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Re: Moisture Content in Timber

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You may want to check out the USDA's Agricultural Handbook No. 72:  "Wood Handbook:  Wood as and Engineering Material."   It contains a good discussion of wood moisture content issues.  Moisture content in service depends on the application (exterior or interior).  For most dry, interior applications the equilibrium moisture content in service should be between 6 and 12%.  It varies with the temperature and relative humidity of the air. 

Wood shrinkage and creep deflection are difficult to predict exactly.  The 1997 NDS provides a "rule of thumb" calculation that has been used for years in Appendix F.1.  It suggests multiplying the bending deflection from long-term load by 1.5 for "seasoned" lumber and 2.0 for "unseasoned lumber."

Depending upon the species, grain orientation, and member dimensions, wood can take a very long time to reach its equilibrium moisture content.  Getting wet once on the job site can be enough eliminate a lot drying that took weeks to accomplish.  The conservative approach would be to assume that wood with a moisture content substantially above its ultimate equilibrium should be considered "unseasoned."


>>> RLFlower(--nospam--at)worldnet.att.net 07/18/00 09:04AM >>>
This one is for all you wood experts:

The 1997 edition of the National Design Specifications, Section 4.1.4
appears to suggest a 19% moisture content as the expected service condition
for structural timber. However, the 1997 UBC Table 16-E footnote 1 says
"Seasoned lumber is lumber having a moisture content of less than 16% at the
time of installation...". If both these statements are accepted as
guidelines for design, then, does that mean that timber cannot be considered
seasoned when used for structural purposes?

This makes a considerable difference in the design of long spaned members.

Regardless of how structural lumber is specified for a project, much of the
structural lumber that is delivered to a construction site is delivered
"wet". Yet, these members can be expected to dry out over time - in fact,
much of the moisture content is lost even before the members are placed in
service. Also, members of relatively smaller cross-section would "air-dry"
more quickly than members of larger cross-section. Yet, I have not found
provisions that consider this fact.

Any comments on this issue would be appreciated.

-Richard L. Flower, P. E.



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