‘Rope effect’ mechanism of self-tapping screws as reinforcement on dowel-type connections

The ‘rope effect’ is a positive effect caused by the failure of fasteners of dowel-type connections. A similar mechanism can be found in connections with screw reinforcement. For self-tapping screws used as reinforcement for connections, bending of the self-tapping screws tends to close developed cracks. Utilising the withdrawal resistance provided by the threads on the pointed end of the screw and the pull-through resistance from the screw head, screw reinforcement enhances the embedment strength and splitting resistance of the wood and increases the ductility of the connection. The influence of different thread lengths and locations on reinforcement effectiveness regarding the embedment strength of the timber was investigated in this study. For cases with threads located only on the pointed end, the splitting resistance increased with longer thread length, indicating that the rope effect was progressively enhanced. It was also found that screws with halved thread length demonstrated similar reinforcement effectiveness to fully threaded screws while the partial threads reduced the risk of damaging the screw during installation. A calculation method to predict the load-carrying capacity of screw-reinforced dowel-type connections was also formulated.