FORESTS AND MEN-part 3

Wood In Use

Above: Building a prefabricated house "The prospective buyer can learn a great deal about quality if he watches the workmen assemble a house."

THE WOOD FOR THE JOB

R. P. A. JOHNSON, CHARLES E. VAN HAGAN.

BECAUSE WOOD is a part of every home and because anybody who can pull a saw or lift a hammer can work with wood, a general knowledge of its properties and uses will come in handy to every person who lives in a home.

The first point in selection of material for satisfactory performance depends on the use of the right wood for the right purpose. The man or woman who intends to work with wood has to determine what kind of service he expects to get from the wood. Will it be strength or hardness, or stiffness or resistance to decay, or beauty or some other property?

The use requirements usually do involve a combination of two or more, and selection involves finding the wood that has the best combination of the desired properties.

The species that have certain special properties that cost more (such as resistance to decay) should not be used unless those properties are definitely needed. For example: People often go to considerable expense to get highly decay-resistant wood for diving boards; they reason that the constant wetting and drying of the board demands it. Actually, however, the most important item in a diving board is strength, for most boards fail mechanically in a year or two if they are in continuous use, as at public beaches. The proper wood for this use, then, would be comparatively inexpensive and strong, selected with little regard to decay resistance.

Similarly, it will be wasteful to pay a premium price for wood with a beautiful grain pattern, like walnut or mahogany, for use in furniture that is to be painted. An inexpensive wood with equal or better painting characteristics but with little figure (yellow-poplar, for instance) would be a more logical choice. There is no economy in paying a high price for wood with a property that is not used.

Thus, wise use of wood in the home requires consideration of the properties needed and a basic knowledge of the main properties of the commercial woods. The final choice of the wood may also be affected by the skill of the worker and the availability of the desired species. The farmer, for example, who wants to use wood growing on his own land has a limited selection and may not be able to choose the ideal wood for a given purpose, but with the actual demands clearly determined, he can make the most satisfactory and economical selection from the wood that he has.

The characteristics vary among species, among the individual trees of the same species, and even among pieces of wood taken from different heights of the same tree. Thus the published values for the different properties are averages and do not hold for every individual piece of wood.

One should also understand that wood does not have the same strength properties in all directions. Strength depends on the direction of the grain. When tension pull is applied parallel to or along the grain, for example, wood may be 300 times as strong as when the tension is applied at right angles to the grain.

The terms "hardwood" and "softwood" are used to distinguish between two general classes of wood and not to indicate the properties of the included species. Hardwood is the name given to the group of trees that are broad-leaved. Softwood is the name given to trees that have needlelike or scalelike leaves and are mostly evergreen (cypress, larch, and tamarack being exceptions).

The hardwoods are not necessarily high in relative hardness; some woods classed as softwoods are actually harder than some classed as hardwoods. The softwoods are used principally in construction; the hardwoods furnish most of the wood for implements, furniture, and other industrial uses.

The weight of wood in itself has an important bearing upon the selection of a species for many uses. Weight also serves as a reliable index of the strength properties of dry wood and affords an accurate comparison between the strength properties of possible species when the degree of dryness and the actual sizes are the same. Generally speaking, the heavier a piece of dry wood, the stronger it is, regardless of the species.

Changes in temperature have little effect upon wood; they cause such small variations in size that for ordinary farm and home uses the effect of temperature can be overlooked.

Changes in moisture content, on the other hand, have a considerable effect on wood, which swells as it takes up moisture and shrinks as it dries. Difficulties may be encountered if this property is disregarded. When proper precautions are taken, however, most of the trouble due to swelling and shrinking can be avoided. The shrinking or swelling in the width of a flat-grained board is nearly twice that of a quarter-sawn, or edge-grained, board of the same width; the shrinkage or swelling lengthwise of the grain in both is negligible.

One can compensate for high shrinkage, if only that kind of wood is available, by using edge-grained pieces, which will prove as satisfactory as flat-grained stock of species that have lower shrinkage values. Much trouble can also be avoided by using only wood that has been dried to approximately the moisture content that the finished piece will have in service. Thorough air drying will take out about half and thorough kiln drying about two-thirds of the shrinkage of wood. That is enough for the ordinary uses.

Warping, which is the result of uneven shrinking or swelling, may occur in wood that is plain-sawed, or cross-. grained, or improperly dried. It can be reduced to a minimum by the use of edge-grained, properly dried material.

Woods that are comparatively free from warping include: Northern and Atlantic white-cedar, eastern and western redcedar, cherry, chestnut, northern white pine, ponderosa pine, sugar pine, western white pine, yellow-poplar, redwood, walnut, and the eastern, Engelmann, and Sitka spruce.

THE STRENGTH PROPERTIES of wood that most concern the woodworker include bending strength, compression strength, stiffness, and toughness.

Bending strength is a measure of the load-carrying capacity of the members that are ordinarily used in a horizontal position and rest on supports.

High bending strength is required in barn rafters, girders, stringers, wagon tongues, and scaffold platforms. If the only available wood is low in bending strength compared with better-suited species, the deficiency can be overcome by increasing the size of the member used. An increase of 10 percent in the height of a beam increases its bending strength by 21 percent. Both the volume and bending strength of a beam, however, increase in direct proportion as the width is increased. Woods high in bending strength for farm and home building include ash, beech, yellow birch, cherry, Douglas-fir, rock elm, hickory, the western larch, locust, hard maple, oak, southern yellow pine, and walnut.

Compression strength of wood is the measure of its ability to resist a load applied in such a direction that it tends to crush the member, as in a post or column. Good compression strength is essential for members used to support houses, garages, barns, storage bins, and the like, because they hold up a load. It is not important in such items as fence posts.

Low compression strength can be compensated for in some instances by the use of proportionately larger members. In the construction of small buildings, then, the size requirements of posts where the length is less than 11 times the smallest dimension are determined by bearing area, stiffness, and stability rather than by actual compression strength. Because these requirements necessitate the use of posts large enough to carry greater actual compressive loads than are ever placed upon them, no particular consideration need be given to the compression strength endwise in selecting a wood for small houses. Where exceptionally heavy loads are involved, as in supports for bins or root cellars, the compression strength of the members should be considered. If the length is greater than 11 times the smallest dimension, the stiffness of the member becomes the controlling factor, and the compression strength can be disregarded. Of the woods used in farm and home building, those high in compression strength include white ash, eastern redcedar, cherry, Douglas-fir, hickory, western larch, locust, hard maple, southern yellow pine, redwood, and walnut.