An innovative technique for the local reinforcement of steel-to-CLT nailed joints

The load‐bearing capacity of the joint area of timber members is the primary weak point when considering the load‐bearing behaviour and stiffness of the overall existing and new timber constructions. Steel-to-timber joints with dowel‐type fasteners may have a range of failure mechanisms defined by the Johansen theory. The key parameters for this failure mode are the embedment strength of the timber or wood‐based material (e.g. Cross-Laminated Timber – CLT) plus the joint geometry (thickness of the members to be connected, spacings and end and edge distances of the fasteners). A promising technique to improve both the load-bearing capacity and the stiffness of joints with dowel‐type fasteners consists in increasing the embedment strength of the wood-based material. This embedment strength improvement is generally performed gluing an high strength layer (e.g. steel plate, high density wood-based material..) to the timber shear plane interface. In this work, a preliminary literature review on the methods typically used for improving the load-bearing capacity and the stiffness of standard nailed connections is presented and the corresponding design method discussed. An original technique for improving the load-bearing capacity and stiffness of steel-to-CLT panel nailed joints, adopting a carbon fibre reinforcement layer glued to the shear plane interface is presented and experimentally tested at laboratory of University of Bologna. In detail, two different load configurations (perpendicular and parallel to the grain) and reinforcement materials (applied with epoxy resin or IPN adhesive) are investigated by means of experimental monotonic tests. The obtained results are analysed in order to evaluate the effectiveness of the reinforcement technique in terms of load-bearing capacity and stiffness with respect to the reference unreinforced configuration. Finally, the applicability of the current design method is verified together with the proposal of an innovative design procedure