It was believed to be areas where ignitable liquids had been poured and ignited, with the high temperatures causing the concrete to fail.

Fire behavior dictates that the area beneath a puddle of liquid, even ignitable liquids, will be cooler than adjacent exposed surfaces during a fire. The floor beneath the liquid pool should not get hotter than the boiling point of the liquid. Spalling has been shown to be the result of different rates of expansion within the concrete. This is influenced by the internal components, such as aggregate, steel mesh and rebar, which absorb heat more readily than concrete. The most drastic change in expansion or contraction of the concrete occurs when water is applied by firefighting streams and is often the cause of spalling. Also consider that non-fire causes for spalling exist and the spalling may have been there prior to the fire

Annealed bed or furniture springs

When furniture or bed springs are subjected to intense or prolonged heat they can lose their tensile strength and collapse. Once thought to be the result of a long duration smoldering event, typically a cigarette, it has been demonstrated that a short duration exposure of over 750° F causes annealing as well. A significant factor is the use of foam padding with synthetic fillers and covers that combust vigorously, as compared to older cotton batted and cotton covered furnishings that were slower burning and prone to long-duration smoldering.

Annealing can be an important indicator, showing a comparison of fire behavior in like fuel packages, similar to using depth of char to quantitate fire flow and intensity patterns on wood.

Ignitable liquids and pour patterns on floors

Narrow or irregular burn patterns on the floor were considered reliable proof of the presence of an ignitable liquid. While this can still be true, there are other situations that cause the same patterns. Floors that are covered with carpet will show the most wear in areas where people walk back and forth, such as from one doorway to an opposite doorway. If there is more carpet wear in these paths, the damage to the floor in post-flashover conditions will tend to penetrate these areas first. To complicate the issue, these paths are also the most commonly used to pour ignitable liquids, as the person backs out toward the exit of the room.

Another source of irregular floor patterns are drop-down fires, more so with synthetic materials, such as when the plastic diffuser panel from a fluorescent light fixtures sags and falls to the floor, subsequently igniting and causing floor level fire damage. The foam padding of furniture, such as couch cushions, can liquefy and run, effectively becoming an ignitable liquid.

A very effective way to determine the presence of an ignitable liquid is by laboratory analysis, which will provide physical proof of the material present.

Spontaneous ignition

Spontaneous ignition is seen most commonly with the use of stain rags that are placed in a configuration that allows a heat build-up with a sufficient infusion of oxygen. If the configuration is too tight, not enough oxygen is available. If too loose, the heat is dissipated faster than it can build up. Spontaneous ignition occurs with natural products such as tung oil and linseed oil. Without these or other natural oils, there will not be spontaneous heating. Hydrocarbon-based oils, such as engine or lubricating oils will not spontaneously heat, nor will paint thinner and oil based paints not containing natural oils.

Fires occurring with paint products that do not have natural oils may be the result of ignition of the ignitable vapors present. The stains that can spontaneously heat also have ignitable vapors that can be ignited by a competent ignition source.

Electrical fire causes

Electricity is blamed for many fires that are caused by other means. Consider a fire involving a desk; there are lamps, computers, calculators, radios, telephones and charging units on the desktop. A portable heater, multi-strip extension cord and small paper shredder are in the foot well. The fire may be attributed to an electrical problem in the absence of tangible proof of the failure that caused the fire, and that may be the case. However, remember to consider the candle, cigarette, and heat from the light bulb and the heater as non-electrical causes.

The fire service has a dilemma with the determination of cause for what appears to be an accidental electrical fire; in the absence of indicators of an incendiary fire, the evidence of accidental cause should not be inappropriately manipulated or damaged, especially once the fire is determined to be accidental. It is understood that damage to evidence of accidental cause may occur while the municipal investigator is determining if the fire was incendiary, but it should not occur after. Because of this, it is sometimes difficult to conclusively determine if a fire was caused by electricity, or a different accidental event.

The company officer or fire investigator most times will not disassemble an electrical appliance to find out if an electrical failure is identifiable. This restraint allows the interested parties; insurance companies, insureds, manufacturers and private investigators to continue the investigation and identify the failure and possibly the responsible party.

In many cases, the determination of electrical cause is made by the company officer because there appears to be no criminal act and fire investigators are not warranted. Sometimes a fire investigator will be requested to respond and determine the cause of what appears to be an accidental fire, but where the company officer feels an investigation is warranted.

While examining the area of origin, there are commonly electrical components present because they are so prevalent in life. The fire will often cause damage to these electrical sources as the fire progresses. Energized electrical wires will fault when the insulation burns away and the conductors contact each other or another object that allows the current to flow improperly. This will often cause a bead on the end of a conductor as the electrical energy causes a separation arc. The presence of beading on exposed conductors is not a reliable indicator of a fire cause, only of a fault. Tripped circuit breakers are also not a reliable indicator of cause, only an indicator that an electrical event has occurred that caused the circuit breaker to trip. Some circuit breakers will trip from heat alone. Fires routinely cause these conditions.

Conclusion

Current, reliable information on fire behavior is available and most likely in fire station libraries. The two most recognized publications are:

  • NFPA 921, Guide for Fire and Explosion Investigations, 2011 Edition; and

  • Kirk's Fire Investigation, Seventh Edition by Dr. John DeHaan.

Copyright © 2003 Fred Herrera, All rights reserved. No portion of this article may be reproduced without the express written permission of the copyright holder. If you use a quotation, excerpt or paraphrase of this article, except as otherwise authorized in writing by the author of the article you must cite this article as a source for your work and include a link back to the original article from any online materials that incorporate or are derived from the content of this article.

This article was last reviewed or amended on Nov 6, 2014.