In inspection, the elements to examine are the rays. In the case of oak and certain other hardwoods these rays are so large that they are readily seen not only on a radial surface, but on the tangential as well. On the former they appear as flakes, on the latter as short lines. Since these rays are between the fibres it naturally follows that they will be vertical or inclined according as the tree is straight-grained or spiral-grained. While they are not conspicuous in the softwoods, they can be seen upon close scrutiny, and particularly so if a small hand magnifier is used.
When wood has begun to dry and check it is very easy to see whether or not it is straight- or spiral-grained, since the checks will for the most part follow along the rays. If one examines a row of telephone poles, for example, he will probably find that most of them have checks running spirally around them. If boards were sawed from such a pole after it was badly checked they would fall to pieces of their own weight. The only way to get straight material would be to split it out.
It is for this reason that split billets and squares are stronger than most sawed material. The presence of the spiral grain has little, if any, effect on the timber when it is used in the round, but in sawed material the greater the pitch of the spiral the greater is the defect.
KNOTS
Knots are portions of branches included in the wood of the stem or larger branch. Branches originate as a rule from the central axis of a stem, and while living increase in size by the addition of annual woody layers which are a continuation of those of the stem. The included portion is irregularly conical in shape with the tip at the pith. The direction of the fibre is at right angles or oblique to the grain of the stem, thus producing local cross grain.
During the development of a tree most of the limbs, especially the lower ones, die, but persist for a time—often for years. Subsequent layers of growth of the stem are no longer intimately joined with the dead limb, but are laid around it. Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber. In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place.[32]
Knots materially affect checking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration. The weakening effect is much more serious where timber is subjected to bending and tension than where under compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress. Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects.
Extensive experiments by the U.S. Forest Service[33] indicate the following effects of knots on structural timbers:
(1) Knots do not materially influence the stiffness of structural timber.
(2) Only defects of the most serious character affect the elastic limit of beams. Stiffness and elastic strength are more dependent upon the quality of the wood fibre than upon defects in the beam.