Knowledge on the moisture behaviour of wood constructions for the targeted use of wood-based materials is necessary, because many mechanical properties, such as strength, stiffness, heat and moisture behaviour as well as bulk density depend on it. In addition, swelling and shrinking can lead to cracks due to stresses exceeding the short term strength criteria, because the expansion coefficients differ with respect to the orthotropic material directions and the moisture content varies over the cross-section due to changing climate. In this diploma thesis numerical simulations of the coupled moisture and heat transfer in wooden cross sections under real climate conditions were carried out. For this purpose, the relative humidity and temperature in Linz (Austria) from November 1st to December 31st were evaluated and the moisture content over the cross-sections was determined for 18 different models (solid, glue laminated, boards, logs).^ This has shown that the moisture content is related to the climate of the seasons and daily climatic changes only influence a rather thin boundary layer. Large cross-sections react much slower to moisture changes than small ones. While the equilibrium moisture content is reached in the outer third of the cross-sections, the inner two thirds (passive zone) with a width greater than 14 cm change only by approximately 2%. The service classes of Eurocode 5 dont take the influence of changing moisture content on the mechanical strength into account sufficiently. No exact information on changing climatic conditions is given. The average moisture content over the cross section was between 12% and 20% during the simulation. It was found that annual extreme values of the moisture content correlate with the cross section area divided by the circumference of the cross section and can be estimated with simple equations.^ Linear elastic stress calculations were conducted based on the obtained moisture content fields. Expansion due to swelling, shrinking and temperature were considered. Furthermore a multi-surface failure criterion was applied. The results show that areas susceptible to cracking occur due to the exceeding of tangential streingth: in the warm season in the outer and in the cold season in the inner regions of the cross-sections. Thus, especially the initial moisture content of small cross-sections should be adapted to the ambient conditions on the building site. The extended finite element method was used to investigate the crack formation of the timesteps with the highest risk of cracking, which were obtained during the linear elastic stress calculation. The maximum lengths of the cracks were obtained and compared to the assumptions of Eurocode 5.^ It was found that the reduction of the cross-sectional width for shear stresses according to Eurocode 5 agrees well with the results of the simulation. For drying cracks, the design according to Eurocode 5 is especially conservative for cross sections with a width of more than 20 cm. In the case of wetting, cracks occur in the inside, which are overestimated by the simulation due to the lack of relaxation effects. In glulam cross-sections, cracks appeared directly next to the glue layers. They were of similar length as in the solid wood sections. In order to better understand the load capacity, serviceability and durability of outdoor wood structures, it is possible to investigate the moisture and crack behavior of wood cross sections under real climate conditions using numerical methods.