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Re: double sided shear walls

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I apologize if this is a duplicate message, but the acknowledgement that I 
received from the list server did not have the message in it, just the 
header, acknowledgement and trailer. 

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Southern Pine grading rules state that Kiln Dry is 15 percent moisture.

WWPA grading rules (1991) states that "Moisture is removed from lumber either 
by air drying or by use of special drying kilns."  [Section 4.00, WWPA 
Grading Rules]  I think that this is the only place in the rules that "kiln" 
is mentioned --- it certainly isn't in the glossary.

Section 4.10 states, "... Except as otherwise provided, any lumber surfaced 
at a moisture content of 15% or less *MAY* be stamped 'MC 15'." [Emphasis 

Supplement No. 3 to the WWPA grading rules gives the following definition: 

"Kiln-Dried (KD) lumber is lumber that has been seasoned in a chamber to a 
predetermined moisture content by applying heat."

It doesn't say what that "predetermined" moisture content should be.  By the 
definition, it could be 25 percent, if the lumber originally had 27 percent 

PS20-94 has the same definition for "Kiln-dried" lumber that WWPA has.

So, the only assurance that we have that lumber had been surfaced at less 
than 19 percent moisture would be if it had an "MC 15" stamp on it or was 
graded as kiln dried Southern Pine under the SPIB grading rules.  But even 
then, the moisture content of the lumber may change as WWPA permits:

"Restrictions on moisture content shall apply at time of shipping as well as 
at time of surfacing, except that when lumber is shipped in open conveyances, 
unprotected from the weather during transit, the seller is relieved of the 
moisture restriction if the buyer is notified and consents to the method of 

Since lumber degrades (and becomes less valuable) as it dries, you can bet 
that most lumber is going to be graded at the moisture content it has at the 
time that it is surfaced.

I have seen lumber in covered storage in lumber yards here in Tucson that 
literally had moisture running out of it.

In addition, 5% of a shipment (1 of every 20 pieces) may actually be below 
the grade stamped on it.

These are also part of the reasons that I am opposed to LRFD for wood 

Hope this helps.

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

Dennis Wish wrote:

. > Roger, possibly you can enlighten me on Kiln Dried lumber. I too live in 
. > the desert where the average humity level is below 10%. I was under the 
. > impression that Kiln Drying is a controled method of reducing the 
. > moisture content below 19% to match the territory it is used in so as to 
. > reduce shrinkage after construction. I thought that Kiln Dried lumber in 
. > the desert delivered wood to the site that was closer to 10% relative 
. > moisture content rather than the traditional 19% and thuse reduced the 
. > occurance of shrinkage, twists, warps and checks. What are the facts 
. > behind Kiln Dried lumber?

. > Dennis S. Wish PE

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