Compression Perpendicular to the Grain in Dry Douglas-Fir and Hem-Fir

Compression perpendicular to the grain (C-Perp) Was studied in five grades of Douglas-fir and three grades of hem-fir laminating lumber randomly selected from four states. Tests of small and large specimens loaded on edge, flatwise, and at the end were made at 10 percent moisture content. Patterns of stress-deformation curves were established and distributions of stress at the proportional limit and at 0.04- and 0.05-inch deformations were analyzed. Average stresses in C-Perp could be estimated with great confidence (corr. coeff.> 0.942) for any deformation from 0.04 to 0.15 inch. Specific gravity was not a good predictor of C-Perp, probably because of the overriding influence of angle of growth rings. Values did not necessarily increase with higher grades of lumber. Hem-fir was about two-thirds as strong as Douglas-fir of corresponding grade. Stresses as the fifth percentile with 75 percent confidence (by log transformation) were developed for 96 combinations of lumber grade, specimen size, and test type. Other data and relationships are shown which should aid in setting allowable-stresses for dry hem-fir and Douglas-fir.

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