Lightweight Wood Truss Construction Gives Up More Lessons

Firefighters attempted to ventilate the fire soon after arrival, but were forced to retreat early as the rapidly spreading fire quickly made the roof deck untenable.

(Photo by Farmington Hills FD)

Lightweight Wood Truss Construction Gives Up More Lessons

FIRE REPORT

The continuing battle against runaway technology.

LIGHTWEIGHT WOOD TRUSS construction is having a major impact on firefighter safety and survival throughout the country. This fact has been emphasized over and over in this and other fire service publications by Frank Brannigan, Vincent Dunn, John Mittendorf, and authors of fire reports from every section of the nation. Originally designed for large, open commercial spans, lightweight truss construction has found its way into bedroom community private dwellings. The factors of less cost and reliance on performance codes are the underlying causes.

In January 1988, the Farmington Hills (MI) Fire Department had these lessons vividly reinforced at a private dwelling fire in our district. The structure collapsed early in the fire operation without causing injury—but we were just lucky.

On that day, we responded to a reported kitchen fire. Upon arrival, firstdue companies reported a heavy fire and smoke condition within a threelevel home with 6,000 square feet of living space, a five-car attached garage, and a converted four-car garage. The lower level was a walkout basement, which included a built-in swimming pool. Response time was four minutes.

An aggressive interior attack was initiated, but heavy fire conditions prevented access to the seat of the fire. Approximately fifteen minutes into the alarm, the second floor collapsed. It is believed that this early collapse was due to the use of lightweight wood trusses to support the entire second floor of this single-family dwelling. The use of these trusses is atypical of most single-family dwellings and not usually considered by fire personnel. Fortunately, firefighters were unable to enter the second floor or search underneath it due to impeding fire conditions—so injuries and perhaps even death to firefighters were averted.

LIGHTWEIGHT TRUSS CONSTRUCTION

Lightweight wood trusses are typically constructed of two-by-threeor twoby-four-inch members forming the bottom chord in tension and the top chord in compression. The chords are joined together by web members of similar lightweight wood construction or, as in this case, two-inch-diameter tubular steel rods cut to size and flattened on each end. The flattened ends of these rods are then inserted into a groove that has been “rabbitted” into the top and bottom chords. A steel pin (up to one inch in diameter) is then placed through predrilled holes in the wood chord member and the tubular steel to hold them in place.

Generally in residential construction, these joists are covered over with drywall. Whether or not the drywall assembly has a fire-resistive rating is irrelevant once the integrity of the covering has been breached. Drywall assemblies are typically rated by testing methods that have the fire burn from the “outside in,” not the “inside out.” All such tests are carried out under controlled conditions that do not always simulate actual structural fire behavior. In this dwelling, tongue-and-groove redwood slats were attached directly to the underside of the joist; this also contributed to the rapid spread of fire.

Soon after roof operations were abandoned, the structure collapsed, sending flames roaring skyward.

(Photo by Farmington Hills FD)

The collapsed area of the structure was extensive.

(Photo by S. Hume)

Pin sizes of connections were inconsistent.

(Photo by S. Hume)

In lightweight wood truss construction of this kind, a large void exists that will allow fire to propagate unimpeded and possibly even assist in propagation by contributing oxygen that has been trapped in the void space. Another problem associated with truss construction is that the members are attacked from the outside and the inside simultaneously. The pins used to hold the web members together are subject to pyrolytic decomposition caused from the heat placed upon them. This can lead to early failure and ensuing collapse. As has been well-referenced in this and other fire service periodicals, failure in any component of the truss can lead to truss failure, and failure of just one truss can lead to total failure of the structure. Each truss is a component of a system, a system that is only as strong as each part which supports it.

All of the above contributed to early collapse during the January fire. One interesting point that we have not been able to clarify is the inconsistency in pin size. Pins in all the web members varied in size, with no standard size used in the top chord as opposed to the bottom chord. Furthermore, it was observed that tubular members in the areas of collapse were still joined together by the pins after the top and bottom chords had burned away.

The tactics utilized to fight fires with this type of construction are dependent upon numerous variables that are beyond the scope of this article. However, die fireground commander, if he is aware that lightweight wood trusses have been used in construction, may base his strategic decisions in part on the following observations:

  1. signs of puffing or pushing smoke (exterior observation) in the area where joists are tied to the structure;
  2. smoke and heat in the interior of the structure with no visible fire;
  3. length of the truss span;
  4. estimated time the fire has been burning without suppression forces making any progress.

The officer must also be aware that the amount of oxygen contained in the void space of lightweight trusses can create a potential for backdraft.

It’s impossible to place a time frame on when a building will collapse. Some texts state that collapse in lightweight construction will occur in ten minutes or less. Any time the fire has been found to be in the truss loft space, consider withdrawal of suppression forces; structural collapse may be imminent.

The fireground commander must consider truss construction in his sizeup of single-family and multiresidential dwellings, just as he must in his size-up of commercial property. Fire personnel are not usually trained to expect this type of construction in single-family dwellings. This is dangerous; singlefamily dwellings are not usually inspected by the fire department, nor are they the subject of preincident planning. This essential construction data must be gathered during actual construction and certainly before occupancy.

The dangers of lightweight wood truss construction are many. Yet, as we have learned, an aggressive attack with constant reevaluation of the fire conditions might eliminate the chances of your personnel becoming statistics on the injury/fatality report. Remember:

  • CONSTRUCTION TYPE IS NOT OCCUPANCY-SPECIFIC.
  • VOID SPACE BETWEEN THE FLOORS CREATES A POTENTIAL FOR BACKDRAFT OR SMOKE EXPLOSION.
  • EARLY COLLAPSE MAY BE IMMINENT IF FIRE HAS EXTENDED INTO THE JOIST SPACE.
  • EXPECT TRUSS CONSTRUCTION IN NEW, LARGE, EXPANSIVE HOMES.

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