Open Joist 2000: Trusses Without Gussets

Open Joist 2000 is the product name for a type of floor-truss system being used in today's building construction. According to the manufacturer, Open Joist Inc. of Trois-Rivieres, Quebec, Open Joist 2000 "is a trimmable all-wood open-webbed finger-jointed...


Open Joist 2000 is the product name for a type of floor-truss system being used in today's building construction. According to the manufacturer, Open Joist Inc. of Trois-Rivieres, Quebec, Open Joist 2000 "is a trimmable all-wood open-webbed finger-jointed floor truss without metal plate...


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Open Joist 2000 is the product name for a type of floor-truss system being used in today's building construction. According to the manufacturer, Open Joist Inc. of Trois-Rivieres, Quebec, Open Joist 2000 "is a trimmable all-wood open-webbed finger-jointed floor truss without metal plate connectors. With the engineering strength of triangulation, Open Joist 2000 uses solid sawn lumber to its maximum capacity."

This truss does not use a metal gusset plate at the vertex of the joints where the web members intersect with the top and bottom cord. Instead, the cord and web are finger-jointed. This thin-wall finger joint is recessed in about five-eighths of an inch and glued into place with a product called phenol-resorcinol formaldehyde resin adhesive, which bonds the web members to the top and bottom cords. The truss top and bottom cords are available in various widths and can span up to 24 inches on center. Different truss depths enable longer spans from 13 to 30 feet long.

The concern for firefighters is how this floor system will hold up to a free-burning fire. Based on information provided by the Carbeck Structural Components Institute (CSCI) on phenol-resorcinol adhesives, "Generally, these adhesives do not ignite at the bond line, but do pyrolyze." To pyrolyze is to subject materials to pyrolysis, defined as "a chemical degradation reaction that is caused by thermal energy" by the International Union of Pure and Applied Chemistry (IUPAC). I interpret that as thermal decomposition.

Phenol-resorcinol adhesives start burning at 1,130 degrees Fahrenheit, according to the information provided by the CSCI. Contents found today inside the average household are burning faster and creating a higher heat release rating than before, thus creating temperatures that can surpass 1,220°F in minutes.

When fire extends into a truss loft, expect a rapid involvement of the area. When this occurs, firefighters cannot control extension and quickly extinguish the fire. Expect the floor system to collapse. This is comparable to a fire in a roof cockloft. The buildup of heated gases creates a pressure surge and vertical ventilation cannot be implemented without risking the lives of firefighters who would have to operate on the floor above. Besides, you do not want to cut through a truss. Another risk to consider is that a violent flashover could occur in the truss loft, causing the ceiling to fail and trapping firefighters beneath. Do we want to jeopardize the lives of firefighters? Absolutely not.

The relation between fire severity and fire resistance is the principle factor in structural collapse. The fire-resistance rating of a structural assembly is determined by subjecting the assembly to a standard fire, ranging from 1,000°F at five minutes to 1,700°F. The manufacturer recommends five-eighths-inch Sheetrock as a protective rated barrier. The assembly, if designated to be load bearing, must support the full design load for the duration of the fire test without allowing any flames to pass through. With that in mind, the open HVAC vent in the ceiling creates an avenue for heat and fire to travel into the truss loft. The fire-resistant ratings of Sheetrock will protect the truss assembly to some degree, but for how long? Just because it has a one-hour rating does not mean you, as a firefighter, can operate for any period under that time rating.

Here are questions for firefighters: Are manufacturers of fire-rated materials keeping up with the market of exported materials that quenches the thirst of the consumer in today's households? Do the manufacturers conduct testing in real-fire scenarios? The latter means operating a hand-held hoseline in conjunction with a free-burning fire to see how the product holds up to extreme heat and cooling.

At a recent fire, an engine company was advancing a hoseline down a long hallway, cooling the superheated thermal layer. As the firefighters approached the doorway of the fire apartment, the Sheetrock in the public hall disintegrated and the fire wrapped around in their direction. The engine company was able to move the line back for protection and to avoid serious consequences.

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