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Change in Dimension Lumber Design Values

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Conrad,

Old (pre-1970) Construction Grade J & P (Joists & Planks) is equivalent to 
today's No. 1 Grade.

Today's "Construction Grade" is limited to Studs (2 to 4 inches thick, 4 
inches wide) and does not apply to J & P.  Be careful that you are referring 
to the correct grouping (J & P, Stud, Light Framing) for your allowable 
stress and that you are not using stresses for Canadian graded (NLGA) lumber 
unless the lumber was actually imported from Canada.

Pre-1970 sizes were also different, with 2-inch nominal being 1-5/8 inches.  
2 X 10's were 1-5/8" X 9-1/2" (IIRC - I'll check in about an hour.)

The change in allowable stress was based on full-size, in-grade tests of 
lumber in the 1980's.  Previously, tests were performed on small (2" X 2") 
clear specimens and the results modified in an attempt to account for 
strength reducing characteristics.  The full-size, in-grade tests showed that 
the allowable stresses based on the small, clear wood specimens was not 
always conservative.

I would evaluate the building based on the current allowable stresses

Hope this helps.

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

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?

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