Forest Service Rules of Timber

FOREST SERVICE RULES OF TIMBER As the result of tests upon structural timbers, the Forest Service proposes the following grades: Grade having a modulus of rupture over 4,000 pounds per square inch.

Grade

timbers having a modulus of rupture under 4,000 pounds per square inch.

having visible defects which render them unfit for structural purposes.

The practical application of these grades is illustrated by the following definitions of terms and tentative rules for timbers, based upon a long series of tests: Shakes A shake is a separation of one annual ring from another, in some cases only a few degrees in length, in others entirely separating two rings. It is thought that shakes are produced in the living tree by stresses caused, by winds and changes of temperature. They are most common in woods that split easily. Shakes are difficult to detect in green timber, and usually do not become visible until the timber is at least partly seasoned. A shake decreases the strength of timber in proportion as a plane tangent to it approaches parallelism with the neutral plane in the beam, since the more nearly parallel the two planes the smaller is the area resisting horizontal shear.

Checks Checks are radial cracks or splits produced, almost without exception, by uneven shrinkage during seasoning. Occasionally, however, they are present in green timber.

Cross-Grains Cross-grain may be divided into three general classes: Diagonal sawing lumber, if the plane of the saw is not approximately parallel to the axis of the log, the grain of the lumber cut is not parallel to the edges, and is termed diagonal.

Spiral

many trees the fibers composing each year's growth are ranged spirally instead of vertically. The greater the pitch of the spiral, the greater is the defect. Spiral grain usually cannot be detected from a casual inspection of the piece, since it does not show in the so-called visible grain of the wood, which in softwood lumber is nothing but a sectional view of the annual rings cut longitudinally. A careful inspection, however, of the medullary rays on the tangential or bastard section, will invariably reveal spiral grain, since the rays necessarily incline with the spiral direction of the fibers around the trunk, and therefore, in section, appear obliquely on the face of the timber. Spiral grain may readily be detected also by splitting a small piece radially.

Burls—Burls are local disturbances in the grain of timber, usually associated with knots or produced by the healing of wounds during the life of the tree.

Pitch Pockets

Pitch pockets are cavities between annual rings, usually filled with resin. They are rarely large enough to affect seriously the strength of structural timbers.

Knots Knots are portions of branches which have been encased in the growing trunk of the tree. In judging their effect upon the strength of timber, it should be borne in mind that the axis of a knot always extends to the center or pith of the tree, and that the visible part of the knot is a section of a somewhat conical mass of wood, the apex of the cone being at the pith of the tree, and the knot, as a whole, more or less intertwined with the wood surrounding it. A spike knot is a longitudinal section of a

whole knot; and a round or elliptical knot is a section, respectively at right angles or at some oblique angle, to the axis of the knot. Sound knots, as a rule, are stronger and harder than the wood fiber surrounding them. Their effect, therefore, upon the strength of the timber depends to a large extent upon the manner in which they are connected to the surrounding wood and upon the degree of stress to which the connecting fibers are subjected. If the knots disturb the grain so that it is decidedly oblique to the edges of the timber, the wood will be subjected to stresses in tension at right angles to the grain, the kind to which it offers the least resistance. In such cases early failure in cross-grain tension almost invariably results.

Class 1

Knots—Class 1 knots must be solid, firmly attached to the surrounding wood, and must cause no marked irregularity in the grain of the timber. Small spike knots will be included in this class.

Class 2

Knots--Class 2 knots must be solid, but are insecurely attached to the surrounding wood, or associated with burl or other irregularity in the grain.

Class 3 Knot.

Class 3 knots are unsound knots; that is, they are softer than the surrounding wood.

Dimensions of Knots—The dimension of a knot on the narrow face of a timber will be the projection of the knot on a line perpendicular to an edge of the timber. On the wide, or vertical, faces the smallest diameter of a knot is to be taken as its dimension.

Small

less than inches in diameter. Large Knots—Knots VA inches or more in diameter.

Position of Defects—The position of defects is designated by means of the three volumes indicated in the diagram (Fig. 7).

Dense Wood

The term "dense wood" is used to define the quality of wood which is desirable in timbers subjected to stresses such as occur in frame structures. The term applies to the wood itself, irrespective of defects. Since dry weight, which is the most accurate index to the mechanical properties of wood, cannot be determined from a casual inspection of the timber, dense—or, in other words, comparatively heavy—wood will be defined as: (1) Wood that shows more than eight rings per inch, or the rings of which contain more than 30 per cent summerwood.

(2) Wood which is resilient—that is, which, when struck with a hammer or similar blunt instrument, gives a sharp, clear sound, while the hammer shows a marked tendency to rebound and the wood to recover from the effects of the blow.

These properties are to be judged from an inspection of the cross-section of the timber.