are exceptions, where knots may be objectionable only in
that they cause nonuniform wear or nonuniform stress distri-
butions at contact surfaces.
Knots have a much greater effect on strength in axial tension
than in axial short-column compression, and the effects on
bending are somewhat less than those in axial tension. For
this reason, in a simply supported beam, a knot on the lower
side (subjected to tensile stresses) has a greater effect on the
load the beam will support than does a knot on the upper
side (subjected to compressive stresses).
In long columns, knots are important because they affect
stiffness. In short or intermediate columns, the reduction in
strength caused by knots is approximately proportional to
their size; however, large knots have a somewhat greater
relative effect than do small knots.
Knots in round timbers, such as poles and piles, have less
effect on strength than do knots in sawn timbers. Although
the grain is irregular around knots in both forms of timber,
the angle of the grain to the surface is smaller in naturally
round timber than in sawn timber. Furthermore, in round
timbers there is no discontinuity in wood fibers, which re-
sults from sawing through both local and general slope of
grain.
The effects of knots in structural lumber are discussed in
Chapter 7.Slope of Grain
In some wood product applications, the directions of im-
portant stresses may not coincide with the natural axes of
fiber orientation in the wood. This may occur by choice inTable 5–9. Average toughness values for a few softwood
speciesa
ToughnessbSpecies
Moisture
contentSpecific
gravitycRadial
(J (in-lbf))Tangential
(J (in-lbf))
Cedar
Western red 9% 0.33 1,500 (90) 2,100 (130)
Yellow 10% 0.48 3,400 (210) 3,700 (230)
Douglas-fir
Coast Green 0.44 3,400 (210) 5,900 (360)
12% 0.47 3,300 (200) 5,900 (360)
Interior west Green 0.48 3,300 (200) 4,900 (300)
13% 0.51 3,400 (210) 5,500 (340)
Interior north Green 0.43 2,800 (170) 3,900 (240)
14% 0.46 2,600 (160) 4,100 (250)
Interior south Green 0.38 2,100 (130) 2,900 (180)
14% 0.4 2,000 (120) 2,900 (180)
Fir
California red Green 0.36 2,100 (130) 2,900 (180)
12% 0.39 2,000 (120) 2,800 (170)
Noble Green 0.36 — — 3,900 (240)
12% 0.39 — — 3,600 (220)
Pacific silver Green 0.37 2,400 (150) 3,700 (230)
13% 0.4 2,800 (170) 4,200 (260)
White Green 0.36 2,300 (140) 3,600 (220)
13% 0.38 2,100 (130) 3,300 (200)
Hemlock
Mountain Green 0.41 4,100 (250) 4,600 (280)
14% 0.44 2,300 (140) 2,800 (170)
Western Green 0.38 2,400 (150) 2,800 (170)
12% 0.41 2,300 (140) 3,400 (210)
Larch, western Green 0.51 4,400 (270) 6,500 (400)
12% 0.55 3,400 (210) 5,500 (340)
Pine
Eastern white Green 0.33 2,000 (120) 2,600 (160)
12% 0.34 1,800 (110) 2,000 (120)
Jack Green 0.41 3,300 (200) 6,200 (380)
12% 0.42 2,300 (140) 3,900 (240)
Loblolly Green 0.48 5,000 (310) 6,200 (380)
12% 0.51 2,600 (160) 4,200 (260)
Lodgepole Green 0.38 2,600 (160) 3,400 (210)
Ponderosa Green 0.38 3,100 (190) 4,400 (270)
11% 0.43 2,400 (150) 3,100 (190)
Red Green 0.4 3,400 (210) 5,700 (350)
12% 0.43 2,600 (160) 4,700 (290)
Shortleaf Green 0.47 4,700 (290) 6,500 (400)
13% 0.5 2,400 (150) 3,700 (230)
Slash Green 0.55 5,700 (350) 7,300 (450)
12% 0.59 3,400 (210) 5,200 (320)
Virginia Green 0.45 5,500 (340) 7,600 (470)
12% 0.49 2,800 (170) 4,100 (250)
Redwood
Old-growth Green 0.39 1,800 (110) 3,300 (200)
11% 0.39 1,500 (90) 2,300 (140)
Young-growth Green 0.33 1,800 (110) 2,300 (140)
12% 0.34 1,500 (90) 1,800 (110)
Spruce, Green 0.34 2,400 (150) 3,100 (190)
Engelmann 12% 0.35 1,800 (110) 2,900 (180)
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.
cBased on ovendry weight and volume at moisture content of test.
Table 5–10. Summary of selected fracture
toughness results
Fracture toughness
(kPa m1/2 (lbf in–2 in1/2))
Mode I Mode II
Species TL RL TL RL
Douglas-fir 320 (290) 360
(330)2,230
(2,030)
Western hemlock 375 (340) 2,240
(2,040)
Pine
Western white 250 (225) 260
(240)
Scots 440 (400) 500
(455)2,050
(1,860)
Southern 375 (340) 2,070
(1,880)
Ponderosa 290 (265)
Red spruce 420 (380) 2,190
(1,990)1,665
(1,510)
Northern red oak 410 (370)
Sugar maple 480 (430)
Yellow-poplar 517 (470)General Technical Report FPL–GTR– 190