WOOD - Materials for Civil and Construction Engineers - page 401

10

8

Tangential

6

4

Radial

Fiber
saturation
point

2

Longitudinal

0

0 10 20 30

FIGURE 10.5 Relation between
shrinkage and moisture content.

Moisture content, %

When the moisture content of wood is above the fiber saturation point,
the wood is dimensionally stable. However, moisture fluctuations below the
FSP always result in dimensional changes. Shrinkage is caused by loss of
moisture from the cell walls, and conversely, swelling is caused by the gain
of moisture in the cell walls. Figure 10.5 shows that the changes in wood
dimensions vary from one direction to another. Dimensional changes in
the radial direction are generally one-half the change in the tangential di-
rection. Swelling and shrinkage in the longitudinal direction is minimal,
typically 0.1% to 0.2% for a change in the moisture content from the FSP
to oven dry. This anisotropy of dimensional changes of wood causes warp-
ing, checking, splitting, and structural performance problems, as discussed
in more detail later in the chapter. It is also the reason that the sawing pat-
tern of boards affects the amount of distortion when subjected to changes
in moisture.
The moisture content in wood varies depending on air temperature and
humidity. However, the natural change of moisture content is a slow process,
so as atmospheric conditions change, the moisture content in wood tends to
adjust to conditions near the average. The moisture content for the average
atmospheric conditions is the equilibrium moisture content (EMC) . The For-
est Service has developed values of the EMC as a function of temperature
and humidity. The EMC ranges from less than 1%, at temperatures greater
than 55°C (130°F) and 5% humidity, to over 20% at temperatures less than
27°C (80°F) and 90% humidity.

Sample Problem 10.1

A 250-mm wide Red Spruce plank is cut in such a way that the width is in the tangen-
tial direction of annual rings. Compute the change in width as the moisture content
changes from 15% to 32%. (The fiber saturation point of Red Spruce is 27% and it
shrinks 7.8% in drying from FSP to oven dry in the tangential direction.)