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 have a silvery appearance and at other times cannot be visually detected. When machined, a fuzzy
or woolly surface may result, particularly in green wood. Stain is sometimes absorbed irregularly
by tension wood during the finishing process, leaving a blotchy appearance. Warping is also a prob-
lem with tension wood due to abnormal longitudinal shrinkage and during drying collapse often
results. The strength properties of tension wood are generally less than for normal wood.
2.6.1.3 Spiral Grain. In some cases, the cells may be arranged in a noticeable
spiral about the stem. In normal wood, the longitudinal cells are almost parallel to the main axis of
the tree stem; with spiral grain, the cells are at a much flatter angle. Poles and piles produced from
these trees, or lumber sawn from these logs, will have a cross or diagonal grain pattern. These
wood products are typically low in strength and stiffness, and tend to twist upon drying. Standards
for poles, limit the amount of spiral grain that can be accepted.
2.6.1.4 Knots. A knot is that portion of a branch or limb which has been sur-
rounded by subsequent growth of the tree. As a knot appears on the cut surface it is merely a section
of the entire knot, its shape depending upon the direction of the cut. The number, shape, size and
location of knots in respect to the geometry of the wood member can affect the strength, and, thus,
the grade of structural products. The method in which the knot size is determined is precisely
defined for structural products.
2.6.2 Anatomical Disruptions. Wood is produced in the tree as an internally contiguous
tissue, but forces acting on trees or products frequently cause disruptions in this tissue. These inter-
nal disruptions can reduce wood strength.
2.6.2.1 Shake. Shake is the longitudinal separation of the wood. Two forms of
shake are recognized. Heart shake is that which starts out at or near the pith and extends radially. It
is also called heart crack or rift crack. A heart shake in which several radial cracks are present is
termed a star shake. Ring shake is concentric with the growth rings. It may partially or completely
encircle the pith, occasionally moving radially to an adjacent growth increment. It is also called cup
shake.
2.6.2.2 Split. Split is the separation of the wood parallel to the fiber direction, due
to the tearing apart of the wood cells. It most often occurs on the ends of a wood member as it dries.
2.6.2.3 Compression Failure. Compression failure is the deformation of the
wood fibers resulting from excessive compression along the grain either in direct end compression
or in bending. It may develop in standing trees due to bending by wind or snow or to internal lon-
gitudinal stresses developed during growth, or it may result from stresses imposed after the tree is
cut. In surfaced lumber, compression failures appear as fine wrinkles across the face of the piece.
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