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Re: Acceptable Level of Overstress

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> . > First, how old is this wood beam, and are you using the correct
> . > allowable stresses?  Older wood generally had higher allowable
> . > stresses, because stronger wood was more common (modern
> . > wood is grown as fast as possible to generate more revenue
> . > and is "weaker").
> Where does all this dangerous misinformation come from?

University professors and other PE's.  Not that this means anything, because
I don't believe any of them would be considered "experts" on the material
properties of wood.  However, there is the fact that allowable stresses for
wood is higher in older codes than it is now.

> Can anyone cite where in the Full-Size, In-Grade testing program that they
> said that they used only "new-growth" wood?
> Can anyone cite where "new-growth" and "old-growth" are defined?

I believe the terms "new-growth" and "old-growth" are misleading.  I think
"slow-growth" and "fast-growth" are better terms.  The timber industry is
steadily advancing with more research on faster-growing hybrids, the correct
planting spacing, the correct age for thinning, etc.  While this generates
faster returns on investments for the tree farmers, the overall strength of
the wood is generally down because the wood density is generally less.  This
is as of about a decade ago - the industry has probably changed since then.

As I understand it, allowable stresses for wood are set by placing
full-sized members in a testing machine and loading them to failure.  The
test results are plotted and the grad student attempts to draw a line
roughly through the center to determine the average strength for a
particular species and grade.  A factor of safety is then applied to this
average to assure that even the weakest members in grade have some assurance
of not failing.  This FS is fairly high - about 2.5 I think.

I agree that they probably do not differentiate between "slow" and "fast"
wood in the testing.  However, the testing done 75 years ago would have a
much higher percentage of "slow-growth" wood, and the average strengths
would be higher.  Wood used in testing and construction today has a much
higher percentage of "fast-growth", so the averages are lower.

Jason W. Kilgore
Project Engineer
Leigh & O'Kane, L.L.C.
Kansas City, Missouri

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