Investigations on the Serviceability Limit State of Dowel-Type Timber Connections

Sammanfattning: Dowel-type steel-to-timber connections are commonly used to transfer a large range of loads. Although they are simple to produce and assemble, the load-carrying behavior and the local stress and strain distribution within the connection area are highly complex. In addition to that, wood is a challenging material from an engineering point of view due to its highly anisotropic structure and behavior and due to its natural origin, which results often in inhomogeneities. The failure characteristics of wood are very different in tension and shear and in compression, where brittle failure and plastic-ductile failure modes occur, respectively. The aim of this thesis is to study the load-carrying behavior of dowel-type steel-to-timber connections in detail. This is achieved by performing experimental tests on single-dowel connections. A large variety of influencing parameters is assessed, which include wood density, connection width, the dowel roughness, and the application of reinforcements in order to prevent brittle behavior. Separate stages in the loading history are identified, starting from an initial consolidation phase, the region of maximum stiffness during load increase, and the point of maximum connection strength. Ductility is of great interest as well as the final failure modes. During the experiments, unloading and reloading cycles are performed, where distinctively higher stiffnesses are observed than during the first loading. The results of the experiments are compared to the design practice in Eurocode 5 for strength and stiffness estimation. Strength prediction is conservative except for slender connections, while stiffness prediction complied with experimental results only for connections of intermediate width. The initial consolidation phase of the experiments is then investigated further. It is concluded, that the properties of the bore-hole surface, where not a smooth but a rough surface with valleys and rifts is encountered, is responsible for the initially low stiffness. The contact behavior is studied by conducting experiments on wood with varying surface characteristics, which are a result of using different cutting tools. A mathematical model for the soft contact behavior is proposed, which is based on the results of the experimental tests. It also includes the evolution of non-reversible deformations in the surface layer. Complementing the experiments, a simulation tool suitable for numerically assessing the mechanical behavior of the connections is developed. It allows to perform simulations by means of the Finite Element method on such connections and provides an enhanced insight into the stress and strain distribution in connections compared to the tests. Hereby, a three-dimensional material model for wood is established, which allows to model the anisotropy of wood in the elastic as well as in the plastic domain, based on the theory of small strains and small displacements. The combination of the developed models for the material as well as the contact behavior leads to realistic simulation results, which are verified by comparing model predictions with the experimental results on connections. It is confirmed, that the computed behavior agrees well with the experimental one and that the features observed during the experiments are well reproduced. Due to the limitations of the simulation tool to small deformations, ultimate load and brittle failure modes cannot be predicted. Nevertheless, the influence of various parameters on both can still be estimated. The modeling approach is suitable for application to more complex situations in the future, such as multi-dowel connections or connection loaded by generalized loads. Especially the contact model, which is a unique feature in the thesis, allows a realistic simulation of the distribution of the forces in such statically indeterminant situations