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Change in Dimension Lumber Design Values

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I have been asked to evaluate a building constructed in 1963.  Design
drawings indicate that Construction Grade D.F. 2x10's are to be used with an
allowable Fb of 1500 psi for the roof framing.  Current code accepted values
(1997 UBC) for Const. Grade D.F. 2x10's are Fb*Csubf=1000*1.2=1200 psi.

The structure was designed for 30 psf snow,  and the current snow load
requirement is 50 psf, (which is reasonable for the area).  I can show that
the structure works using the original design values, but not the current
ones.  The building appears to be performing adequately, but I need
something to justify the use of the higher original allowable stresses.
I've been through the changes to the 1991 NDS, but the literature I can find
isn't much help.

Does anyone have an explanation (with documentation) for the change in
allowable stresses?

Thanks

Conrad Guymon, P.E.


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