Strength graded structural timber is an important product for the Norwegian sawmilling industry. In load bearing wood members, like floor joists, roof trusses and glulam beams it is required to use strength graded wood. This timber has to be strength graded according to EN 14081. The grading process focuses on the most important physical and mechanical properties like density, modulus of elasticity and bending strength. Consequently, the capability to predict and document the strength strongly influence on the value creation in the wood-based industry. An efficient and accurate grading is important to comply with building material standards. Furthermore, more precise documentation of the strength properties in individual boards increases the competitive power of structural wood in relation to other building materials. For machine grading, the timber is sorted into strength classes based on respective approved setting values given in the standards.
Currently, C24 is the dominating strength class used for structural timber in Norway and the highest strength class applied is C30. For some applications, e.g. roof trusses, loadbearing beams with long spans and timber for glulam beams higher strength classes are of interest. Several studies performed in Norway show that a large proportion of the timber resource has a strength that is higher than the currently applied strength classes. This along with the development of more precise grading machines in recent years implies that there is a potential to grade structural timber into higher strength classes. The present paper reports results from a study of machine grading Norway spruce structural timber into strength class C40.
Boards of Norway spruce (Picea abies (L.) Karst.) sampled from eight sites with varying stand characteristics were machine graded into strength class C40 and C24. The yield of C40 was on average 23.6 %. The yield of C40 showed large variation between sites, from 1.4 % to 60.1 %. The results show that a presorting of logs is needed to produce timber with a high yield of C40.
Keywords: structural timber, machine grading, C40
Authors
Flæte P.O.
Senior researcher, Department of Material and Processes, Norwegian Institute of Wood Technology, Blindern, Oslo
Aanerød R.S.
Research assistant, Department of Material and Processes, Norwegian Institute of Wood Technology, Blindern, Oslo
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