Metal-plate-connected wood trusses are the predominant type of truss used in residential construction. They are typically fabricated from 2×4 or 2×6 dimension lumber. Trusses are categorized as “parallel” chord for use in floor and flat roof applications or “pitch” chord for sloped roof applications.
In a roof truss, the three sides (or perimeter elements) of the triangle are called “chords.” The “webs” are wood pieces connecting the top and bottom chords. Chords and webs are the “members” or elements of the truss. The “connectors” joining chords and webs in modern trusses are usually metal-toothed plates.
Manufacture of Trusses
Trusses are designed using sophisticated software that accurately calculates the structural load conditions in accordance with building code requirements. During construction is the best time to see how a truss roof system is configured and distributes loads. Almost as soon as trusses are set in place, maybe even the same day, the roof sheathing is attached. This quick construction time prevents inspectors from having a good opportunity to see the framing from outside the building. Fortunately, everything the fire service and building inspector need to see is visible from inside the structure during the complete building framing inspection. The web and chord elements are fabricated to exact dimensions. The pieces are arranged in their final orientation and the metal plates are applied using equipment capable of exerting high pressure to embed the metal-plate teeth. Trusses are inspected for proper plate orientation and plate-teeth penetration depth prior to shipment to the job site.
Performance Requirements for Code Acceptance
Metal-plate-connected wood trusses are designed and manufactured in accordance with ANSI/TPI1 National Design Standard for Metal Plate Connected Wood Truss Construction¹, as specified by the building code (International Residential Code 2003, Section R802.10.2, International Building Code 2003, Section 2303.4). More details regarding metal-plate-connected wood trusses can be found in the Metal Plate Connected Wood Truss Handbook².
¹ National Design Standard for Metal Plate Connected Wood Truss Construction, ANSI/TPI1, Truss plate Institute, Madison, WI., 2002. http://www.tpinst.org
² Metal Plate Connected Wood Truss Handbook, 3rd Ed., Structural Building Components Association, WI., 2002. http://www.sbcindustry.com/firepro.php
Fire Incidents Involving Trusses
The National Institute of Occupational Safety and Health maintains a partial database of firefighter fatalities. Each fire is reported separately with details on the fire and circumstances leading to the fatality. Additionally, the reports provide a summary of fire ground management/command activities that could be improved upon. This information is extremely valuable to the fire service as a learning aid.