Mill variation in bending strength and stiffness of in-grade Douglas-fir No. 2 2 × 4 lumber

Visually graded Douglas-fir (DF) has wide variability within grade. Variability for 2 × 4 lumber has likely increased because harvest has shifted from federal forests to private plantation forests that contain high percentages of juvenile wood. To investigate resource variability, six No. 2, 2 × 4 DF packages were acquired from six mills. From each package, 124 samples were destructively tested in bending. Mean modulus of elasticity (MOE₁₅) ranged from 9.8 to 13.4 GPa, and three mills did not meet the grade requirements (11.0 GPa). Bending strength (F _b) ranged from 5.5 to 11.3 MPa, and three mills did not meet the grade requirements (9.3 MPa). ANOVA of MOE₁₅ % and F _b showed significant differences at the 0.05 significance level. MOE₁₅ explained from 50 to 78 % of the variability in strength. These data suggest that variation between mills is high and destructive testing is critical for more accurate characterization of lumber properties.