Moisture Induced Stresses in Timber Structures
Abstract: Wood is a hygroscopic material; always striving to reach equilibrium with the surrounding air, and consequently the moisture content is different in different climate conditions. The strength of wood is directly dependent on the moisture content, lower moisture content leads to increasing strength. Generally, state moisture equilibrium is seldom obtained because the moisture transport in wood is very slow. The fact that the exchange of moisture is a slow process means that the distribution of moisture content in the cross section becomes nonuniform. Moisture movements (shrinkage or swelling) will therefore be restrained and internal stresses are developed. These moisture-induced stresses affect short and long-term strength mainly in structures where the load-bearing capacity is determined by the strength perpendicular to grain. This thesis contains five separate papers, describing 1) how to measure internal stresses 2) internal stresses due to climate exposure perpendicular to grain, 3) the effect of internal stresses on strength perpendicular to grain, 4) internal stresses due to climate exposure along the grain and 5) the effect of reinforcement on the load carrying capacity of curved beams. In the first paper, a contact free measuring technique is presented. By using a digital camera and an image application, it was possible to replace the conventional gauges commonly used when measuring displacements. This method makes it possible to e.g. use more measuring points and to measure whole strain fields. In the second, third and fourth paper tests were conducted on glulam specimens where the fundamental behaviour regarding the internal stress state and strength perpendicular to grain was investigated when exposing them to different climates. The specimens were moisture sealed, ensuring a one-dimensional moisture flow, and exposed to single climate change, cyclically varying climate and natural outdoor sheltered climate. During the test period, the specimens were tested at different occasions to determine the internal stress distribution and the effect on the short-term strength. The main conclusions from these papers are that locally, the characteristic tensile strength perpendicular to grain can be exceeded during a longer period and that the short-term strength is dependent whether the specimens are in a drying or a moistening phase. In the fifth paper the influence on the load carrying capacity of curved glued laminated beams due to reinforcement and climate induced stresses were examined. The experimental work showed that it was possible to achieve the same strength after failure when reinforcing the beam with screws. It also showed that the strength was increased by approximately a factor 1.5 when reinforcing the beams prior to test, compared to unreinforced beams. The effect of moisture induced stresses is that if the beam is in a moistening phase the strength is almost halved compared to that of seasoned specimens.
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