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Re: Kiln Dry Lumber

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----- Original Message -----
From: Roger Turk <73527.1356(--nospam--at)>
To: SEAOC Listservice <seaint(--nospam--at)>
Sent: Wednesday, June 02, 1999 9:55 PM
Subject: Re: double sided shear walls

> 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.
> ----Forwarded Message(s)----
> Dennis,
> Southern Pine grading rules state that Kiln Dry is 15 percent moisture.
> WWPA grading rules (1991) states that "Moisture is removed from lumber
> 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
> is mentioned --- it certainly isn't in the glossary.

Comment from Bruce Pooley: WWPA Standard no. 17, Grading Rules for West
Coast Lumber, revised January 1, 1996 defines Kiln Dry in section 3-d on
page 22. It allows the MC15 marking as for Southern Pine or KD 15 when the
lumber is kiln dried to a MAXIMUM of 15%. It also allows the lumber to be
marked KD 19 when the lumber is kiln dried to a MAXIMUM of 19% moisture
> Section 4.10 states, "... Except as otherwise provided, any lumber
> at a moisture content of 15% or less *MAY* be stamped 'MC 15'." [Emphasis
> added.]
> 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
> definition, it could be 25 percent, if the lumber originally had 27
> moisture.
> 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
> at time of surfacing, except that when lumber is shipped in open
> unprotected from the weather during transit, the seller is relieved of the
> moisture restriction if the buyer is notified and consents to the method
> shipment."
> 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
> time that it is surfaced.

Comment from Bruce Pooley: I would like to clarify that the term "degrades"
as used here refers to warping, twisting, crook and bow. Seasoning checks
may also form and/or existing checks may increase in size. These factors may
change the grade, but in most cases the grade will remain the same.
Dimensional changes due to shrinkage do not effect the grade. Reduction in
cross section are offset by increases in fiber strength and in most cases
the bending, shear, compression perpendicular to grain, and Modulus of
elasicity strength increase.
> I have seen lumber in covered storage in lumber yards here in Tucson that
> literally had moisture running out of it.

Comment from Bruce Pooley: I think the lumber yard should return the lumber
due to excessive moisture if a reinspection shows that the lumber was above
the 19% MC specification. If it was due to not covering the lumber shipment,
then the lumber yard should wise up and make sure the lumber is covered.
> 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
> construction.

Comment from Bruce Pooley: Isn't LRFD based on statistical representation of
the material? I think that this variation is accounted for in the
development of the betas, phis and other factors.

> 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
> . > the desert where the average humity level is below 10%. I was under
> . > 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
> . > reduce shrinkage after construction. I thought that Kiln Dried lumber
> . > 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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