Withdrawal resistance and design method of hybrid anchored screws on Douglas-fir glulam

Steel-timber composite floors rely heavily on screws as shear connectors between H-steel beams and timber slabs, where withdrawal resistance is critical for structural performance. Recently, the mortar-glulam anchoring method has proven effective in enhancing screw withdrawal resistance. In this study, 18 pull-out tests were conducted on glulam-anchored and grout-glulam hybrid anchored screws, investigating the effects of screw diameter (6 - 12 mm), effective anchorage length (20 - 40 mm), and grout anchorage ratio (grout length/total length, 0.25 - 0.5) on withdrawal performance. Results indicated that the grout-glulam method provides stronger constraint, leading to a 12.70% average increase in withdrawal resistance. The relationship between anchorage length and withdrawal resistance was established, and prediction equations in EN 1995-1-1 Eurocode 5: Design of Timber Structures - Part 1-1: General - 4 Common rules and rules for buildings (EC 5) and DIN1052: Design calculation and dimensioning of timber structures General rules and rules for building construction (DIN 1052) for glulam-anchored screws were calibrated with test data. A novel design formula for hybrid anchorage is proposed, and experimentally verified, providing relatively favorable predictions with an error ranging from -3.55% to 6.67%. These findings advance hybrid anchorage mechanics understanding and provide practical tools for optimizing hybrid connections.