Moisture-induced Distortion in Norway Spruce Timber - Experiments and Models

Doktorsavhandling, 2002

Timber is a natural material that is environmentally friendly, renewable, light, strong and, in the eyes of most people, beautiful. Timber does, however, have one important drawback when it is used in a modern mechanised building process - its lack of straightness. This is one of the main reasons for the building industry to choose materials other than timber. The main objective of this work was to identify the cause of moisture-induced distortion and create statistical and analytical models to predict distortion on the basis of material parameters. The work also gives recommendations on the parameters that should be focused on in order to develop methods to remove material prone to distortion. Studies were conducted in which the distortion in a large number of studs was measured at several moisture content levels. The changes in moisture content were made without outer restraints in order to study the effect of the material without the effect of restraints. Several material parameters were also measured on the studs; spiral grain angle, annual ring curvature, juvenile wood, knots, annual ring width, compression wood, cracks, wane, density, shrinkage and skew sawing. Spiral grain angle and annual ring curvature together explain about 65% of the variation in twist. An analytical model for twist created by Stevens and Johnston (1960) was used. The same level of coefficient of determination was found as that produced by the statistical method. This model was based on annual ring curvature, spiral grain angle and tangential shrinkage strain. None of the measured parameters was able to explain bow and spring statistically. Detailed studies to determine how the longitudinal shrinkage varies within a stud were conducted. A simple model was developed to calculate bow and spring on the basis of the variation in longitudinal shrinkage. This model shows that the most of the bow and spring caused by changes in moisture content can be explained by the variation in longitudinal shrinkage. The most important parameters which can be used to remove timber prone to distortion are, in the case of twist, annual ring curvature and spiral grain angle and, in the case of bow and spring, the variation in longitudinal shrinkage.