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Components –
Metal-plate-connected wood trusses are the predominant type of truss used in residential
construction. They are typically fabricated from 2x4 or 2x6 dimension lumber.
Trusses are categorized as “parallel” chord for use in floor and
flat roof applications or “pitch” chord for sloped roof
applications.
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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.
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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 Construction[1], 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 Handbook[2].
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.
For more detailed information on wood trusses,
click here to download the seven page PDF file.
[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
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