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Re: 3x plates, shrinkage

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In a message dated 6/30/99 7:06:34 PM EST, smthengr(--nospam--at) 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
 Jeff Smith

I haven't heard of any complaints about shrinkage, but I would be expecting 
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 

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