to 0.9, 0.5 to 0.95, and 0.45 to 0.75, respectively. Changes
in shear strength resulting from increases in juvenile wood
content can be adequately predicted by monitoring changes
in density alone for all annual ring orientations. The same is
true for perpendicular-to-grain compressive strength when
the load is applied in the tangential direction. Compressive
strength perpendicular-to-grain for loads applied in the ra-
dial direction, however, is more sensitive to changes in ju-
venile wood content and may be up to eight times less than
that suggested by changes in density alone (Kretschmann
2008). The juvenile wood to mature wood ratio is lower for
higher grades of lumber than for lower grades, which indi-
cates that juvenile wood has greater influence in reducing
the mechanical properties of high-grade structural lumber.
Only a limited amount of research has been done on juve-
nile wood in hardwood species.Compression Failures
Excessive compressive stresses along the grain that produce
minute compression failures can be caused by excessive
bending of standing trees from wind or snow; felling of
trees across boulders, logs, or irregularities in the ground;
or rough handling of logs or lumber. Compression failures
should not be confused with compression wood. In some
instances, compression failures are visible on the surface
of a board as minute lines or zones formed by crumpling or
buckling of cells (Fig. 5–10A), although the failures usu-
ally appear as white lines or may even be invisible to the
unaided eye. The presence of compression failures may be
indicated by fiber breakage on end grain (Fig. 5–10B). Be-
cause compression failures are often difficult to detect with
the unaided eye, special efforts, including optimum lighting,
may be required for detection. The most difficult cases are
detected only by microscopic examination.
Products containing visible compression failures have low
strength properties, especially in tensile strength and shock
resistance. The tensile strength of wood containing com-
pression failures may be as low as one-third the strength of
matched clear wood. Even slight compression failures, vis-
ible only under a microscope, may seriously reduce strength
and cause brittle fracture. Because of the low strengthassociated with compression failures, many safety codes
require certain structural members, such as ladder rails and
scaffold planks, to be entirely free of such failures.Pitch Pockets
A pitch pocket is a well-defined opening that contains free
resin. The pocket extends parallel to the annual rings; it
is almost flat on the pith side and curved on the bark side.
Pitch pockets are confined to such species as the pines,
spruces, Douglas-fir, tamarack, and western larch.
The effect of pitch pockets on strength depends upon their
number, size, and location in the piece. A large number
of pitch pockets indicates a lack of bond between annual
growth layers, and a piece with pitch pockets should be in-
spected for shake or separation along the grain.Bird Peck
Maple, hickory, white ash, and a number of other species are
often damaged by small holes made by woodpeckers. These
bird pecks often occur in horizontal rows, sometimes encir-
cling the tree, and a brown or black discoloration known as
a mineral streak originates from each hole. Holes for tappingFigure 5–9. Properties of juvenile wood.
Figure 5–10. Compression failures. A, compres-
sion failure shown by irregular lines across grain;
B, fiber breakage in end-grain surfaces of spruce
lumber caused by compression failures below dark
line.Chapter 5 Mechanical Properties of Wood