Lightweight Truss Systems: A Killer of Firefighters

Gary Morris reports that testing reveals the effects of fire on lightweight truss systems and pose early collapse potential.


Firefighting has always been inherently risky. This is especially true once fire penetrates a building’s concealed spaces and begins to chew away at structural members and the structure begins its collapse sequence. What is unknown to the incident commander is when a collapse will occur...


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Firefighting has always been inherently risky. This is especially true once fire penetrates a building’s concealed spaces and begins to chew away at structural members and the structure begins its collapse sequence. What is unknown to the incident commander is when a collapse will occur.

Recent scientific testing by the National Institute of Standards and Technology (NIST) on the effects of fire on lightweight truss systems should cause the fire service to rethink its approach to firefighting with these types of systems. The tests reveal very early collapse potential.

Lightweight truss systems typically are constructed of two-by-four-inch wood materials. The system is engineered to hold up a substantial deadweight load for its design and construction. They are most often used to construct attic systems, but may also be found in floor systems.

Lightweight truss systems are mass-produced off site and brought to a building location. These systems use metal plates, often called “gusset plates,” to hold the wood components together and replaces the old method of “nailing.” These plates have dozens of small teeth (typically no more than three-eighths of an inch in depth) that are pneumatically pressed into the wood during the assembly process (see photo 5). It is these plates that have been found to contribute to the early collapse potential. As fire penetrates a space containing a lightweight truss system, the metal gusset plates rapidly absorb heat, even prior to direct flame impingement. As the metal heats, wood charring quickly occurs around the teeth of the plate. Because of the superficial penetration of the teeth, the charring rapidly causes the loss of hold. Eventually, the plate will literally fall off. This leaves the truss system held in place purely by friction.

Once any single component of the truss fails, the entire truss system is severely compromised. Any weight or movement across the roof surface or ventilation activities can cause an immediate collapse of the roof. The same will occur if the system is disturbed from below – such as pulling ceiling, or operating hose streams into the attic space or floor assembly. It should also be noted, when lightweight construction is involved, history indicates large portions of the roof will collapse.

Over the past two decades, the National Fire Protection Association (NFPA) has cited numerous cases of lightweight construction contributing to firefighter fatalities. In one case, in 1988, two Orange County, FL, firefighters died while fighting an attic fire in a strip mall. Firefighters were operating from below and collapse occurred before the second-due company arrived on scene!

In another case, the Phoenix, AZ, Fire Department experienced a “near miss” in July 1989 when three of four members of a ladder crew fell through the roof at a residential fire. One member disappeared completely into a well-involved attic space for several seconds before he crawled back to the opening and was dragged out by fellow firefighters. All survived with minor injuries only because they were wearing self-contained breathing apparatus (SCBA), with their facepieces on their faces, and were fully encapsulated in protective clothing.

The investigation revealed a lightweight truss system. The gusset plates had fallen off many of the truss components. By contrast, of the surviving two-by-four-inch truss members, the worst char depth had penetrated less than 20% of the thickness of the truss members. Had the roof system used a conventional “nailing” method, with deep penetrating nails, the roof system most likely would not have collapsed in such a short time (see photo 6).

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