STUDY OF THE RELATIONSHIP BETWEEN FLATWISE AND EDGEWISE MODULI OF ELASTICITY OF SAWN TIMBER AS A MEANS TO IMPROVE MECHANICAL STRENGTH GRADING TECHNOLOGY

In most of the strength grading machines for sawn timber, the flatwise bending modulus of elasticity (MOE) of timber pieces is measured. Emp loying regression functions, their edgewise MOE is estimated on the ba sis of the flatwise MOE and the edgewise bending strength of the weake st part of each piece is calculated to allocate each piece to a standa rdised strength class. With regard to improvements in the accuracy of timber strength grading machines, it was studied to which extent struc tural wood characteristics and grading parameters affect the relations hip between flatwise and edgewise bending MOE. Edgewise and flatwise M OE have been determined both in knotty and in knot-free sections of bo ards of Norway spruce. The flatwise MOE was determined in a three-poin t bending test as it is typically employed in strength graders. The ed gewise MOE was determined in a four-point bending test. Additionally, the MOE and density distributions over the timber cross sections were determined to model the total MOE under consideration of these distrib ution patterns. Shear deformation accounts for a substantial portion o f the difference between flatwise and edgewise MOE. The effect of knot s on the MOE could not be defined precisely. Growth ring structure and juvenile wood in the boards lead to 5 to 10% lower flatwise MOE value s as compared to the edgewise MOE.