Performance of timber board models for prediction of local bending stiffness and strength—with application on douglas fir sawn timber

Abstract

Efficient utilization of structural timber requires accurate methods for machine strength grading. One of the most accurate methods presented this far is based on data of local fiber orientation on board surfaces, obtained from laser scanning., in combination with data of axial resonance frequency and board density. The method, by which local bending stiffness is calculated and used as indicating property to bending strength, has been evaluated by means of large samples of timber boards, and settings for machine strength grading of Norway spruce sawn timber have been determined and approved for use on the European market. In this paper, two potential improvements of this method are examined. The first one consists of replacing a model based on simple integration over cross sections of boards for calculation of local bending stiffness by a 3D solid finite element model from which local bending stiffness is derived. The second improvement concerns replacement of a simple model for the fiber orientation in the interior of board by a more advanced one taking location of pith and growth direction of knots into account. Application of the alternative models on a sample of more than 200 Douglas fir boards, size 40 × 100 × 3000 mm, cut from large logs, show that each of the evaluated model improvements contributes to improved grading accuracy. When local bending stiffness is calculated utilizing the herein suggested finite element model in combination with the improved model of fiber orientation in the interior of boards, a coefficient of determination to bending strength as high as 0.76 is obtained. For comparison, a coefficient of determination of 0.71 is obtain using the simpler original models. Thus, strength grading of structural timber based on the suggested improvements would make it possible to increase the yield in higher strength classes.