What the wood does: stiffness, knots, grain and moisture

Clear wood versus structural timber

Tabulated strength values for small clear specimens are averages of defect-free pieces. Structural timber differs: defects (knots, slope of grain, pith, compression wood, wane) govern failure, design uses low percentiles (exceeded by about 95 of 100 pieces) instead of means, and size matters. A clear-wood value of about 105 MPa for white oak does not mean a structural beam is rated at 105 MPa. Regions are named below.

Two properties are used throughout. MOR (modulus of rupture) is bending strength from the peak load of a beam test; MOE (modulus of elasticity) is bending stiffness. Grading cares about both and about density.

How well do features predict strength?

A 2023 review in Forests (Arriaga et al.) reports, for European spruce, correlations with bending, tension and compression strength of:

Knots explained about 57% of strength variation in pine but only 26% in spruce, so rules should be species-specific. Stiffness predicts strength better than any single visible feature; combining it with knots does better still.

From correlation to explained variation

The share of variation explained is the square of the correlation. For a correlation of 0.75:

• 0.75 x 0.75 = 0.5625, so about 56% of the variation is explained;
• about 44% is not, so a grade setting must absorb scatter.

Knots: size and position

A knot is the base of a branch embedded in the stem. It breaks fibre continuity, deflects grain around it and concentrates stress. Position matters as much as size. In a finite-element study of Norway spruce beams (Saad and Lengyel, 2023, Hungary), a knot at mid-span cut load-bearing capacity by about 39%, and the largest loss in the vertical comparison, about 69%, occurred with the knot at the lowest, tension-side position. Knots in the compression zone had only a marginal effect. These are model results for particular beams, not design values.

Sloping grain

A Canadian teaching lecture (North America) gives the fraction of straight-grain strength remaining as 93% at a slope of 1 in 20, 81% at 1 in 10 and 55% at 1 in 5. The lecture does not name which strength property this refers to, and the table could not be checked against the US Wood Handbook, so treat the figures as indicative. European visual rules use their own limits (lesson 3).

Worked example: slope of grain

A piece has a straight-grain bending strength of 40 MPa, and its grain slopes 1 in 5.

• Remaining fraction 55%: 40 x 0.55 = 22 MPa.
• At 1 in 10, 81%: 40 x 0.81 = 32.4 MPa.
• Difference between the two: 10.4 MPa, which is 26% of the straight-grain strength.

Moisture: strong for stiffness, weak for structural strength

Below the fibre saturation point (about 28 to 30%) wood gets stronger and stiffer as it dries. Notes from VTT Finland (European practice, EN 384) give approximate changes per percentage point of moisture content between 8% and 20%:

• compression strength of sawn timber: about 3% per point;
• bending and tension strength of sawn timber: about 0%, because weak points such as knots react little to moisture; EN 384 applies no moisture adjustment for bending and tension, only for compression;
• MOE: roughly 1.5% to 2% per point (source labels garbled; approximate).

Worked example: kiln drying and stiffness

A board dried from 20% to 12% moisture content loses 8 points. With 1.5% to 2% per point, stiffness gains 8 x 1.5 = 12% to 8 x 2 = 16%. A board with an MOE of 10.0 GPa at 20% would be about 11.2 to 11.6 GPa at 12%, but its bending strength, controlled by the weakest knot, barely changes.

Key points

• Structural strength is controlled by defects and expressed as a low percentile; clear-wood tables are upper bounds.
• For European spruce, MOE correlates with strength (0.7 to 0.8) much better than knots (0.4 to 0.6), density (0.5 to 0.6) or slope of grain (0.1 to 0.2); explained variation is the correlation squared.
• Knot position is as important as size; tension-side knots are the most damaging in modelled spruce beams.
• Slope of grain cuts strength quickly (indicative: 81% left at 1 in 10, 55% at 1 in 5).
• Drying raises stiffness by roughly 1.5 to 2% per moisture point between 8% and 20%, but structural bending and tension strength barely change.