Bending properties and strength grading of Norway spruce: variation within and between stands1

Current strength grading of Norway spruce (Picea abies (L.) Karst.) structural timber is only able to describe parts of the great variability in density and bending properties. This study assesses whether information about the origin of the timber can be used to predict its strength and stiffness, alone or in combination with machine strength grading. Three hundred and seventy-three boards from 45 trees sampled from three stands in eastern Norway were studied. Substantial parts of the variability of density, modulus of elasticity (MOE), and bending strength or modulus of rupture (MOR) of the boards studied were explained by origin (differences between sites, relative tree size (diameter at breast height), and longitudinal position in stem). Origin also gave a reduction in residual variance in addition to what was obtained by machine grading based on resonance frequencies. For MOR, the improvement was larger than what was obtained by adding density, whereas for MOE, the density was more important than information about origin.