Withdrawal Resistance of Self-Tapping Screws in Laminated Bamboo Lumber Considering Insertion Angle and Direction

This study examined the withdrawal behavior of 6 mm self-tapping screws (STSs) in laminated bamboo lumber (LBL). A total of 55 specimens were tested. For the 6d effective penetration length, insertion–grain angles of 0°, 30°, 45°, 60°, and 90° were considered. Edgewise and flatwise configurations were tested at 30–90°, while the 8d groups were tested only at 90°. Maximum withdrawal load, withdrawal strength, withdrawal stiffness, load–displacement response, and failure modes were evaluated. For the 6d series, ANCOVA showed significant effects of insertion angle, insertion direction, and their interaction on withdrawal strength after accounting for specimen-level density variation. The edgewise series had its highest observed mean strength at 60°, but the within-direction angle effect was not statistically significant (p = 0.103). The flatwise response remained relatively stable from 45° to 90°. At 90°, both 8d groups had higher maximum withdrawal loads than the corresponding 6d groups, and STS fracture occurred as the load approached the measured mean tensile resistance of the screw. An endpoint-based Hankinson-type interpolation was defined by the 0° and 90° group means and assessed against the intermediate-angle data. It did not reproduce the non-monotonic edgewise response or the plateau-like flatwise response. The interpolation therefore serves as a simple two-endpoint benchmark for the mean withdrawal-strength response.