Moisture Induced Stress Perpendicular to Grain in Timber Structures

Sammanfattning: Timber structures and wooden element are in general affected by moisture and moisture variation in different ways. Their hygroscopic properties result in adsorption of water when wetted and in desorption when dried. Because the volume of wood is not constant but varies with the moisture content, uneven distribution of moisture will result in induced eigen-stresses because of internal restraint hygroexpansion. Since cross-sectional forces must self-balance, drying causes tensile and compressive stresses near the surface and the middle of a beam, respectively; the opposite applies for wetting. In the design of timber structures today, effects of moisture are considered in a rather summary way. A structural element is assigned to a certain service class and each class has its specific constant strength reduction factor (i.e. the wetter the climate, the larger the reductions). Nevertheless, one concern with such a method it that noting is said about the moisture variation—the selection of service class is only based on anticipated moisture equilibrium levels. It is verified by experiment that moisture induced eigen-stress, which adds to the mechanical stress from external loading, may cause stress fields that significantly reduce the load bearing capacity. In view of the significant importance of moisture on the behavior of wood structures, there is an apparent lack of precise characterization of the climatic condition as a basis for design of timber structures. In order to improve design codes it has been suggested that moisture effects should may be considered as an action to be combined with other ordinary loads (such as snow and live load) instead of being recognized in form of strength reduction factors. In order to further explore this line of suggestion, investigation of climatic conditions and determination of moisture and stress profiles were performed, and the outcome was evaluated by application of e.g. statistical extreme value analysis. This thesis presents background and new findings on moisture induced stress perpendicular to grain. The five appended papers address climate modeling, moisture penetration, development of eigen-stresses and consideration of moisture as an action. In short, some of the findings are: higher stress-levels are induced in larger cross-sections than in smaller ones, summer seasons cause more induced eigen-stresses than winter seasons, induced tensile stress can reach well above the characteristic strength (0.5 MPa), proper surface coating effectively reduces moisture gradients (and thus hinder potential stresses to fully develop), and finally, it is suggested that a load combination factor for moisture induced stress should be equal to 0.2.