Reinforcement of Timber Dowel-Type Connections Using Self-Tapping Screws and the Influence of Thread Configurations

The application of self-tapping screws as reinforcement on glulam connections has been proven effective. However, the implication of different thread configurations on the effectiveness of reinforcement remains unknown. This paper conducted experiments using screws with various thread configurations in embedment-strength tests and tensile connection tests. Results show that self-tapping screws with one third of thread achieved similar improvement in the embedment strength and mechanical properties of connections as fully threaded screws. This implies that properly reducing the thread length on self-tapping screws ensures easier screw installation than using fully threaded screws. The influence of screw-to-dowel distance was also investigated and two distances (0.5 d and 1 d) were adopted, with 'd' being the diameter of the dowel. The difference in embedment strength due to different screw-to-dowel distances was insignificant. The group with screws placed in contact (0.5 d) with the dowel achieved 5% higher embedment strength than the group with screws placed at a 1 d distance. The connection tests showed good agreement with the embedment-strength tests. This confirms that self-tapping screws with reduced thread can enhance the load-carrying capacity and ductility of connections to a level similar to connections reinforced by fully threaded screws.