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Re: Moisture Content in Timber

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Richard,

19 percent moisture content (at time of grading) is an arbitrary value that 
separates timber called "dry" from timber called "green."  Lumber with a 
moisture content when graded of 19 percent or less is considered as being 
graded dry.  Lumber with a moisture content when graded of more than 19 
percent is considered as being graded green.  The 19 percent moisture content 
has no relation to the expected in-service moisture content of lumber.

The in-place equilibrium moisture content of lumber is another story.  The 
equilibrium moisture content is dependent on the ambient temperature and 
humidity conditions of the area and averages vary from 11 percent along the 
Gulf and South Atlantic coast to 6 percent for Arizona, western New Mexico, 
Nevada, parts of Utah, Colorado, Idaho, Oregon and California.  For the rest 
of the U. S., the average equilibrium moisture content is 8 percent.  (Table 
12-1, "Wood Handbook," Forest Products Society, 1999.)

As lumber drys out, it also degrades.  Splits, checks, twists, warps develop 
that may qualify the piece of lumber one or two grades lower.  Since a mill 
can get more higher grade pieces out of a "green" log than a "dry" log, much 
lumber is graded green or close to the 19 percent moisture content.

Table 16-E of the UBC provides coefficients that is supposed to take into 
account "creep" of certain materials and does not restrict the use of wood 
with higher moisture content.  Wood with higher moisture contents will creep 
more than wood of less moisture content.  I have reservations about the 
values of the coefficients in table and use values that I feel are more 
realistic based on experience and judgement here in the desert southwest.  
For wood, I use 2*DL + LL to calculate deflection, and, if the wood can be 
cambered, I specify a camber approximately equal to the deflection due to 
2*DL.  For prestressed concrete, I consider that the final camber will be in 
the range of 3 times initial camber (assuming that the stress condition under 
full dead load has a higher compressive stress at the bottom of the member 
than at the top.)  There are a number of hump-backed prestressed bridges in 
the Tucson area that were designed in the late 1950's that support this 
assumption.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Richard Flower wrote:

>>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.<<

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