Construction Concerns: Unpredictable Fire

By Gregory Havel

One thing that most Fire Fighter I and II courses do not teach is the simple fact that fire behavior is not always predictable. We can predict—probably—where the fire is going to go and what it will do—probably—when it gets there, but these predictions are without certainty.

Part of fire behavior is determined by the structure in which it occurs. Is the structure combustible or noncombustible, fire-resistive or not, small compartment or large open space, rooms finished with low-flame spread or other type materials? Also, are the materials lightweight or does it have enough mass to provide some inherent fire resistance.

Part of fire behavior is determined by the fuel, whether it is cellulose- or petroleum-based, and the geometry of the fuel (the size and shape of its pieces or particles, their arrangement, and their location within the compartment).

Part of fire behavior is determined by the ventilation, i.e., whether the compartment is closed with limited air supply or has openings and an available supply of fresh air and an opening for the escape of the products of combustion. Forcible entry into the structure, wind speed and direction, or removal of upper floor windows for use as secondary exits changes the ventilation characteristics of the building and the fire. Which openings are used for which purpose depends on the temperature differential between the fire compartment and the incoming fresh air; wind direction and velocity; and the volume of, and the path followed by, the combustion products that are trying to escape.

We do not have absolute control over most of these.

Photo 1 shows an acquired structure (platform framed with joists and rafters of sawn lumber) used for live fire training in compliance with NFPA 1403, Standard on Live Fire Training Evolutions. After the individual room fire attacks had been completed and overhauled, fires were set on each floor to burn down the remaining structure. These fires were located in stacks of pallets and arranged so that the structure should have burned down symmetrically. Even though the wind (five miles per hour) was blowing from the B-C corner to the D-A corner, most of the products of combustion vented from the missing patio doors and windows on the C side and from the roof ventilation hole.


The fire weakened the connections between walls and the roof. On the A side, the roof collapsed into the rooms below. On the C side, the roof slid over the exterior wall until it hit the ground, leaning against the C side wall. This placed an eccentric load on the balcony and exterior wall. This eccentric load initiated the following chain reaction that took less than 10 minutes under fire conditions:

  • The balcony collapsed.
  • The exterior wall at the second floor was pushed inward.
  • The exterior wall at the first floor was already partially burned away and collapsed.
  • The second floor collapsed onto the first (ground level) floor.


The A side rooms remained standing about 10 minutes longer than the C side rooms.

The balance between fuel geometry and ventilation can be so precise that in two identical room fires with apparently identical fuel loads, fuel geometry, and ventilation openings, one room fire will flash over when the door is opened, and the other will not. This was demonstrated in tests by Underwriters Laboratories (UL) (see the links below). Most of today’s room contents fires fueled with plastics, plastic foam, and synthetic fabric burn faster and hotter than the Standard Temperature-Time Curve (Photo 2) used in testing wall and floor-ceiling assemblies for a fire resistance rating in hours. With the fuel in the right geometry and with exactly the right ventilation conditions, it is also possible for a fire comprised of cellulose-based room contents to burn faster and hotter than the Standard Temperature-Time curve.

Laboratory testing, and tests in structures, conducted by UL and the National Institute of Standards and Technology have shown that fuel geometry and ventilation play significant roles in fire behavior. Visit HERE and click on the links at the bottom of the Web page for more information on this research.

Also visit HERE for online courses that present the results of ongoing and past research projects. Additional courses will be available as research data analysis is complete.

This research, combined with the knowledge of the behavior of lightweight and manufactured wood construction materials in fires, suggests that the transitional fire attack should be the method of choice at most residential structure fires. The fire and the products of combustion are the greatest hazard to any remaining building occupants. Knock down the fire from outside with ventilation; evaluate the structure for stability, location of other fire, and possible location of trapped people; and conduct interior operations for search, rescue, fire suppression, and overhaul if the structure is still sound enough to support them.

Fire instructors at all levels of training and education need to emphasize that fires react to many variables and that the fire triangle and fire tetrahedron represent complex chemical and physical reactions. The presence of a fire alarm system with smoke detectors combined with an automatic fire sprinkler system is the best guarantee that the occupants of the compartment will survive, that the fire in a compartment will behave as we expect that it will, and that the fire will still be manageable when initial fire department response arrives.

In the 21st century, firefighters can no longer consider their training to be complete with Fire Fighter I and II. To protect ourselves and our teammates, to keep abreast of changing technology in building construction and the scientific research on fire and fire dynamics, and to work more effectively, we must become educated in the science of firefighting beyond the hands-on practical skills that we learned early in our careers. This initial training taught us the “How” of firefighting. To educate ourselves in the “Why” and “When” of fire behavior, strategy, and tactics, we need to participate in an ongoing continuing education program; if our organization does not presently have one, we can assist in developing one. At the very least, each of us must have a continuing education program for himself and be willing to pass along to our teammates and department members what we learn.

Download this article as a PDF HERE


Gregory Havel is a 30-year fire service veteran, a member of the Town of Burlington (WI) Fire Department; and a retired deputy chief and training officer. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College. He has more than 30 years of experience in facilities management and building construction and has presented classes at FDIC.






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