Experimental and numerical investigations on adhesively bonded hardwood joints

Adhesively bonding timber structural elements provides new opportunities as it is well adapted for the nature of the material. Experimental and numerical investigations were carried out on full-scale double-lap joints composed of hardwood adherends (beech). Although beech is currently only rarely used as a construction material – due to the difficult processing and the lack of design methods – its abundant availability in Central Europe and its high mechanical resistance make it a promising material for structural applications. Hardwood differs significantly from softwood in terms of different mechanical properties that originate from the fundamentally different anatomy. To help filling the research gap on the strength of adhesively bonded hardwood joints, the influence of the overlap length (40 mm–320 mm) on joints strength was studied. It was found that joint strength increased with the overlap length up to an apparent maximum of approximate 200 mm. The subsequent numerical investigations allowed determining the stress state within the joints and provided input for a probabilistic capacity prediction method. The approach considers the size effect in the strength of timber and previously provided excellent results for brittle failure of softwood joints. The method provides a framework for the design of adhesively bonded hardwood joints; however, due to the fundamental differences in the material properties, further investigations are needed in order to adapt the approach into a reliable design procedure.