Abstract
Finger joints in structural timber and glulam lamellae are often used to enable production of long members or to allow for re-connection of parts of a member after removal of weak sections. According to the European Standard EN 15497, certain margins are required between knots and a finger joint in structural timber, which means that a considerable amount of clear wood becomes waste when finger joints are applied. The purpose of this paper was to investigate the possibility of reducing the quantity of waste using different criteria for placement of finger joints. The investigation was based on (1) application of methods of colour scanning and tracheid effect scanning to detect knots and grain disturbance on board surfaces, and (2) interpretation of the requirements of EN 15497 regarding where finger joints may be placed. The standard’s requirement when producing finger joints is that the minimum distance between a knot and a finger joint is three times the knot diameter. The standard allows for the minimum distance between a knot and a finger joint to be shortened to 1.5 times the diameter when the local fibre orientation is measured. Utilizing this in simulated production resulted in reduction of waste from 7.4 to 4.0%, when using finger joints simply to produce timber of long lengths. If finger joints are also used to re-connect parts of members after removal of weak sections, even larger savings can be made. Furthermore, it is concluded that knowledge of fibre orientation obtained from scanning could be used not only to decrease the waste in production but also to increase the quality of finger joints.
Highlights
1.1 BackgroundThe engineering properties of softwood timber vary considerably between species, and within single species, between origins, stands, trees, boards and within boards
The minimum distance between the finger joint and a knot in structural timber is set (EN 15497) to three times the diameter of the knot except where an appropriate automated system guarantees that in the range of the finger joint the grain orientation is parallel to the longitudinal direction of the board
Since such knot surfaces may be disregarded with respect to the knot diameter defined in Sect. 1.2 this is, not an issue for the applications considered in this research
Summary
1.1 BackgroundThe engineering properties of softwood timber vary considerably between species, and within single species, between origins, stands, trees, boards and within boards. The properties of structural timber of different strength classes are defined in the European standard EN 338 (2016). In this standard, characteristic values of several strength and stiffness properties are specified, in addition to characteristic density. Machine strength grading of timber according to the European standard EN 14081-2 (2018) concerns only requirements on three different grade determining properties, namely the bending or tensile strength, the MoE. European Journal of Wood and Wood Products (2019) 77:1063–1077 in bending or tension and the density In addition to these requirements, there are limitations regarding geometrical imperfections and visually identified defects, as specified in EN 14081-1 (2016). Regarding accuracy in grading it is, in most cases, the ability to predict strength rather than density or MoE, that limits the yield and, improved grading accuracy with respect to strength leads to higher yield in high strength classes (Olsson and Oscarsson 2017)
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