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RE: Wood Allowable Stresses

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That "young growth versus old growth" may have something to do with the most
recent values, I don't know for sure, but I do know that the stress values
used in versions of the NDS prior to the 1991 edition were based on the
testing of clear specimens, i.e. the specimens did not include defects which
are nearly always present in most structural lumber we build with, such as
knots, compression wood, etc.  Values for allowable wood stresses prior to
the new testing methodology may well have been more liberal than we thought,
but not necassarily cause for concern.  I know based on some literature and
some testing I did for my thesis that the "factor of ignorance" for most
wood stresses is somewhere around 3, so possibly before that factor might
have been, say 1.5.

Hope this helps.

Albert J. Meyer, Jr., P.E.
Martin Engineering
238 North 22nd Street
Philadelphia PA 19103-1004
(215) 665-8570
(215) 561-5064 Fax


-----Original Message-----
From: Gary Ehrlich [mailto:GEhrlich(--nospam--at)mcecorp.com]
Sent: Thursday, July 25, 2002 9:49 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wood Allowable Stresses



I thought one of the reasons for the dropping allowable stresses is the fact
that the wood coming from younger forests isn't as strong as that which came
from older forests.  So I wouldn't see a problem using the higher stresses
to evaluate an existing building, which was presumably built with the
higher-quality older wood.

Gary J. Ehrlich, P.E.
Project Engineer
Meyer Consulting Engineers Corp.
451 Hungerford Drive, Suite 113
Rockville, MD  20850
(301) 738-5690
(301) 738-5695 (fax)

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