Quantcast Strength Affecting Characteristics

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2.6 STRENGTH AFFECTING CHARACTERISTICS. Grading rules and materials standards for
wood products to be treated exclude or set maximum allowable limits for certain anatomical charac-
teristics and defects that reduce wood strength. Thus, it is important to be cognizant of those fea-
tures which could detract from the total performance of the treated wood product if they were not
detected during the inspection process. A few of the more important terms which are not discussed
elsewhere are given below. Additional definitions are provided in most standards and by the
American Society For Testing and Materials.
2.6.1 Abnormal Wood.  In both hardwoods and softwoods, depending on age and loca-
tion in the tree, some wood is found which is not representative for that species. This wood is called
abnormal wood. Its strength properties are significantly lower than that of normal wood, therefore,
the amount of abnormal wood in a structural product is usually limited.
2.6.1.1 Juvenile Wood.  Juvenile wood is that material formed near the center or
pith of the tree, and is prevalent in the softwoods or conifers and it also occurs in hardwoods.
Juvenile wood is characterized by wide growth rings with shorter, thin-walled cells, and fewer
latewood cells, thus resulting in a lower density and reduced strength values. There is also a tenden-
cy towards greater spiral grain. The shrinkage characteristics of juvenile wood are also different
from those of normal wood, thus increasing warp problems. When drying, the wood often checks
across the grain (Figure 2-14). The change from juvenile wood to normal wood is gradual, thus
making identification of juvenile wood difficult.
2.6.1.2 Reaction Wood.  Reaction wood forms in leaning or bent trees and is an
attempt by the tree to straighten itself out.
Compression Wood. In softwoods, reaction wood is called
compression wood. It is formed on the lower side of leaning trees. The part of the growth ring with
reaction wood is usually wider than the rest of the ring, has a high proportion of latewood, and as a
result the tree develops an eccentrically shaped stem and the pith is not centered. Compression
wood, especially the latewood, is usually duller and more lifeless in appearance (Figure 2-14). It
presents serious problems in wood manufacturing since it is much lower in strength than normal
wood of the same density. Also, it tends to shrink excessively in the longitudinal direction causing
cross grain checking. Compression wood is the cause of shelling in poles and piles (Figure 2-15). It
is sometimes the cause of structural failures in critical applications, such as ladders. The softwood
lumber grading rules restrict the extent of compression wood in lumber. Specifications and stand-
ards for wood poles usually require that the outer one inch of all poles be free from compression
wood visible on either end.
Tension Wood. In hardwood trees, reaction wood is called tension
wood and forms predominately toward the upper side of the leaning tree. Tension wood is usually
not as evident as compression wood. It may form irregularly around the entire stem and, as a result,
there is less tendency for the pith to be off center. Tension wood is often difficult to detect. It may
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