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Re: Wood shrinkage

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The lumber grading rules allow maximum of 19% moisture content (MC) for Dry
and if Kiln Dried, 15% MC for framing (dimension lumber such as 2x4's to
2x12's and 4x's). I would suggest that you use these as the "initial"
moisture content.

The Wood Handbook: Wood as an Engineering Material by the US Dept. of
Agriculture, Forest Service contains additional information you are looking
for. In Chapter 14, Table 14.1 framing is recommended to be 12% average,
individual pieces are 9 to 14 % for most areas of the country that includes
Iowa. (Other areas are the dry Southwest and the damp coastal areas.) This
can be used to for the equilibrium moisture content that can be expected in
the wood after construction. I would suggest using these values for the
"final" moisture content.

Interior wood materials are recommended to be 8% average and 6 to 10% for
individual pieces.

Dimensional change coefficients are included there as well. For Douglas fir
Coast, radial coefficient is approximately 0.165% and tangential is 0.267%.
These coefficients are percent dimension change per 1 percent change in
moisture content based on dimension at 10 percent MC.

Bruce Pooley
Timber Design
Lakewood, CO
----- Original Message -----
From: <JPRiley485(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Friday, February 25, 2000 2:38 PM
Subject: Wood shrinkage

> Architect wants to build residential structure in Iowa.
>      4 stories (1 at grade, 3 supported)
>      Wood stud bearing walls
>      Approx. 15,000 sf per floor
> Fire safety issues have been hashed out between the architect and the
> building official.  The building official required the architect to hire a
> structural engineer on this project "because it is 4 stories."  For an
> essentially identical 3-story building, he was allowed to do the
> himself.
> My concern is with the following UBC section:
> Wood stud walls and bearing partitions shall not support more than two
> and a roof unless an analysis satisfactory to the building official shows
> that shrinkage of the wood framing will not have adverse effects on the
> structure or any plumbing, electrical or mechanical systems, or other
> equipment installed therein due to excessive shrinkage or differential
> movements caused by shrinkage.  The analysis shall also show that the roof
> drainage system and the foregoing systems or equipment will not be
> affected or, as an alternate, such systems shall be designed to
> the differential shrinkage or movements.
> It's probably within the architect's overall responsibility to coordinate
> building materials so that we don't buckle a plumbing pipe, or something
> that.  And hopefully, the "analysis" of the effects of shrinkage will not
> entail much more than a shrinkage estimate (by me) and an evaluation of
> effects on the various systems (by someone other than me).  Seem
> Estimating shrinkage:  I believe longitudinal shrinkage in
> is negligible, possibly 0.1%.  The significant shrinkage occurs in the
> wall plates and rim board.  In the Timber Construction Manual there is a
> formula for estimating shrinkage based on initial and final moisture
>  And I found this shrinkage calculator on the Internet:
>    It boils down to
> establishing initial and final moisture content . . . help.
> John P. Riley
> Riley Engineering
> Blue Grass, Iowa