EFFECTS OF INCISING ON LUMBER STRENGTH AND STIFFNESS - RELATIONSHIPS BETWEEN INCISION DENSITY AND DEPTH, SPECIES, AND MSR GRADE

This report describes the relationship of incising-induced strength lo ss in bending as a function of preservative treatment and incising pat tern, density, and depth of penetration for various machine-stress-rat ed (MSR) grades of full-size 2 by 4 Douglas-fir, Hem-Fir, and Spruce-P ine-Fir (South) dimension lumber. This study may represent a worst-cas e ''incising effects'' scenario: although the incising patterns and de pths 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, s uch 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 elastic ity, 15% to 25% loss in modulus of rupture, and 30% to 50% loss in wor k to maximum load. The effect on properties at the lower end of the di stribution, such as the allowable stress design value F-b was equal to or less than that on mean properties for the three species groups eva luated. While these results specifically apply to only MSR-graded stan dard 38-mm- (nominal 2-inch-) thick lumber and to lumber incised in th e dry condition prior to treatment, they do imply that the new U.S. de sign adjustments for C-i in modulus of elasticity of 0.95 and F-b of 0 .85 may not be sufficient for incised and treated material used in dry in-service conditions.