WOOD - Materials for Civil and Construction Engineers - page 394

10
WOOD
Wood, because of its availability, relatively low cost, ease of use, and dura-
bility, if properly maintained, continues to be an important civil engineering
material. Wood is used extensively for buildings, bridges, utility poles, floors,
roofs, trusses, and piles. (See Figures 10.1 and 10.2.) Civil engineering appli-
cations include both natural wood and engineered wood products, such as
laminates, plywood, and strand board. In order to use wood efficiently, it is
important to understand its basic properties and limitations. In the United
States, the Forest Service of the Department of Agriculture has broad man-
agement responsibility for the harvesting of wood from public lands and for
assisting private sources with the selection of products for harvesting. This
agency has produced an excellent document describing the characteristics
and properties of wood (USDA-FS, 1999).
This chapter covers the properties and characteristics of wood. In the
design of a wood structure, joints and connections often limit the design el-
ements. These are generally covered in a design class for wood construction
and, therefore, are not considered in this text.
Wood is a natural, renewable product from trees. Biologically, a tree is a
woody plant that attains a height of at least 6 m (20 ft), normally has a single
self-supporting trunk with no branches for about 1.5 m (4 ft) above the
ground, and has a definite crown. There are over 600 species of trees in the
United States.
Trees are classified as either endogenous or exogenous, based on the
type of growth. Endogenous trees, such as bamboo, grow with intertwined
fibers. Wood from endogenous trees is not generally used for engineering ap-
plications in the United States. Exogenous trees grow from the center out by
adding concentric layers of wood around the central core. This topic con-
siders only exogenous trees.
Exogenous trees are broadly classified as deciduous and conifers, pro-
ducing hardwoods and softwoods, respectively. The terms hardwood and
softwood are classifications within the tree family, not a description of the
woods' characteristics. In general, softwoods are softer, less dense, and easier