Moisture
effects on lumber within a structure can cause problems such as
Warping
Buckling or cracking of other components such as drywall,
siding, or plaster
Added stress on connections
Additionally, biological attack and decay are of greater
concern at higher moisture levels.
Below is a demonstration of the structural effects of shrinkage
on a two story exterior wall section.
Demo 3: Global Effects of Shrinkage
HOW SHRINKAGE IS CALCULATED
Below the FSP,
there is a linear relationship between changes in MC% and the decrease
and increase of lumber dimensions. Table M-2 lists typical values for
the amount of shrinkage (%) which occurs as lumber MC% goes from FSP to
oven-dry. Values are provided for the radial and tangential shrinkage of
various wood species. Longitudinal shrinkage is not listed because it is
so small, between 0.1% and 0.2% for all species.
Table M-2: Typical Shrinkage from FSP to Oven-dry
(% based on green dimensions)
Species
Radial (%)
Tangential (%)
Ash,
white
4.9
7.8
Birch,
yellow
7.3
9.5
Douglas
fir, coast
4.8
7.6
Douglas
fir, interior west
4.8
7.5
Hemlock,
western
4.2
7.8
Larch,
western
4.5
9.1
Pine,
loblolly
4.8
7.4
Pine,
longleaf
5.1
7.5
Pine, red
3.8
7.2
Spruce,
red
3.8
7.8
Spruce,
Sitka
4.3
7.5
Linear
interpolation is used to calculate the amount of shrinkage that occurs
when MC% changes but does not go all the way from FSP to oven-dry. The
following example shows how shrinkage is calculated.
The moisture
content of wood is not always constant. As seen in the demonstration
above, moisture content can change while the wood is in use. The
moisture content of in service lumber (and lumber in general) will
change with environmental conditions. Prolonged exposure to a
level of relative humidity and temperature will cause the wood to change
its moisture content. When wood in in service it is not
possible to determine the moisture content by getting an oven dry
sample; you would not want to cut specimens out of beams and studs in
your house would you?. Therefore, the following table has been
developed that related temperature and relative humidity to moisture
content.
Temperature
Moisture content (%) at various relative
humidity values
oC
oF
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
-1.1
30
1.4
2.6
3.7
4.6
5.5
6.3
7.1
7.9
8.7
9.5
10.4
11.3
12.4
13.5
14.9
16.5
18.5
21.0
24.3
4.4
40
1.4
2.6
3.7
4.6
5.5
6.3
7.1
7.9
8.7
9.5
10.4
11.3
12.3
13.5
14.9
16.5
18.5
21.0
24.3
10.0
50
1.4
2.6
3.6
4.6
5.5
6.3
71.0
7.9
8.7
9.5
10.3
11.2
12.3
13.4
14.8
16.4
18.4
20.9
24.3
15.6
60
1.3
2.5
3.6
4.6
5.4
6.2
7.0
7.8
8.6
9.4
10.2
11.1
12.1
13.3
14.6
16.2
18.2
20.7
24.1
21.2
70
1.3
2.5
3.5
4.5
5.4
6.2
6.9
7.7
8.5
9.2
10.1
11.0
12.0
13.1
14.4
16.0
17.9
20.5
23.9
26.8
80
1.3
2.4
3.5
4.4
5.3
6.1
6.8
7.6
8.3
9.1
9.9
10.8
11.7
12.9
14.2
15.7
17.7
20.2
23.6
32.2
90
1.2
2.3
3.4
4.3
5.1
5.9
6.7
7.4
8.1
8.9
9.7
10.5
11.5
12.6
13.9
15.4
17.3
19.8
23.3
37.8
100
1.2
2.3
3.3
4.2
5.0
5.8
6.5
7.2
7.9
8.7
9.5
10.3
11.2
12.3
13.6
15.1
17.0
19.5
22.9
43.4
110
1.1
2.2
3.2
4.0
4.9
5.6
6.3
7.0
7.7
8.4
9.2
10.0
11.0
12.0
13.2
14.7
16.6
19.1
22.4
49.0
120
1.1
2.1
3.0
3.9
4.7
5.4
6.1
6.8
7.5
8.2
8.9
9.7
10.6
11.7
12.9
14.4
16.2
18.6
22.0
54.6
130
1.0
2.0
2.9
3.7
4.5
5.2
5.9
6.6
7.2
7.9
8.7
9.4
10.3
11.3
12.5
14.0
15.8
18.2
21.5
60.0
140
0.9
1.9
2.8
3.6
4.3
5.0
5.7
6.3
7.0
7.7
8.4
9.1
10.0
11.0
12.1
13.6
15.3
17.7
21.0
65.5
150
0.9
1.8
2.6
3.4
4.1
4.8
5.5
6.1
6.7
7.4
8.1
8.8
9.7
10.6
11.8
13.1
14.9
17.2
20.4
71.1
160
0.8
1.6
2.4
3.2
3.9
4.6
5.2
5.8
6.4
7.1
7.8
8.5
9.3
10.3
11.4
12.7
14.4
16.7
19.9
76.7
170
0.7
1.5
2.3
3.0
3.7
4.3
4.9
5.6
6.2
6.8
7.4
8.2
9.0
9.9
11.0
12.3
14.0
16.2
19.3
82.2
180
0.7
1.4
2.1
2.8
3.5
4.1
4.7
5.3
5.9
6.5
7.1
7.8
8.6
9.5
10.5
11.8
13.5
15.7
18.7
87.8
190
0.6
1.3
1.9
2.6
3.2
3.8
4.4
5.0
5.5
6.1
6.8
7.5
8.2
9.1
10.1
11.4
13.0
15.1
18.1
93.3
200
0.5
1.1
1.7
2.4
3.0
3.5
4.1
4.6
5.2
5.8
6.4
7.1
7.8
8.7
9.7
10.9
12.5
14.6
17.5
98.9
210
0.5
1.0
1.6
2.1
2.7
3.2
3.8
4.3
4.9
5.4
6.0
6.7
7.4
8.3
9.2
10.4
12.0
14.0
16.9
104.4
220
0.4
0.9
1.4
1.9
2.4
2.9
3.4
3.9
4.5
5.0
5.6
6.3
7.0
7.8
8.8
9.9
110.0
230
0.3
0.8
1.2
1.6
2.1
2.6
3.1
3.6
5.2
4.7
5.3
6.0
6.7
115.6
240
0.3
0.6
0.9
1.3
1.7
2.1
2.6
3.1
3.5
4.1
4.6
121.1
250
0.2
0.4
0.7
1.0
1.3
1.7
2.1
2.5
2.9
126.7
260
0.2
0.3
0.5
0.7
0.9
1.1
1.4
132.3
270
1.0
0.1
0.2
0.3
0.4
0.4
This table is adopted from Table 3-4 of the Wood Handbook
published by the Forest Products Society.
EXAMPLE
How much tangential shrinkage will occur if Douglas fir lumber is
dried from FSP to 19%MC?
Solution
FSP for Douglas fir: 26% (Table M-1)
Tangential shrinkage from FSP to oven dry: 7.5% (D. fir, interior
west) (Table M-2)
Use linear interpolation to calculate % tangential shrinkage at
19% MC:
The lumber will shrink 2.0% in the tangential
direction from FSP to 19% MC.
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