From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 30 May 2001 16:34:43 -0400
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
I would evaluate the building based on the current allowable stresses
Hope this helps.
A. Roger Turk, P.E.(Structural)
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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