Factors Which Affect the Strength of Lumber

Five basic factors influence the strength of wood:

• wood specific gravity
• slope of grain
• presence of knots or holes
• moisture content
• time

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 specific gravity. (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.	Change in Strength When Specific Gravity is Doubled Property Change Compression to grain × 2 MOR × 2.5 Compression to grain × 4

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!	Slope of Grain % of Retained Strength 0 100% 1 in 20 93% 1 in 10 81% 1 in 5 55%
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 or Holes

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 encased, or 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:

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%.

©1998


All Rights Reserved