Warp of sawn timber of Norway spruce in relation to end-user requirements : quality, sawing pattern and economic aspectsWoxblom, Lotta
Warp, i.e. twist, crook, bow and cup, has been identified as an important factor for sawn timber quality, especially for structural purposes. Twist, crook and bow affect the efficiency of construction and serviceability of wooden products. The prerequisites for improving the quality of solid timber products in terms of shape stability, with focus on the connection between end-user demands, raw material properties and processing methods, are studied in this thesis.
In the first study, acceptance levels for warp set by the building industry were compared with the properties of current production at five sawmills in southern Sweden. The quality of a product, wall studs, at time of delivery to the end-users was described, and an evaluation of fulfilment of the end-user requirements showed that one-third of the graded studs did not fulfil the requirements on warp. Twist was the most severe type of warp.
The effectiveness of altered sawing patterns to reduce warp was studied in the second study. Growth ring orientation and distance from pith considerably affected the shape stability of sawn timber. Removing the central part of the log greatly affected warp, especially twist, in the studs. The different forms of warp were also related to wood characteristics. Percentage of corewood in the studs, distance from pith and grain angle significantly affected twist. For crook and bow, no significant relationships with wood properties could be established. Stand age, tree height class and longitudinal position within the tree did not significantly affect warp.
In the third study, a sub-sample of studs from the second study was used to illustrate how a change in moisture climate affects sawn timber. There were differences in sensitivity to changes in moisture content for studs produced by different sawing patterns. Largest changes in twist and bow occurred in studs containing pith. Crook changed more in quarter-sawn studs than in flat-sawn studs.
The economic aspects of the possible reduction in warp through changed sawing pattern and an adapted choice of raw material are discussed in the final study.
Keywordswarp; twist; sawing pattern; corewood; grain angle; moisture cycling; wood quality; end-user requirements; picea abis
Published inActa Universitatis Agriculturae Sueciae. Silvestria
1999, number: 126
Publisher: Swedish University of Agricultural Sciences
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