Factors Which Affect the Strength of Lumber
Five basic factors influence the strength of wood:
Wood Specific Gravity or Density
As a general rule the greater the specific gravity or density of a wood, the greater the strength.
Shown here are a number of properties of wood, plotted in relation to
(Adapted from: Panshin, A.J. and P. deZeeuw, 1964. Textbook of Wood Technology - Vol. I. New York: McGraw-Hill Inc., p.187.)
|Note that strength values rise significantly with a doubling of wood specific gravity.||
Slope of Grain
When the grain direction in a wood member is parallel to the two edges of the piece the wood is said to have straight grain. However, if the grain direction in a piece is not precisely parallel to the board edges, strength will be lower than if edges and fiber direction are parallel.
This 2 × 10 split along the grain direction revealing a slope of grain of about 1 in 20.
|An indication of the degree to which slope of grain affects strength is provided by the table to the right. Even a slope of grain of 1 in 20 results in a 7 percent loss of strength. A slope of grain of 1 in 1 (a 45° angle) results in a 91 percent loss of strength as compared to straight grain!||
A slope of grain of almost 1 in 3 contributed to failure of the side
rail of this ladder, and in severe injury to the person who was using it
when it failed.
(Photo: John Haygreen)
|Knots affect strength in two ways: First the slope of grain around a knot causes a marked reduction in strength in the vicinity of the knot.|
|Note that the slope of grain around this knot, and the one pictured above, is severe, and sometimes as great as 1 in 1.|
Second, if a knot is
loose, it contributes nothing
to strength, particularily if the wood is stressed in tension.
For these two reasons, edge knots are assumed to reduce the useful cross section of a beam in which they occur by an amount equal to the knot diameter. In effect, the presence of knots impacts strength as much as do actual holes in the wood.
The effect of knots or holes upon strength is dependent upon location of these defects. A knot located near the neutral axis of a member affects strength only slightly; however, a similar knot located at the tension face of a member is likely to significantly affect strength. The following table shows how strength is affected by various knot sizes:
Effect of Knots or Holes on Strength of Bending Members
(Bending Strength as a Percent of Knot Free Wood)
|Knot or Hole on Tension Edge or Wide Face||Knot or Hole on Neutral Axis|
|Nominal 2 x 10||Nominal 2 x 6||Nominal 2 x 10||Nominal 2 x 6|
|The preceding table shows why holes in structural members, such as floor joists, should be made as small as possible. The hole drilled near the center of this 2 x 10 has reduced its bending strength only about 6 percent...|
|...whereas this hole, that was drilled much larger than need be, has reduced the bending strength of this joist by almost 20%.|