BEWARE OF HIDDEN DANGERS IN WOOD TRUSS FIRES

BEWARE OF HIDDEN DANGERS IN WOOD TRUSS FIRES

BY JOHN J. NOVAK

Today`s fire service is faced with many dangers and hazards, some of which can be seen and some of which are hidden from view. A hazard that seems to be growing more common every day is lightweight wood truss construction.

On October 1, 1990, at 1700 hours, Toms River (NJ) Fire Department Stations 25 and 26 were dispatched to a trash fire behind a Wawa convenience store. The fire apparently was set by juveniles in discarded cardboard boxes piled next to the building. As a chief in the department, I responded from only two blocks away. On arrival, I found that the trash fire had extended into the unprotected attic through the vents in the soffits. Units from Stations 25 and 26 were still on the road, returning from a previous alarm. Therefore, the initial response of two engines and a ladder tower was very quick. The first handline was operating inside the building within seven minutes of the initial alarm.

CONSTRUCTION A CRITICAL FACTOR

I was well aware of the type of construction and knew that we were facing a serious collapse hazard. I told the officer of the first-due engine that he had two to three minutes to get the fire under control and then I was going to pull everyone out. The quick interior attack, due to the stroke of luck of having responding units already en route when the call was received, brought the fire under control before an evacuation was ordered and a switch to exterior firefighting began.

Further investigation revealed that the 50-foot by 30-foot building had approximately 25 lightweight wood trusses placed approximately 24 inches on center. Half of these trusses had sustained fire damage to the point that the truss members were burned away. In some areas, gusset plates had fallen out. However, as the fire burned overhead, the store occupants were unaware of it.

The knowledge of the hazards involved with this type of construction played a critical role in the overall strategy and tactics of this operation (Thank you, Mr. Brannigan).1 When confronted with a fire in a concealed truss space, the fire officer should consider some critical factors during the mental size-up. These factors include a risk analysis, fire conditions that can be seen, and the reflex time of the fire department.

LIFE SAFETY: THE FIRST CONSIDERATION

Life safety should be the first consideration in the risk analysis. Are occupants in danger? Are all occupants accounted for? If the answer to this question is no, then the fire attack must progress in the rescue mode until an “All Clear” is given to the incident commander (IC). Once the occupants are accounted for, the IC must consider the life safety of the firefighters and be willing to write off property to provide for firefighter safety.

EXTERIOR CONDITIONS

Exterior conditions viewed on arrival are an important factor when determining strategy. Fires in the truss void space can go unnoticed by occupants in habitable spaces. Such was the case with the Wawa fire, because on arrival smoke and flames were venting from the ridge vent while shoppers were still in the store unaware. When dealing with fires in buildings with high ceilings and large concealed truss spaces that make up the attic, conditions at the roof will give a good indication of the fire`s severity. Conditions under the attic in the habitable space are poor indicators of fire conditions in the attic space, because tremendous amounts of heat and fire can accumulate in the large open area in the attic.

REFLEX TIME

The reflex time of the fire department is the span between the time the fire department receives notification of the fire and the time an actual firefighting operation (such as the first operating fire stream) is underway.

In the past, we used to have a considerable amount of time when dealing with sawn wood used as rafters and joists. The dimensions of the lumber were anywhere from two inches by six inches to two inches by 10 inches. The mass-to-surface ratio was large; there was a lot of “fat” to burn before a rafter or joist lost its strength. With the lightweight wood trusses, the mass-to-surface ratio is small. These structural members are very lean and have no extra “fat.” When the fire is in the truss space, all bets are off. The old adage that you have a specified amount of time no longer holds.

Today`s fire officers need to educate themselves as never before. The hazards and dangers are out there, whether they can be seen or not. Today`s successful fire officers seek out training and education and apply their knowledge during prefire planning, company training, and when it counts–on the fireground. n

Endnote

1. Francis L. Brannigan, SFPE, author of Building Construction for the Fire Service, Third Edition, published by the National Fire Protection Association.

JOHN J. NOVAK is a 20-year veteran of the fire service, having served as a volunteer and career firefighter. He is special operations officer at the Toms River (NJ) Fire Department, Company 2, and currently works for Crash/Fire/Rescue N.J.A.N.G., Atlantic City International Airport. He is an instructor at the Dover Township Fire Academy and has an A.A.S. in fire science from Ocean County College, New Jersey.

No posts to display