Custom Search
|
|
|
||
 CHAPTER 2. WOOD AS A CONSTRUCTION MATERIAL
Lumber, heavy timbers, poles, piles and many other wood products are derived from trees. Trees
are biological organisms which we see and enjoy every day, but we seldom consider the complexity
of the wood produced by them. An understanding of some of the basic characteristics and proper-
ties of wood can help in making sound decisions in selecting, specifying and, finally, using the
many different wood products. In addition, a better understanding can also help solve problems
when they arise in current applications.
This chapter will present an overview of the following particulars of wood:
Visible Characteristics
(1)
Microscopic Structure
(2)
(3) Chemical Composition
Physical Properties
(4)
Mechanical Properties
(5)
Strength Affecting Characteristics
(6)
Also, these sections will emphasize design criteria and preservative treatment of wood products
when applicable.
2.1 VISIBLE CHARACTERISTICS. Many characteristics can be readily observed with the naked
eye or with the use of a 10x hand lens (Figure 2-1). These gross features are often good indicators of
a species structural properties, and are helpful in identifying some of the more common species or
species groups.
2.1.1 Softwoods And Hardwoods. The terms "hardwood" and "softwood" do not directly
describe the hardness or softness of wood. Instead, they refer to the leaf form or mode of seed
production on trees from which the wood is cut. The terms are especially confusing because some
true hardwoods have softer or lighter wood than common softwoods. Softwoods also are very
strong for their weight in comparison to the hardwoods. Softwoods are trees such as the pines,
spruces, larch, true firs, hemlock, redwood, yew, cypress, Douglas-fir and cedars which have needle
or scale-like leaves and, except for cypress and larch, maintain these leaves throughout the year.
They are also called evergreens. Because they bear scaly cones to produce seeds, they are also
called conifers. Softwoods are most commonly used for construction lumber, heavy timbers, poles
and piles where strength, not appearance, is important.
Hardwoods are trees such as the oaks, gums and many others which have broad deciduous leaves.
That is, their leaves change color in the fall and drop to the ground--at least in temperate climates.
Hardwoods often are used for railroad ties, heavy timbers and pallets.
2.1.1.1 Commercial Species Groups. Softwoods are specified by "species
groups". These species groups are based on strength characteristics of the lumber. For example, if
southern pine is specified, the shipment may contain any of the major species, namely loblolly,
2-1
|
 
|
|
|
||
