Loading rate effects on the withdrawal strength of self-tapping screws inserted into engineered wood products

Timber-to-timber connections play an important role in the robustness of timber structures. Their performance and structural integrity primarily depend on the effective transfer of loads between elements, typically achieved through metal fasteners, such as nails, screws, bolts or dowels. During their design life, these connections may be subjected to dynamic loads, such as during earthquake, wind and progressive collapse events. However, the response of fasteners under such loads is under researched, especially for connections used in modern mass timber buildings. In this study, the pull-out (parallel- and perpendicular-to-grain) and pull-through withdrawal resistances of 7 mm- and 8 mm-diameter self-tapping timber screws, driven into Laminated Veneer Lumber (LVL) and Glued Laminated Timber (glulam) samples, were experimentally evaluated under four loading duration, with failure occurring between 200 s (quasi-static) and 0.2 s (representative of seismic and progressive collapse events). The screws were 100 mm and 120 mm long for the 7 mm and 8 mm diameters, respectively. In total, 960 tests were performed. Withdrawal resistance increased with loading rate for all test configurations, with average and characteristic withdrawal forces increasing by up to 16.5% and 19.1%, respectively, from quasi-static to seismic and progressive collapse loading conditions. The increase was found to be independent of both screw diameter and the engineered wood product into which the screws were inserted. Stress level correction factors were expressed as linear functions of the logarithm of the failure time. A direct comparison with load duration factors specified in international design standards indicates that most current load duration factors underestimate the observed rate sensitivity of screw withdrawal resistance. Through a simplified reliability analysis, a load duration factor of 1.14 is proposed for the Australian wind load combination and screw withdrawal, with a gust wind speed duration of 0.2 s.