SCREW_STIFFNESS - Load displacement behaviour of multi-axially loaded self tapping timber screws

Since about 20 years, self-tapping timber screws have been frequently applied in modern timber engineering. The reasons for their success are their simple and economic installation without pre-drilling, as well as their flexibility in terms of geometry, realising an application for various different purposes. Comprehensive R&D activities concerning the nature of self-tapping screws allow a comparatively good estimation of their loadbearing capacity nowadays – irrespective the specific direction they are loaded. The situation for their axial and lateral stiffness stands in contrast, since corresponding approaches currently neglect essential influencing parameters such as the axis- and load-to-grain angles (as consequence of application and loading) and the specific screw thread geometry. Consequently the displacement in case of multiaxial loading cannot be determined with an appropriate accuracy. As the stiffness significantly influences the loadbearing behaviour of a connection – and thus that of the whole structural system – the project “SCREW_STIFFNESS” aims to extend the related (and insufficient) state-of-knowledge. The corresponding work is carried out in the frame of three working packages with the goal to describe the load-displacement relationship of a self-tapping screw in case of multiaxial loading. Worth mentioning, that this bases on the examination of the axially and laterally loaded single screw’s properties. The approaches for this description are empirical and mechanical models, verified with results gained from several comprehensive experimental campaigns. The main focus of this research project lies on increasing the efficiency and the optimisation potential of connection and reinforcement solutions with self-tapping screws (in particular screws optimised in terms of thread geometry and system connectors).