Moisture induced effects on the capacity of screws and threaded bar connections in timber structures.

The competitiveness of timber structures is largely governed by the design and effectiveness of its connections. Moisture-induced cracks in wood connections are a major problem in load bearing connections. Threaded bars and long self-tapping screws repres ent new possibilities in the quest to avoid this cracking, but they may also, constitute a bigger risk of cracking. Our long-term tasks will be to explore and quantify the combined effect of moisture cycles and external loading on this cracking tendency a nd then incorporate the results in design rules and in numerical models suitable for analysis by the Finite Element Method. The transfer of the external loading from the screws to the wood will involve shear stresses, while wood cracking is governed by t he tensile strength normal to grain. An interaction effect occurs between shear and normal tensile strength. Long threaded screws may represent interesting means to stop crack propagation. The project focuses on effects of the angle between the grain dire ction and the screw axes, combined with the effects of the edge distances, when exposed to moisture changes and external loading. The main part of the research project will be concerned with acquisition of experimental data. The data will be evaluated in view of the theories available in the literature. Anisotropic stress and strain distribution, anisotropic strength criteria, theories related to moisture dependent shrinking and swelling as well as some of the viscous theories are candidates for further s tudies. The scope is a PhD project for one person, followed by one year of a researcher. The planned outcome of the project is; a basis for development of connections with high capacity and durability and minor vulnerability for serious cracks, more effe ctive joining of wooden parts, 1 Ph D thesis, at least 2-3 MSc theses, 3 refereed international publications, and 2 presentations at international conferences during the project period.