The Statistical Reliability of Predicting Bending Strength from Strength-Ratio Tables

The 600 actual strength ratios ranged from 13 to 100, averaging 64.4, with a standard deviation of 16.1. The predicted values ranged from 5 to 100, averaging 66.3 plus or minus 22.9. The mean actual strength ratio for rough lumber (63.9) was lower than the predicted strength ratio (67.2), probably because the defects were obscured by the roughness of surface and the dark color which developed during storage. Although there was a significant degree of correlation between predicted and actual values, the Y intercepts were so high and the degree of correlation so low that the predicting system needed to be adjusted or abandoned at least for predicting the strength of oak and cottonwood beams. The mean moduli of rupture for red oak and cottonwood were 9,480 psi and 5,880plus or minus1,184 psi, respectively. The specific gravities for 900 cottonwood samples ranged from 0.32 to 0.62, averaging 0.43 (the standard deviation, 0.05) OD/OD. For a similar number of red oak specimens the range was 0.54 to 0.90, averaging 0.68 t 0.05. Both values were higher than the means assigned to the two species in standard reference tables. Equations for the effect of crossgrain on bending strength gave values that were about 15 percent higher for oak, and 20 percent higher for cottonwood, than strength ratios published by the American Society for Testing and Materials.

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