Withdrawal resistance of structural self-tapping screws parallel-to-grain in common Canadian timber species

Abstract

An experimental investigation of the withdrawal resistance of self-tapping screws (STS) inserted parallel-to-grain in Douglas fir and eastern white pine, dry (7% – 6% MC) and wet (28% MC), is conducted. Withdrawal tests with 10 mm screws showed that at an embedment length of 240 mm in dry Douglas fir, 280 mm in wet Douglas fir, and 320 mm in dry eastern white pine, the screws consistently failed by yielding in tension. Withdrawal tests of 12 mm screws and a constant embedment length of 280 mm showed that a longer embedment length is needed compared to the 10 mm screws to ensure tensile screw failure consistently occurs. For both the 10 mm and 12 mm screws, the withdrawal resistance was lower for the wet wood as compared to dry. The CSA O86 equation for lag screws significantly under-predicts the withdrawal resistance of STSs inserted parallel-to-grain.