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factory-made roof trusses

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Nels,

Get a copy of MasterSpec Section 06192 (Metal-Plate-Connected Wood Trusses) 
from your architect-client.  MasterSpec is licensed, otherwise I would offer 
to send you a copy.

Since most pre-fab truss fabricators use computer analysis provided by the 
plate manufacturer, which kicks out *anything* that is permitted by the code, 
I would specify that only No. 2 or better grade is to be used.  (I have seen 
No. 3 called for on their calcs and I have seen No. 3 in lumber yards --- No 
way would I ever permit No. 3 to be used on any of my jobs, except as 
firewood.)

Make sure that you clearly specify the loads that are to be applied to the 
top chord and the loads that are to be applied to the bottom chord.  I 
investigated one failure where non-structural wood box beams at 6' centers 
were connected adjacent to every fourth truss, causing the web member to 
chord connections to fail.  (The trusses also lacked lateral bracing of 
compression web members --- see next paragraph.)

Designs that I have seen typically will not spell out lateral bracing of web 
members in compression.  I specify that *all* bracing that is required as a 
condition of design is to be clearly detailed on the fabricator's plans.

Be aware that truss fabricators are neither licensed, trained nor franchised 
by the metal plate manufacturers.  The metal plate manufacturers merely sell 
the fabricators the metal plates and the engineering.  Therefore, the quality 
of work produced by different fabricators can vary tremendously.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

. > I have a project for which I need to specify pre-fabricated wood roof 
. > trusses (gang-nailed, pronged side-plate, or whatever is the current
. > least-cost roof- framing method). This is something I haven't had to do 
. > before and I have no data or names of manufacturers. Any recommendations 
. > on sources or fabricators, and design data?
. > 
. > Nels Roselund
. > Structural Engineer
. >