PHYSICAL PROPERTIES OF WOOD The physical properties of wood which determine its usefulness, vary with the species, the rate and place of growth, the seasoning condition, and even with individual trees. Two trees are no more exactly alike in either botanical or physical characteristics than are two human beings; hence tabulations purporting to compare the weight, strength, stiffness, or other properties of various woods can be accepted as true only within rather wide limits, and this caution especially applies to the tables in this chapter.
Similar variability, however, is found in other construction materials; and the factors of safety allowed for their use are as great as, or greater than, those for wood.
The commercial terms, "hardwood" and do not correspond to the physical characteristics of hardness or softness, and are of little real value in this respect. As ordinarily used, the term "softwood" is given to all trees of the family that the botanists call "coniferous" or "needle-leaved." These are the pines, firs, spruces, hemlocks, cypress, larch, redwood, tamarack, cedars, etc. The term "hardwood" is commonly applied to the species which botanists call "broad-leaved," represented by the oaks, maples, hickories, elms, 11 ashes, basswood, beech, birches, walnut, etc. The slightest experience with wood shows that these terms give little indication of the physical properties of the species to which they refer. There are hardwoods softer than the so-called softwoods, and softwoods harder than the socalled hardwoods, although as a group the softwoods average much. softer than the hardwoods. Comparisons of this sort may be readily made from the tables given in this chapter.
Useful Properties of Wood The properties of wood most important from the standpoint of the ordinary user are: Weight, strength, stiffness, toughness, hardness, and shrinkage. For some purposes, light weight and stiffness are essential where neither great strength nor toughness is required. For other purposes, strength is by far the most important consideration; and for still other uses, hardness is the determining quality. In some places, it makes little difference how much a piece of wood shrinks; in other places, even a little shrinkage will impair the usefulness of the article. Toughness is essential for many purposes, but not at all necessary for other uses. There is, thus, a very wide range in the requirements of wood users, which is met by a great diversity of species and physical properties.
The statements in this chapter regarding the physical properties of wood are based upon a series of tests by the United States Forest Service to obtain data for the comparison of the more important species. All the figures are
derived from tests of small, clear pieces of wood in green condition. Tests of this character afford the best basis for the comparison of various woods; but the figures obtained in this manner do not correspond with the results of tests upon larger-sized material or upon material in the various stages of seasoning ranging from air-dried to kiln-dried. Neither is it safe to assume that the rank of the several species as to weight, strength, stiffness, toughness, and hardness is exactly as indicated by the tables, since many factors such as growth, situation, length of fiber, etc., influence the properties of a given piece of wood. In a broad sense, however, the figures do have a real comparative value, and they are of especial interest since it is the first time that they have been presented in this fashion.
Weight The weight of wood is usually expressed by a comparison of the weight of a given volume of wood with that of an equal volume of water, or by what is known as "specific gravity." If the specific gravity of a certain kind of wood is stated as .30, it means that a given volume of this wood weighs .30 times as much as an equal volume of water. Since a cubic foot of water weighs 62.5 pounds, a cubic foot of wood of specific gravity .30 weighs .30X62.5=18.75 pounds. A piece of wood whose specific gravity is .50 weighs .50X62.5.31.25 pounds per cubic foot. Similarly, the weight per cubic foot of any kind of wood may be quickly ascertained when the specific gravity is known.
Table 1 gives the specific gravity of a number of hardwoods and softwoods when "oven-dry," arranged in order from the lightest to the heaviest in each class. By "oven-dry" is meant the condition produced by drying wood at a temperature of 212° F. (the boiling point of water) until it ceases to lose moisture.
The average specific gravity of the softwoods is .39; and that of the hardwoods, .53; hence these hardwoods average 36 per cent heavier than the softwoods. Several of the softwoods are lighter than any of the hardwoods; but the heaviest of the softwoods, as larch, shortleaf pine, tamarack, and longleaf pine, are heavier than many hardwoods. On the other hand, Table 1 contains 17 hardwoods which are at least twice, or more than twice, as heavy as the lightest of the softwoods. Any of these woods, of course, is much heavier when green. For example, the weight of thoroughly dried northern white cedar is 18 lbs. per cubic foot, compared with 28 lbs. when green; and that of osage orange, 48 lbs. per cubic foot, compared with 62 lbs. when green.