Load-carrying behaviour of steel-to-timber dowel connections

This paper details tests on steel-wood-steel connections with thin dowels carried out at the University of British Columbia in Vancouver, Canada. Connections configurations were selected in such a way that the influence of the main parameters such as dowel slenderness ratio, dowel strength, fastener spacing and end distance and the number of dowels could be studied. An important issue of the paper is the effect of fabrication tolerances. MULTIPLE SHEAR STEEL-TO-TIMBER JOINTS The multiple-shear dowel connection with slotted-in steel plates is one of the most efficient timber joints for large cross sections, which are often used in timber engineering. During the last years a large research project was carried out at the Swiss Federal Institute of Technology in Zurich (ETH), Switzerland to optimise the design of this type of connection (Mischler 1998a). One connection of this type is the BSB-joint which is represented in Figure 1. The BSB-joint is well known in Europe for its high performance in glued laminated timber and in high strength composite lumber products (Mischler 1999). This high performance is mainly achieved through the use of thin slender steel dowels and the very tight fabrication tolerances. To manufacture the BSB-joint, a special automated production line with a high precision drilling machine was developed. dowels diameter 6.3 mm inserted steel plates glued laminated timber or structural composite lumber Figure 1: Multiple-shear dowel connection with slotted-in steel plates type BSB EUROPEAN YIELD MODEL FOR STEEL-TO-TIMBER CONNECTIONS The load-carrying behaviour of dowel type fasteners can be described by the so-called European Yield Model (EYM). This model is based on a plastic limit state analysis. Johansen (1949) who first published this model assumed an ideal rigid-plastic behaviour of both the timber and the steel dowel. It is also assumed that no premature brittle failure in the timber such as splitting, plug shear or tensile failure in the reduced cross section occurs before the fasteners reach their ultimate resistance. In the connection, the dowels act similar to a beam on a deformable foundation. The support conditions significantly affect the load-carrying behaviour of the dowel. In a connection with two thin side steel plates as shown in Figure 2 Form B the dowel behaves like a simple beam with the steel plates as supports. In a multiple-shear connection however the dowel acts like a continuous beam (Figure 2 Form A). This type of connection is typically made with slotted-in steel plates as shown in Figure 1. The other important factor in the EYM is the dowel slenderness ratio λ. Stocky dowels with effective slenderness λef < limit dowel slenderness λy remain straight. Slender dowels with λef > λy are bent in the connection. Depending of the support conditions, 2 or more plastic hinges are formed (See Figure 2). 1 Research Associate, Dept. of Civil Engineering, Swiss Federal Institute of Technology ETH, Zurich, Switzerland 2 Associate Professor, Dept. of Civil Engineering, University of British Columbia, Vancouver, BC, Canada 3 Associate Professor, Dept of Wood Science, Univ. of British Columbia, Vancouver, BC, Canada Form A: Multiple shear connection Form B: Steel-to-timber connection d t ef = λ 3 h u y d f M 4 ⋅ ⋅ = λ 3 h u y d f M 2 2 ⋅ ⋅ ⋅ = λ y ef For λ < λ d 2F 2F