Effects of Incising on Lumber Strength and Stiffness: Relationships Between Incision Density and Depth, Species, and Msr Grade
Keywords:Incising, preservative, treatment, mechanical properties, lumber, Douglas-fir, Hem-Fir, Spruce-Pine-Fir (South), MSR, grade
AbstractThis report describes the relationship of incising-induced strength loss in bending as a function of preservative treatment and incising pattern, density, and depth of penetration for various machine-stress-rated (MSR) grades of full-size 2 by 4 Douglas-fir, Hem-Fir, and Spruce-Pine-Fir (South) dimension lumber. This study may represent a worst-case "incising effects" scenario: although the incising patterns and depths selected for study represent commonly used industrial practices, the incising process itself was performed on dry lumber, which is not the standard practice of the treating industry. As we had expected on the basis of Canadian results, incising affected bending properties, such as modulus of elasticity, modulus of rupture, and work to maximum load. Our results show that the combined incising-preservative effect on mean property values for lumber incised in the dry condition prior to treatment was in the range of a 0 to 10% loss in modulus of elasticity, 15% to 25% loss in modulus of rupture, and 30% to 50% loss in work to maximum load. The effect on properties at the lower end of the distribution, such as the allowable stress design value Fb was equal to or less than that on mean properties for the three species groups evaluated. While these results specifically apply to only MSR-graded standard 38-mm- (nominal 2-inch-) thick lumber and to lumber incised in the dry condition prior to treatment, they do imply that the new U.S. design adjustments for Ci in modulus of elasticity of 0.95 and Fb of 0.85 may not be sufficient for incised and treated material used in dry in-service conditions.
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