Prediction of Bow and Crook in Timber Studs Based on Variation in Longitudinal Shrinkage
Keywords:Compression wood, warp, model, moisture content
This paper presents a model that describes the change in magnitude of bow and crook between two moisture contents below the fiber saturation point. The model shows that the variation in the longitudinal shrinkage coefficient over the cross section and along the stud could explain most of the change in bow and crook in the studs. It was possible to identify evenly curved bow and crook, as well as S-shaped bow and crook. The results show that the model predicts changes in bow better than changes in crook.
The results of measurements of distorted geometry along the length of 12 studs are presented. The equipment for measuring distorted geometry is described. The distorted geometry was measured at two moisture contents. The studs were then sawn into sticks (10 X 10 X 200 mm), a total of 3,600. The longitudinal shrinkage coefficient in these sticks was obtained for a change in moisture content from 18% to 8%. The sticks were also classified visually into three groups depending on their compression wood content: no compression wood, mild compression wood, mild compression wood, or severe compression wood.
The variation in the longitudinal shrinkage coefficient was large in the studied sticks (x 0.0111, SD 0.0111). The sticks classified as containing severe compression wood had a significantly larger longitudinal shrinkage coefficient than the sticks classified as no or mild compression wood. Moreover, sticks classified as mild compression wood had a significantly larger shrinkage coefficient than the sticks classified as no compression wood.
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