Stress-laminated timber (SLT) decks in bridges are popular structural systems in bridge engineering. SLT decks are made from parallel timber beams placed side by side and pre-stressed together by means of steel rods. SLT decks can be in any length by just using displaced butt joints. The paper presents results from friction experiments performed in both grain and transverse direction with different levels of pre-stress. Numerical simulations of these experiments in addition to comparisons to full-scale experiments of SLT decks presented in literature verified the numerical model approach. Furthermore, several alternative SLT deck configurations with different amounts of butt joints and pre-stressing rod locations were modelled to study their influence on the structural properties of SLT decks. Finally, some recommendations on design of SLT bridge decks are given.
IOP Conference Series: Earth and Environmental Science
Summary
An efficient butt-joint bonding technology allows to build new types of timber structures. Under the name Timber Structures 3.0 a connection has been developed which connects timber elements with an end-grain to end-grain butt joint bonding. Therefore, it is now possible to build continuous, point supported flat slabs in cross laminated timber (CLT). Multiple CLT slabs are connected rigidly together and are only supported by columns. Some major challenges had to be solved in terms of bending strength of the glued connection and shear resistance of the part of the slab above the column. The research in both topics is successful and more projects were built in the last two years using this technology. Starting point was a real scale structure at ETH Zurich, followed by a working platform for a timber construction company and finally four three storey residential buildings. The research team is continuing to optimize the different elements of this innovative technology and will soon provide engineers with guidelines to design their own biaxial, point supported timber flat slabs.
This paper presents an experimental study to evaluate the use of spatially arranged self-tapping screws (STS) as shear connections for cross-laminated timber panels. Specifically, simple butt joints combined with crossed STS with different inclinations were investigated under quasi-static monotonic and reversed-cyclic loadings. The influence of the number and angle of insertion of screws, screws characteristics, friction and loading on the joint performance was explored. The yield load, load-carrying capacity and related slips, elastic stiffness, and ductility were evaluated considering two groups of tests performed on a total of 63 specimens of different size. Performance of connections with respect to the energy dissipation and loss of strength under cyclic loads was also investigated. It was shown that the spatial insertion angle of screws plays a key role in the performance of joints, not only because it relates to the shank to grain angle, but also because it affects the amount of wood involved in the bearing mechanism. Design models of STS connections are presented and discussed, and the test results are compared against analytical predictions. While good agreement for load-carrying capacity was obtained, the existing stiffness model seems less adequate with a consistent overestimation.
Until today, all known timber building systems allow only slabs with a uniaxial load bearing action. Thereby, in comparison to normal reinforced concrete slabs, timber slabs are often thick, expensive and complicated to build. The reason for this is that there is no efficient connection technology to rigidly connect timber slab elements to each other. Alternative solutions are hybrid structural systems with concrete or steel, however, this combination of materials results in some disadvantages especially in terms of weight, ecology, construction time and costs. In the framework of a large research project a new timber slab system has been developed and already tested in first real applications. The developed slab system is designed for housing, commercial and industrial buildings. The slab system works as a flat slab carrying vertical loads biaxial and consists of timber slab elements like CLT glued together on site with a high performance butt-joint bonding technology. Research about the central slab element, the butt-joint bonding and fire tests have already been performed. The research showed the feasibility of this innovation. In 2015 a first prototype was built in Thun, Switzerland. A large three year research project started 2016 with the goal to reach market maturity.
Cross laminated timber (CLT) connections in shearwalls require an understanding of the shear strength and stiffness of panel-to-panel connections within the wall. This research measures the strength and stiffness of three different panel-to-panel CLT connections considering both monotonic and cyclic loading. Connections included a laminated veneer lumber (LVL) spline, a half-lap connection and a butt joint with overlapping steel plate. All connections were ductile in nature. The butt joint with steel plate demonstrated the highest connection strength of the connections tested. The cyclic stiffness of the laminated veneer lumber spline was less than the monotonic stiffness, while the halflap joint experienced a sharp drop in load after ultimate load was achieved. Full details of the monotonic and cyclic behaviour will be discussed, including load, stiffness and ductility terms.
