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RE: Historic Wood Structure; How to Determin

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Robert, 
I would agree with Roger's rules on this issue. Stress values in wood
were re-evaluated back in the early or mid 80's based on a 20-year study
of wood quality in construction. As I recall the availability of the
same quality wood used in the early part of the 20th century is not as
abundant today as it was then, therefore, in the 1991 code (I think this
was the year) stress values for almost all species of wood were reduced.
This means that older structures were constructed of better quality of
wood deserving higher values.

One comment on testing for strength in wood. While I assume that you can
do various bearing tests on wood, grading is, as I recall is based on
each member of wood used. Grading is a statistical evaluation of defects
in the wood based on a given length of board. If you cut the board
(joist or beam) down, you can change the grade of the wood by removing
or reducing the number of defects in the resulting member.

I might be wrong on this point, but maybe Buddy Showalter can lead us to
the document that the study was based on. I don't believe it is possible
to determine the grading of a piece of lumber accurately if the grade
stamp is removed or cut off the end of the member.

Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant


-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com] 
Sent: Friday, April 25, 2003 4:37 AM
To: seaint(--nospam--at)seaint.org
Subject: Historic Wood Structure; How to Determin

Robert,

Prior to 1970's, the lowest grade of structural wood was "Construction 
Grade."  Comparing the visual grading rules of 1960's "Construction
Grade" to 
today's grading rules, these are equivalent to today's No. 1 grade,
therefor, 
I would use the allowable stresses for today's No. 1 Grade to evaluate
the 
structural capacities of the existing members.  (Also, I believe that
the 
UCBC states that No. 1 grade stresses be used.)

HTH

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

 


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