Adhesion of structural bonded oak timber joints: Direct shear test and analytical cohesive model

Timber structural members have been widely used in the construction industry, particularly in buildings that are considered a fundamental part of the world's historic built heritage. Timber structural elements may need retrofitting and repair for various reasons, such as natural/environmental ageing and increase of applied loads. A possible solution is represented by gluing new timber planks to the existing member, increasing its crosssectional area and/or repairing/substituting its damaged portions. The bond behavior between the existing member and new timber planks plays a crucial role in the effectiveness of the strengthening solution. In this paper, the bond behavior of glued timber joints made with oak hardwood planks and a polyurethane adhesive is studied using single-lap direct shear tests. Sixty-six joints with six different bonded lengths are considered. Digital image correlation is used to measure the relative displacement between planks. Axial load capacity and peak average shear stress of joints are studied. An analytical model based on a cohesive material law (CML) is used to describe the experimental responses. The results obtained show the effectiveness of this strengthening solution, provide indications on the joint effective bond length, and give insights into the bonded interface stresstransfer mechanism.