This study uses partially threaded self-tapping screws to enhance the mechanical properties of damaged and undamaged dowel-type timber connections. The damaged connections have a 1.5mm wide artificial crack across the middle row of the fasteners. Test results showed that screw reinforcement can restore the rotation capacity of damaged connections. The rotational capacity of reinforced connections without cracks is 45.6% higher than unreinforced connections while the improvement on moment-resisting capacity is slight. Digital image correlation (DIC) was used to detect the movement of the connections and validated that the fasteners rotate around the centre of rotation in reinforced connections. Screw reinforcement also demonstrated the ability to control crack propagation, with the reinforced groups showed a reduction of crack length by at least 37% when compared to the unreinforced groups. A calculation method is proposed to calculate the characteristic moment-resisting capacity of damaged and undamaged screw reinforced connections. The calculated values are proven to be conservative when compared with the characteristic value based on the experimental results. Highlights Self-tapping screws restored the rotational capacity of connections with cracks Partially threaded screws showed a trend to effectively control crack propagation A theoretical prediction method is demonstrated
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.
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Self-tapping screws have the potential to reinforce timber elements. However, their application is restricted due to a lack of design guidance. Compared with fully threaded screws, partially threaded screws with a shorter thread length have less drive-in torque, enabling easier installation. This study showed that partially threaded screws are as effective as fully threaded screws in enhancing the mechanical properties of single-dowel timber connections with artificial cracks. A total of 120 embedment tests were conducted, using screws with two thread lengths (33% and 100%) on single-dowel connections. The connections were also prepared with three different widths of artificial crack to represent splitting of the wood due to moisture variation. The results showed that the screws with 33% thread length were as effective as screws with 100% thread length at reinforcing the connections when the crack width was 1.5 mm or 4.5 mm. However, for a crack width of 6 mm, the embedment strength of the connections reinforced with screws with a 33% thread length was significantly lower (about 15.7%) than the connections reinforced with screws with a 100% thread length. This research demonstrates the potential of using self-tapping screws with a partial thread to reinforce existing timber elements with cracks.
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