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Re: Change in Dimension Lumber Design Values

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Conrad:

It seems to me that you could use the repetetive member factor Cr (1.15) and the
duration factor CD (1.25 for Roof LL, I think it might be 1.15 for snow load.)
If I'm right about the numbers that would give F'b = 1000*1.2*1.15*1.15 = 1590
psi.  HTH

Chris Willcox, P.E.

Conrad Guymon wrote:

> 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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