WOOD - Materials for Civil and Construction Engineers - page 407

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T ABLE 10.3 Sample of Stress Grading of Softwood for Structural Applications

Design Values, 2 psi

Tension Compression Minimum

Grade Parallel to Parallel to Modulus Modulus of

Bending 3 Designation Grain Grain of Elasticity Elasticity

900f-1.0E 900 350 1,050 1,000,000 510,000

1650f-1.3E 1,650 1,020 1,700 1,300,000 660,000

1950f-1.5E 1,950 1,375 1,800 1,500,000 760,000

2250f-1.7E 2,250 1,750 1,925 1,700,000 860,000

2400f-2.0E 2,400 1,925 1,975 2,000,000 1,020,000

2850f-2.3E 2,850 2,300 2,150 2,300,000 1,170,000

3000f-2.4E 3,000 2,400 2,200 2,400,000 1,220,000

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Courtesy of American Forest & Paper Association, Washington, D.C.
Stresses apply to lumber used at 19 percent maximum moisture content. When lumber is
designed for use where the moisture content will exceed 19 percent for an extended period
of time, the values shown herein shall be multiplied by certain wet service factors.
Bending values are applicable to lumber loaded on edge. When loaded flatwise, these
values may be increased by multiplying by certain flat use factors.
For a complete list of grade designations and more detailed design values see
reference (American Forest & Paper Association, 2005).

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10.6 Defects in Lumber

Lumber may include defects that affect either its appearance, mechanical
properties, or both. These defects can have many causes, such as natural
growth of the wood, wood diseases, animal parasites, too rapid seasoning,
or faulty processing. Common defect types are shown in Figure 10.8.

Knots are branch bases that have become incorporated into the wood of
the tree trunk or another limb. Knots degrade the mechanical prop-
erties of lumber, affecting the tensile and flexural strengths.
However, the presence of sound, tight knots may increase the com-
pressive strength, hardness, and shear characteristics of the wood.
Shakes are lengthwise separations in the wood occurring between an-
nual rings. They develop prior to cutting the lumber and could be
due to heavy winds.
Wane is bark or other soft material left on the edge of the board.
Sap Streak is a heavy accumulation of sap in the fibers of the wood,
which produces a distinctive streak in color.