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RE: Wood Truss Rafter Center Uplift

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It is not likely that you will get anywhere near L/240 deflection with a
standard truss.  I typically specify L/720 for snow load deflection.  One I
had a  truss manufacturer will get excited about it; but I told him to look
at the calc's. he had done on a few of his recent typical trusses.  On some
of them, the calculated deflection turned out to be less than L/1000.  I
don't know how accurately their computer models predict joint slip, but
because of the shape of a standard truss, I think you can expect less
calculated deflection than the usual L/240 criteria.

Roger Davis
SDS Architects, Inc
205 N. Dewey Street
Eau Claire, WI 54703
715-832-1605
rdavis(--nospam--at)sdsarch.com


-----Original Message-----
From:	Jake Watson [mailto:jwatson(--nospam--at)inconnect.com]
Sent:	Wednesday, February 02, 2000 9:06 PM
To:	seaint(--nospam--at)seaint.org
Subject:	Re: Wood Truss Rafter Center Uplift


An additional thought: if you "ignore" the interior walls, how will you deal
with the large deflection at center span?  At L/240 deflection a 50' span
deflects 2.5", that's an awful lot for sheet rock to move.  My point is, the
trusses will bear on those walls, even if you don't intend them to.  Think
relative rigidity as well.  If the walls are stiffer than the trusses, the
walls will take the load.  You can design for 2.5" of movement, but it won't
be easy.
Jake Watson, E.I.T.
Salt Lake City, UT