- Bendtsen, Alan
Forest Products Laboratory, Forest Service, U.S. Department of Agriculture
Nine mechanical properties and the specific gravity of black, red, and white spruce and balsam fir were measured on material collected at random from the growth range of each species.
Significant changes, with 95 percent confidence, were observed in 31 to 40 instances (four species, five properties, and two moisture contents) that are commonly used in lumber and plywood design.
Design values for modulus of elasticity and proportional limit in compression perpendicular to grain are based upon sample estimates on the mean. Average values for these properties are, in 9 of 16 instances, significantly higher than previous estimates. In only one case, the proportional limit of white spruce in the green condition, this study revealed a significant reduction.
In the modulus of rupture, compressive strength parallel to grain, and shear strength, design is based upon a 5 percent exclusion limit derived from the sample average and standard deviation. All changed significantly, with three exceptions: Modulus of rupture and compressive strength for dry white spruce and shear strength for green red spruce. A simultaneous statistical test of the mean and standard deviation was used to measure significant differences for these properties.