Strain distribution of dowel-type connections reinforced with self-tapping screws

Current limited guidance on the selection of screws together with undefined design specifications restricts the effectiveness of self-tapping screws as reinforcement on timber members to control crack propagation. Using digital image correlation (DIC), this study visualized the surface strain distribution of screw-reinforced dowel-type connections to understand the influence of thread configuration and screw-to-dowel distance on controlling crack propagation. The experiment was based on single-dowel embedment tests using 16- and 20-mm-diameter steel dowels. Three thread lengths (0%, 33%, and 100% thread) and six screw-to-dowel distances (0.5d, 0.75d, 1d, 1.5d, 2d, and 4d) were investigated. Results show that a screw with 33% thread on the point end can be as effective as screws with 100% thread to control crack propagation under the same geometrical parameters of the connections. Results also reveal that a screw placed further from the dowel (e.g., at 2d distance) can delay the crack-controlling effect. Self-tapping screws placed at 2d can still improve the embedment strength and ductility; however, further doubling this distance (4d) did not enhance the embedment strength, but a higher ductility was still achieved.