Experimental investigation on parallel-to-grain wood-to-wood joints with self-tapping screws

The paper presents the main outcomes of an experimental research performed on parallel-to-grain wood-to- wood joints made with self-tapping screws in single shear. Screws have a nominal diameter of 12 mm and have been investigated with and without washers. Joints with 8 mm dowels (obtained from the shank of the self-tapping screws) have been also investigated for comparison purpose. Specimens have been designed to fail according to failure modes of the European Yield Model that don’t correspond to a uniform stress distribution in wood elements. In detail the failure modes taken into consideration are: rigid rotation of fasteners (mode c), development of a single plastic hinge in fasteners in the point-side wood elements (mode d) or in the head-side wood elements (mode e), development of two plastic hinges in fasteners (mode f). In designing, the strength increase due to the “rope effect” has been taken into account. After testing, specimens have been processed to determine the actual failure modes of fasteners, their plastic rotations and the maximum embedding deformations close to shear planes. The main results, in terms of strengths, actual failure modes and plastic deformations, are compared for the four different failure modes and for the three investigated fasteners: the 12 mm self-tapping screws with washers, without washers and the 8 mm dowels derived from the shank of self-tapping screws