PSI - Issue 78

Martina Sciomenta et al. / Procedia Structural Integrity 78 (2026) 253–260

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When comparing the stiffness results with the values reported in the literature (Blass et al.) for configurations made of solid spruce T28, with the same screw penetration length in both the side and central members, the findings of the present study indicate a systematically lower stiffness (e.g., beech with VGZ screws: –37%). In terms of resistance, from the test results the characteristic values were identified according to EN 14358:2016 (CEN, 2016). A comparison was then carried out with the values specified by the manufacturer for the use of screws in softwood. The results show that, for beech with VGZ screws, the resistance is 7% lower, whereas VGS screws exhibit an increase of 36%. For chestnut, both configurations exhibit a reduction in resistance of approximately 23%. Finally, the hybrid series shows higher resistance, with increases of 28% for VGZ screws and 31% for VGS screws. In addition to providing higher maximum load-bearing values compared to VGZ screws, VGS screws exhibited a significant increase in load-carrying capacity at large displacements ( v > 12 mm). This behavior can be attributed to the contribution of the screw head pull-through resistance, as reported by Aicher et al., which is influenced by geometric and mechanical parameters such as side member thickness and wood density. Therefore, further experimental and analytical investigations are required to more precisely quantify the benefits associated with this screw type in terms of overall load-bearing capacity. 5. Conclusions This study investigated the mechanical performance of timber-to-timber screw connections in three different configurations: Glulam Beech, Glulam Chestnut and Hybrid Beech-Silver Fir. The results highlight how material composition influences key mechanical properties, including ultimate load ( max ' F ), slip modulus (K s ), yield force ( y F ), yield displacement ( y v ), and ductility ( D ). Case study VGZ9x160 board with 40 mm • Screws in hybrid beech-silver fir had significantly higher ultimate load, stiffness, and yield force than those in glulam beech. • Screws in glulam chestnut had a lower ultimate load and yield force but showed higher ductility (meaning better deformation capacity before failure). • Screws in hybrid configuration sacrifices ductility for increased strength and stiffness, whereas screws in chestnut provide more flexibility. Case study VGS9x160 board with 40 mm • Screws in hybrid beech-silver fir had slightly lower ultimate load, and yield force than those in glulam beech but slightly greater stiffness and ductility. • Screws in glulam chestnut had a lower ultimate load and yield force but showed higher ductility. • Screws in hybrid configuration sacrifices ductility for increased strength and stiffness, whereas screws in chestnut provide more flexibility. From the results obtained it can be deduced that the use of hardwood elements is a valid choice for timber-to-timber composite (TTC) systems. Acknowledgements This research was developed in the framework of the Industrial Research and Experimental Development Projects GENESIS: "GEstioNE del rischio SISmico per la valorizzazione turistica dei centri storici del Mezzogiorno" - Project no. ARS01_00883 CUP: D96G18000160005 funded by the Ministry of Education, University and Research (MIUR) within the PNR 2015-2020 Specialization area: Cultural Heritage. The authors would like to thank the company Rothoblaas Srl for providing the screws and for their interest and technical support in this research. A sincere thanks to Dr. Roberto del Tosto for his kind contribution in the specimen processing.