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

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Abstract

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.

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