Low cycle ductile performance of screws used in timber structures

Self-tapping screws are largely used in both lightweight and massive timber constructions, being able to carry out simultaneously axial (withdrawal) and lateral (shear) loads. In Europe, the requirements of such fasteners are described in EN 14592; however, the standard used until today addresses only their applications under monotonic conditions and it lacks of prescriptions when the screws are used to withstand cyclic loads. Recently, the standard has undergone a significant revision and some specific rules for screws used in seismic areas have been introduced. Specifically, the revised version of EN 14592 defines three low cycle ductility classes, to which a fastener must be assigned based on the outcomes of monotonic and cyclic bending tests. Following the new seismic provisions included in EN 14592, this paper describes the results of an independent study aimed at assessing the low cycle ductile performance of self-tapping screws that are currently available on the market. Approximately 200 experiments are presented and critically analysed, demonstrating that screws with a diameter of 6 mm must be assigned to the lowest low cycle ductility class. Conversely, all screws with diameter greater than or equal to 8 mm have the potential to be assigned to the highest low cycle ductility class.