Dowel-Type Connections in Beech LVL

Abstract

The presented thesis focuses on the structural behaviour of dowel-type connections in laminated veneer lumber (LVL) made of beech wood (Fagus sylvatica L.). In Europe, beech wood is available in large quantities. However, despite its favourable mechanical properties in terms of strength and stiffness compared to softwood species, it is today mostly used for non-structural purposes. The industrialised production process of beech LVL and the homogeneity resulting from the assembly of thin rotary-peeled veneers ensure controllable and reliable strength and stiffness properties. Thus, beech LVL is a wood-based material with a great potential for applications in high Performance structural elements, as for instance in large-span truss structures. In timber structures, the design is often governed by the connections. Therefore, optimising the connections can significantly enhance the performance of the entire structure. In the presented thesis, dowel-type connections are investigated both experimentally and theoretically. Due to their versatility, dowel-type connections are commonly applied in trusses and other timber structures. Two particular connection designs are investigated in detail: Dowelled connections with one slotted-in steel plate and bolted connections with thin outer steel plates. The prevailing design approach for dowel-type connections is based on ideal rigid-plastic material assumptions. This so-called Johansen Yield Model (JYM) is implemented in current design codes, such as Eurocode 5. However, in reality timber generally shows a rather brittle behaviour, particularly when subjected to shear forces or tension parallel and perpendicular to the grain. Based on extensive investigations focused primarily on solid wood and glulam made of softwoods, the Johansen design approach has been modified in order to take into account these secondary effects due to the brittle nature of timber. The first part of the thesis presents experimental investigations on dowel-type connections in beech LVL. This includes embedment tests and tensile connection tests on dowelled and bolted connections. It was found that introducing cross-layers in the LVL significantly improves the performance of the connections compared to the reference tests on LVL without cross-layers. Given sufficient spacing between the fasteners, premature splitting and shear failures are prevented in cross-layered specimens. This allows the designated failure modes according to the JYM to fully develop, leading to higher load-carrying capacities and improved ductility. Furthermore, the tests have shown that a significant rope effect can be activated in cross-layered LVL. In the second part, a numerical model is developed to assess the load-bearing behaviour of dowel-type connections in beech LVL. The model is based on the JYM, but substitutes the assumed ideal rigid-plastic material behaviour with more realistic stress-displacement behaviour. The material models are derived from test results on beech LVL and steel fasteners. The model generates a full load-displacement curve for the respective connection design and illustrates the different components contributing to the total resistance of a dowel-type connection. It is shown that with cross-layered LVL, a nearly plastic embedment stress distribution can be reached. In configurations including a rope effect, it is shown that the contribution of the rope effect significantly increases in the post-yielding stage while continuously changing the shape of the designated failure mode. Subsequently, the current design approach according to Eurocode 5 for dowel-type connections is evaluated based on the experimental investigations and the numerical model approach. The evaluation shows that the load-carrying capacity of dowel-type connections in beech LVL is significantly underestimated. As premature brittle failures are prevented in cross-layered beech LVL, higher embedment stresses and increased bending angles of the fasteners are reached. The rope effect occurring in cross-layered beech LVL is also underestimated by the current design provisions. Lastly, an experimental investigation on beech LVL truss structures with dowel-type connections is presented. The impact of an optimised connection design on the global load-displacement behaviour is studied. The results show that applying cross-layered LVL significantly improves the overall performance, and activating a rope effect in the connections can further increase the load-carrying capacity and the ductility of the truss structure.