Wood Handbook, Wood as an Engineering Material

that the spiral grain slope on the flat-grained surface of
Figure 5–5D is 1 in 12 and the diagonal-grain slope is 1 in

18. The combined slope is

or a slope of 1 in 10.

A regular reversal of right and left spiraling of grain in a tree
stem produces the condition known as interlocked grain.
Interlocked grain occurs in some hardwood species and
markedly increases resistance to splitting in the radial plane.
Interlocked grain decreases both the static bending strength
and stiffness of clear wood specimens. The data from tests
of domestic hardwoods shown in Table 5–3 do not include
pieces that exhibited interlocked grain. Some mechanical
property values in Table 5–5 are based on specimens with
interlocked grain because that is a characteristic of some
species. The presence of interlocked grain alters the rela-
tionship between bending strength and compressive strength
of lumber cut from tropical hardwoods.

Annual Ring Orientation

Stresses perpendicular to the fiber (grain) direction may be

at any angle from 0° (T direction) to 90° (R direction) to

the growth rings (Fig. 5–6). Perpendicular-to-grain proper-

ties depend somewhat upon orientation of annual rings with

respect to the direction of stress. The compression perpen-

dicular-to-grain values in Table 5–3 were derived from tests

in which the load was applied parallel to the growth rings (T

direction); shear parallel-to-grain and tension perpendicular-

to-grain values are averages of equal numbers of specimens

with 0° and 90° growth ring orientations. In some species,

there is no difference in 0° and 90° orientation properties.

Other species exhibit slightly higher shear parallel or ten-

sion perpendicular-to-grain properties for the 0° orientation

than for the 90° orientation; the converse is true for about an

equal number of species.

The effects of intermediate annual ring orientations have

been studied in a limited way. Modulus of elasticity, com-

pressive perpendicular-to-grain stress at the proportional

limit, and tensile strength perpendicular to the grain tend to

be about the same at 45° and 0°, but for some species these

values are 40% to 60% lower at the 45° orientation. For

those species with lower properties at 45° ring orientation,

properties tend to be about equal at 0° and 90° orientations.

For species with about equal properties at 0° and 45° orien-

tations, properties tend to be higher at the 90° orientation.

Reaction Wood

Abnormal woody tissue is frequently associated with lean-

ing boles and crooked limbs of both conifers and hard-

woods. Such wood is generally believed to be formed as

a natural response of the tree to return its limbs or bole to

a more normal position, hence the term reaction wood. In

softwoods, the abnormal tissue is called compression wood;

it is common to all softwood species and is found on the

lower side of the limb or inclined bole. In hardwoods, the

abnormal tissue is known as tension wood; it is located on

the upper side of the inclined member, although in some

instances it is distributed irregularly around the cross sec-

tion. Reaction wood is more prevalent in some species than

in others.

Many of the anatomical, chemical, physical, and mechanical

properties of reaction wood differ distinctly from those of

normal wood. Perhaps most evident is the increase in densi-

ty compared with that of normal wood. The specific gravity

of compression wood is commonly 30% to 40% greater than

that of normal wood; the specific gravity of tension wood

commonly ranges between 5% and 10% greater than that of

normal wood, but it may be as much as 30% greater.

Compression wood is usually somewhat darker than normal

wood because of the greater proportion of latewood, and it

frequently has a relatively lifeless appearance, especially in

woods in which the transition from earlywood to latewood

is abrupt. Because compression wood is more opaque than

Table 5–12. Strength of wood members with

various grain slopes compared with strength

of a straight-grained membera

Maximum slope

of grain in

member

Modulus

of

rupture

(%)

Impact

bending

(%)

Compression

parallel to

grain

(%)

Straight-grained 100 100 100

1 in 25 96 95 100

1 in 20 93 90 100

1 in 15 89 81 100

1 in 10 81 62 99

1 in 5 55 36 93

aImpact bending is height of drop causing complete failure

(22.7-kg (50-lb) hammer); compression parallel to grain is

maximum crushing strength.

Figure 5–6. Direction of load in relation to direction of

annual growth rings: 90° or perpendicular (R), 45°, 0° or

parallel (T).

Chapter 5 Mechanical Properties of Wood