WOOD - Materials for Civil and Construction Engineers - page 398

Latewood Latewood
Earlywood Earlywood

Growth direction

(a) (b)

FIGURE 10.3 Cross section of a typical tree stem: (a) annual rings (photo cour-
tesy of American Forest & Paper Association, Washington, D.C.) and (b) earlywood
and latewood.

its natural state, sapwood is not durable when exposed to conditions that
promote decay. Heartwood is not a living part of the tree. It is composed of
cells that have been physically and chemically altered by mineral deposits.
The heartwood provides structural strength for the tree. Since the heartwood
does not contain sap, it is naturally resistant to decay.
The pith is the central core of the tree. Its size varies with the tree species,
ranging from barely distinguishable to large and conspicuous. The color ranges
from blacks to whitish, depending on the tree species and locality. The pith
structure can be solid, porous, chambered, or hollow.

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10.1.2 Anisotropic Nature of Wood

Wood is an anisotropic material in that it has different and unique proper-
ties in each direction. The three axis orientations in wood are longitudinal
or parallel to the grain, radial or cross the growth rings, and tangential or
tangent to the growth rings, as illustrated in Figure 10.4. The anisotropic na-
ture of wood affects physical and mechanical properties such as shrinkage,
stiffness, and strength.
The anisotropic behavior of wood is the result of the tubular geometry
of the wood cells. The wood cells have a rectangular cross section. The cen-
ters of the tubes are hollow, whereas the ends of the tubes are tapered. The
length-to-width ratio can be as large as 100. The long dimension of the ma-
jority of cells is parallel to the tree's trunk. However, a few cells, in localized
bundles, grow radially, from the center to the outside of the trunk. The pre-
ponderance of cell orientation in one direction gives wood its anisotropic
characteristics. The hollow tube structure is very efficient in resisting com-
pressive stresses parallel to its length, but readily deforms when loaded on
its side. Also, fluctuations in moisture contents flex the tube walls but have
little effect on the length of the tube.