WOOD - Materials for Civil and Construction Engineers - page 396

to cut than hardwoods. However, exceptions exist such as Balsa, a very soft
and lightweight wood that is technically a hardwood.
Deciduous trees generally shed their leaves at the end of each growing
season. Commercial hardwood production in the U.S. comes from 40 differ-
ent tree species. Hardwoods are generally used for furniture and decorative
veneers, due to their pleasing grain pattern. The cost of hardwoods limits their
construction application.
Conifers, also known as evergreens, have needlelike leaves and nor-
mally do not shed them at the end of the growing season. Conifers grow
continuously through the crown, producing a uniform stem and homoge-
nous characteristics (Panshin & De Zeeuw, 1980). Softwood production in
the U.S. comes from about 20 individual species of conifers. Conifers are
widely used for construction. Conifers grow in large stands, permitting
economical harvesting. They mature rapidly, making them a renewable re-
source. Table 10.1 shows examples of hardwood and softwood species
(USDA-FS, 1999).

10.1 Structure of Wood

Wood has a distinguished structure that affects its use as a construction ma-
terial. Civil and construction engineers need to understand the way the tree
grows and the anisotropic nature of wood in order to properly design and
construct wood structures.

■ Growth Rings

The concentric layers in the stem of exogenous trees are called tree rings or
annual rings , as shown in Figure 10.3a. The wood produced in one grow-
ing season constitutes a single growth ring. Each annual ring is composed of
early wood, produced by rapid growth during the spring, and latewood from
summer growth. Latewood consists of dense, dark, and thick-walled cells
producing a stronger structure than early wood, as shown in Figure 10.3b.
The predominant physical features of the tree stem include the bark, cam-
bium , wood , and pith , as shown in Figure 10.4. The bark is the exterior cov-
ering of the tree and has an outer and an inner layer. The outer layer is dead
and corky and has great variability in thickness, dependent on the species
and age of the tree. The inner bark layer is the growth layer for bark but is
not part of the wood section of the tree. The cambium is a thin layer of cells
situated between the wood and the bark and is the location of all wood
growth.
The wood section of the tree is composed of sapwood and heartwood .
Sapwood functions as a storehouse for starches and as a pipeline to trans-
port sap. Generally, faster growing species have thick sapwood regions. In

10.1.1