Wood Handbook, Wood as an Engineering Material

associated with the knot. The influence of knots depends on

their size, location, shape, and soundness; attendant local

slope of grain; and type of stress to which the wood member

is subjected.

The shape (form) of a knot on a sawn surface depends upon

the direction of the exposing cut. A nearly round knot is pro-

duced when lumber is sawn from a log and a branch is sawn

through at right angles to its length (as in a flatsawn board).

An oval knot is produced if the saw cut is diagonal to the

branch length (as in a bastard-sawn board) and a “spiked”

knot when the cut is lengthwise to the branch (as in a quar-

tersawn board).

Knots are further classified as intergrown or encased

(Fig. 5–3). As long as a limb remains alive, there is continu-

ous growth at the junction of the limb and the bole of the

tree, and the resulting knot is called intergrown. After the

branch has died, additional growth on the trunk encloses

the dead limb, resulting in an encased knot; bole fibers are

not continuous with the fibers of the encased knot. Encased

knots and knotholes tend to be accompanied by less cross-

grain than are intergrown knots and are therefore generally

less problematic with regard to most mechanical properties.

Most mechanical properties are lower in sections containing

knots than in clear straight-grained wood because (a) the

clear wood is displaced by the knot, (b) the fibers around the

knot are distorted, resulting in cross grain, (c) the disconti-

nuity of wood fiber leads to stress concentrations, and (d)

checking often occurs around the knots during drying. Hard-

ness and strength in compression perpendicular to the grain

Table 5–8. Average toughness values for a few hardwood

speciesa

Toughnessb

Species

Moisture

content

Specific

gravityc

Radial

(J (in-lbf))

Tangential

(J (in-lbf))

Birch, yellow 12% 0.65 8,100 (500) 10,100 (620)

Hickory (mocker- Green 0.64 11,400 (700) 11,700 (720)

nut, pignut, sand) 12% 0.71 10,100 (620) 10,700 (660)

Maple, sugar 14% 0.64 6,000 (370) 5,900 (360)

Oak, red

Pin 12% 0.64 7,000 (430) 7,000 (430)

Scarlet 11% 0.66 8,300 (510) 7,200 (440)

Oak, white

Overcup Green 0.56 11,900 (730) 11,100 (680)

13% 0.62 5,500 (340) 5,000 (310)

Sweetgum Green 0.48 5,500 (340) 5,400 (330)

13% 0.51 4,200 (260) 4,200 (260)

Willow, black Green 0.38 5,000 (310) 5,900 (360)

11% 0.4 3,400 (210) 3,700 (230)

Yellow-poplar Green 0.43 5,200 (320) 4,900 (300)

12% 0.45 3,600 (220) 3,400 (210)

aResults of tests on clear, straight-grained specimens.

bProperties based on specimen size of 2 cm square by 28 cm long; radial indicates

load applied to radial face and tangential indicates load applied to tangential face of

specimens. c

Based on ovendry weight and volume at moisture content of test.

Figure 5–3. Types of knots. A, encased knot; B,
intergrown knot.

Chapter 5 Mechanical Properties of Wood