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Change in Dimension Lumber Design Values

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Roger, US DOC PS 20-99 standardized the sizes for both dimension lumber and
timbers (5x5 and larger). here's a link to the pdf file for ps 20-99. check out
table 3.

Buddy Showalter, P.E.

From: Roger Turk <73527.1356(--nospam--at)>
Subject: Change in Dimension Lumber Design Values
To: SEAOC Listservice <seaint(--nospam--at)>

I wrote:

 > 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.)

Confirmed.  The properties are:

   A     = 15.44 in^2
   I     = 116.1 in^4
   S     = 35.82 in^3

Also, to the best of my knowledge, the new lumber stresses had nothing to do 
with new growth/old growth wood.  In the late 1970's, tension tests on 
full-size, in-grade lumber indicated a significant reduction (40 - 60 
percent) in allowable tension stress than the small, clear specimens 
indicated.  (ICBO came out with an emergency code change in 1979, reducing 
the allowable tension stress 60 percent.)  I believe that it was this 
discovery that caused the 1980's full-size, in-grade tests.  For smaller 
pieces, the allowable stress increased; for larger pieces, the allowable 
stress was reduced.

Prior to 1970, each lumber grading agency wrote their own grading rules and 
determined allowable stresses.  Although the difference was not great, it was 
not unusual to see different stresses promulgated by WWPA and WCLIB for the 
same grade and species of wood.  The early 1970's were a transition in the 
lumber industry regarding "dimension" lumber, in which the grading, size and 
allowable stress was standardized.  This uniformity does not extend over to 
Beams & Stringers, or Posts & Timbers which are not "dimension" lumber.


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

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