Prediction of tensile strength of sawn timber: models for calculation of yield in strength classes

Abstract

In Europe, strength classes for structural timber and glulam lamellae are defined by minimum requirements of characteristic values of the grade determining properties (GDPs). To fulfill these minimum requirements of characteristic values in the daily production at sawmills, indicating properties (IPs) to GDPs are calculated for each board and based on predetermined limits of the IPs (settings) boards are assigned to the graded class, or rejected. The aims of this paper is to address and discuss two different grading procedures/models that can be applied when settings for IPs that reflects a local board property are derived and to show how the yield in different T-classes depend on the model applied. It is not always that a board's weakest cross-section is evaluated in a destructive test. An IP representing a local board property can therefore be determined either as the lowest property of the tested part of the board or as the lowest property along the whole board when applied to derive settings. Results presented in this paper show that too low settings and too large yields are obtained when the latter IP is employed. Similarly, IPs reflecting a global board property, like axial dynamic MOE, also give too low settings and too high yield in strength classes. This paper is the second and closing part of a series of two paper on prediction of GDPs and procedures for grading sawn timber into T-classes. Keywords Grading of timber Á Laser scanning Á Fibre direction Á Dynamic modulus of elasticity Á Norway spruce List of symbols E a,90,nom Lowest local axial modulus of elasticity (MOE) of the destructivly tested part of the board, calculated on the basis of observed fibre directions E a,90,nom,test Lowest local axial MOE of the destructivly tested part of the board, calculated on the basis of observed fibre directions, i.e. E a,90,nom,test = E a,90,nom E a,90,nom,whole Lowest local axial MOE of the whole board, calculated on the basis of observed fiber directions E b,90,nom Lowest local bending MOE of the destructivly tested part of the board, calculated on the basis of observed fibre directions

Prediction of tensile strength of sawn timber: definitions and performance of indicating properties based on surface laser scanning and dynamic excitation

“…Both would be permitted with respect to requirements of EN14081-2 [4] and EN 14081-2 [5] but they give quite different results in terms of yield in strength classes. Since this is the case, the alternative models must be described, compared and discussed in detail and this is an objective of Prediction of tensile strength of sawn timber: Models for calculation of yield in strength classes [19] in this series of two papers. Another objective of [19] is to present…”