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Re: Change in Dimension Lumber Design Values

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Conrad,

Use the old design values for stress with the current equations for allowable capacities except when the Adjustment Factors (other than Load Duration) are greater than 1.0. I would not use size factors greater than 1.0, but I'd get some more opinions from the list on that one. Yes, use the current values for snow, wind, seismic, live loads when evaluating adequacy.

The reason the new allowable stress values are much lower is because the wood used today is much younger and weaker than the wood chopped down from the old forests, even from the same species... I'm sure some of the wood experts on the list from APA and other places can elaborate beyond my crude summary.

-gerard
San Francisco, CA

>>> conrad(--nospam--at)karren.com 05/30/01 10:51AM >>>
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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