BENDING STRENGTH, STIFFNESS, AND STRESS-GRADE OF STRUCTURAL PINUS RADIATA: EFFECT OF KNOTS AND TIMBER DENSITY

Abstract

tested in bending to determine modulus of elasticity (MOE) and modulus of rupture (MOR). Knots contributing to failure were measured and classified and their knot area ratio calculated. The air-dry density of the timber was also determined. Accuracy of prediction of MOR using MOE was shown to be independent of timber density but was improved by including knot area ratio, and sometimes knot position, in the regression. Reductions in timber density and increases in knot size reduce theoretical mechanical stress-grade yields. Therefore a reduction in the market value of sawn structural timber is predicted if forest management aims to increase the rate of growth of plantation trees without compensatory attention to density and branch size. When the radiata pine timber used in the study was graded to the normal Australian visual grading rules, it generally failed to meet normal bending strength requirements. A re-assessment of the grading rules is recommended.