Modelling of strengthened connections for large span truss structures

Abstract: Because of its large ratio between strength and weight timber is a material very suitable for large span structures. For large spans of more than about 30 m, usually truss structures are adopted. The problem of large span timber truss structures are the expansive and complex nodes that have to be adopted, normally single or multiple slotted-in steel plates in combination with a number of dowels. In order to find a more efficient and economical solution for timber nodes, a novel truss node has been developed at SP-Trätek in Borås. The node consists of a single large- diameter dowel which connects the diagonals with the chords. Additionally, self-tapping reinforcing screws are applied to prevent premature splitting failure and also to increase the strength and the stiffness of the node. In the course of this thesis a full-scale test series was carried out to investigate the behaviour of the node. 15 glulam specimens with reinforced single large-diameter dowel connections were tested in load controlled tensile tests. Besides various configurations of self-tapping screws also lateral prestressing was used as a reinforcement measure. It was found that the bearing capacity of the connection can be significantly increased by applying self-tapping screws. Reinforcing screws effectively impede splitting of the timber if placed near the loaded end of the connection, where major lateral deformations occur. However, reinforcing screws cannot prevent the formation of a shear plug, which was observed to be the ultimate failure mode. Lateral prestressing of the connection also proved to be an effective reinforcement measure, as splitting can be prevented completely. By applying large lateral prestresses the failure mode changes to a combined shear and tensile failure which results in a higher bearing capacity.