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RE: Moisture Content in Timber

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Generally design is based on MC at time of fabrication-time of use
considerations.  19% is the defining line for load and strength
considerations.  The National Design Specification (NDS) covers this.  Since
long term deflection of limber is moisture dependent for Table 16-E a factor
of 16% was used.  You can get lumber in 3 grades; wet or more than 19%, dry
16% to 19%, and MC15 which is 15% MC or less. Every market (I am in So.
Cal.) is either green or dry.  MC 15 is a special order that you must ask
hard for.  For typical construction I use Green at time of Fabrication Dry
in Use.  For trusses you can get Dry lumber.  Exception timbers are not
usually available dry. 

George Richards, P. E.

-----Original Message-----
From: Richard Flower [mailto:RLFlower(--nospam--at)worldnet.att.net]
Sent: Tuesday, July 18, 2000 8:05 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Moisture Content in Timber 


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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