From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Fri, 25 Feb 2000 16:44:16 -0800
>Estimating shrinkage: I believe longitudinal shrinkage in Doug-Fir/Larch(N)
>is negligible, possibly 0.1%. The significant shrinkage occurs in the stud
>wall plates and rim board. In the Timber Construction Manual there is a
>formula for estimating shrinkage based on initial and final moisture content.
Sawn lumber joists and rim boards are the main source of vertical dimension
loss in "platform" framed buildings. Framing lumber is ordinarily surfaced
unseasoned, or "green", at more than 19 percent moisture content.
Lumber continues to shrink cross-grain after this surfacing, at the rate of
1 percent shrinkage for every 4 "points" further reduction in moisture
content. Lumber ideally should not be installed until moisture content falls
below 19 percent; a reduction in nail allowable value appears in the 1997
NDS at Table 7.3.3 for lumber nailed when moisture is above 19 percent, even
though it is dryer in service later. A reduction of moisture from 19 percent
to 8 percent is enough to shrink 9.25 inches of 2x10 joisting to 9.00
inches. (ie, a 2.7 percent shrinkage resulting from 11 points loss of
moisture.) This is explained in the 1991 Western Lumber Grading Rules at
section 4.00. Glue-laminated beams are fabricated of lumber substantially
wetter than in service, and shrink accordingly, but slowly.
Window and door headers are often 12-in deep lumber in 8-ft wall heights.
Watch out for vertical shrinkage of these if any load bears on them.
Factory-made, fully dry wood products are offered for plates and headers in
lieu of shrink-prone sawn lumber, as well as for joists and rim boards.
Besides vertical metal pipes being affected by accumulated vertical
shrinkage, so are story to story uplift-resisting connectors. Watch out for
concentrated overturning-generated vertical forces, in both up and down
directions, from multi-story stacked shear panels.
Charles O. Greenlaw SE Sacramento CA