Embedment behavior of low-to-medium diameter bolts in Douglas fir glued laminated timber

Reliable embedment properties are essential for the design of bolted glulam connections, yet most available equations were developed for sawn timber. This study investigated the embedment behavior of Douglas-fir glued-laminated timber by full-hole tests on 13 specimen series with a constant 1 mm hole clearance. The matrix was designed to isolate the effects of bolt diameter (8 to 16 mm), load-to-grain angle (0 to 90°), and member thickness (30 to 40 mm). Increasing bolt diameter markedly increased embedment stiffness, from 4.20 to 8.46 kN/mm in the 35 mm parallel-to-grain series, while strength changed only slightly. For the 12 mm reference series, both yield strength and stiffness decreased from 0° to 60° and then partially recovered at 90°, confirming pronounced anisotropic behavior. Increasing thickness improved the overall response, although the strength trend over 30–35–40 mm was not strictly monotonic. Existing Eurocode 5 and NDS equations showed noticeable deviations, especially for off-axis loading. A modified Hankinson-type model gave the closest agreement with the measured yield embedment strengths and offers a more reliable basis for the design and assessment of low-to-medium diameter bolted glulam connections.