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Throughout the United States, engineered wooden I-beam joists are becoming more prevalent in residential construction for several reasons. For example, they can be more cost effective than dimensional lumber, especially in longer lengths; they can span longer distances without vertical supports; spacing can be wider on center, thus reducing material; they are lighter and easier to handle; and they are straighter, with no crowning.
That's all fine for the builder and for the homeowner, who can take pride in reduced lumber consumption and the ecological friendly nature of engineered wood. But what about the life safety of firefighters? How will lightweight structural support beams hold up when exposed to heat and flame?
In my 31-plus years in the fire service, I never once was concerned about the structural integrity of a floor I was operating on, whether advancing a hoseline into an inferno or conducting a initial primary search under a heavy smoke condition. Every residential floor was comprised of solid dimensional lumber. The structural stability of such a solid-mass floor enabled firefighters to accomplish all phases of structural firefighting: primary, secondary and tertiary searches; occupant rescue and removal; fire suppression; defining fire travel and extension; overhauling and salvage; and protection from secondary fire damage. Time and time again, the same routine interior attack strategy and tactics were implemented for occupied residential occupancies. This planned and coordinated attack could vary, dependent upon the type of occupancy and the severity of the fire.
Time is not on our side. A floor system constructed with these engineered products can collapse within minutes after exposure. Lightweight construction presents two major concerns: very fast-moving, very hot fires, especially when vinyl exterior siding is involved; and the rapid collapse of the floor and roof when fire and or heat impinges on the trusses or wooden I-beam joists.
The following information is provided by the National Institute of Standards and Technology (NIST): "Predicting a potential structure collapse is one of the most challenging tasks facing an incident commander at a fire scene. Usually, lack of information on the construction of the building, fire size, fire location, fire burn time, condition of the building, fuel load and floor weight load make the task nearly impossible. Many factors influence the failure of structural elements -- the floor load, fire intensity, fire duration, fire-resistance rating of wood assembly consisting of wood I-beams or wood trusses protected by half-inch gypsum board (20-minute rating), the fire resistance and ability of a structural member to maintain its load during an actual fire."
Today, the fire service is faced with a new life-threatening dilemma, the collapse potential and the structural stability of engineered structural elements when exposed to heat and fire. In four recent fires in four states, the lives of five firefighters were lost in residential fires due to structural floor collapse. The common thread in these incidents was the fire location, which originated in the basement area, lightweight residential construction, type of floor system which comprised of wooden I-beams used as floor joists and open-web gang plate floor trusses. There was no fire-rated thermal barrier to the underside of the unprotected engineered I-beam joists or the open-gang plate floor trusses.
The following information is from a recent National Institute for Occupational Safety and Health (NIOSH) firefighter fatality investigation report: "Tests conducted by the Illinois Fire Service Institute at the University of Illinois suggested that engineered wooden I-beams can fail in as little as four minutes and 40 seconds under controlled conditions." While objective time-to-failure studies of various lightweight flooring systems being used today have been conducted only in limited cases, evidence from two investigations suggest and seem to support the fact that engineered I joists quickly lose strength and integrity when damage and weakened by heat exposure and flame impingement. The weakened floor joist can be difficult to detect from above, as the floor surface may still appear intact.