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# shrinkage

• To: <seaint(--nospam--at)seaint.org>
• Subject: shrinkage
• From: <buddy_showalter(--nospam--at)afandpa.org>
• Date: Thu, 01 Jul 1999 10:03:07 -0400

```there's a handy little applet at the following address to calculate dimensional
changes due to change in MC. using Mr. Cochran's example below it calculates
0.13"

http://www.cwc.ca/english/wood_design/design_tools/dDimCalc0399/dDimCalc.htm

fyi, we're planning to incorporate this module onto the WoodWorks Design Office
2000 release later this year.

********************************

From: Mlcse(--nospam--at)aol.com
Subject: Re: 3x plates, shrinkage
To: seaint(--nospam--at)seaint.org

In a message dated 6/30/99 7:06:34 PM EST, smthengr(--nospam--at)sirius.com writes:

<< Within the last few months I have heard contractors mentioning that the use
of 3x sill and top plates is causing quite a problem with shrinkage, as much
as 1/4" per 2-1/2" plate thickness. The problem being that "dry" 3x material
is "not available". If this is in fact true, then it might help if we remind
the suppliers of the UBC requirements and that 3x material is in demand. Any
thoughts?

Jeff Smith
>>

I haven't heard of any complaints about shrinkage, but I would be expecting
some.
A 1/4" seems a bit much for a single 3x sill though.   Only an average of  6%
change in dimension is expected when going from Moisture Content = 30% to
Moisture Content = 0%.  See Breyer's book on Design of Wood Structures (3rd
edition). Maximum expected 3x sill expected shrinkage = 0.06 (2.5) = 0.15
inches.  Most likely the wood is reaching equilibrium around 10-12% moisture
content and the resulting, therefore shrinkage  = .002(2.5")(30-10) = 0.10
inches.

If you are talking about a double 3x top plate + a 3x sill plate, then I can
see the overall shrinkage being around 1/4" to 5/16" inch.  Typically we have
been only using the 3x at the foundation sill plate (code required).  I don't
believe the code requires it at the double top plate or sill plate at upper
floors unless you are splicing plywood on the 3x member or need the area by
design for a tension chord or drag.

I believe shrinkage is going to be one components which you are going to have
to consider when calculating holdown assembly displacements when calculating
shearwall deflections.

Michael Cochran

```