Properties of axially loaded self-tapping screws with focus on application in hardwood

Abstract

The raising stock of hardwood and the increasing popularity to build with timber are strong drivers to use hardwood as structural material on a regular basis. Despite its outstanding mechanical properties, economic boundary conditions in producing structural hardwood products consequence, however, rather a niche application. Efficient fastener and joint solutions which are optimised for hardwood are one possible solution to increase the share of hardwood in structural applications. Within this contribution a new, innovative and for hardwood optimised screw is benchmarked with a commercial screw. The influence of wood anatomy, thread-grain angle and predrilling on the withdrawal strength of single screws are analysed. For groups of screws, the minimum edge-distance and spacing perpendicular to the grain are determined and various possibilities for end-grain joints investigated. A generic approach for predicting the withdrawal strength of axially loaded self-tapping screws inserted in structural timber or glued laminated timber products from either soft- or hardwood is presented. Models to account for the influence of density, thread-grain angle and predrilling are proposed and regulations for the geometric requirements and execution of groups of screws in end-grain joints discussed.