PSI - Issue 78

Andrea Fabbri et al. / Procedia Structural Integrity 78 (2026) 1183–1189

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climate change mitigation (Sathre and O’Connor, 2010). From a circular economy perspective, replacing energy intensive materials with timber and reusing it as an energy source at the end of its service life can significantly reduce CO₂ emissions (Gustavsson et al., 2021). Recent advances in manufacturing and prefabrication techniques have expanded timber's application potential. Complex, modular, and high-tech structures are now feasible (Bukauskas et al., 2019; Hua et al., 2022). Moreover, the use of unconventional or low-grade timber species (Burton et al., 1998; Brito and Calil Jr., 2010), including innovative materials like glubam (Xiao et al., 2014), opens new opportunities for lightweight and sustainable design. In this context, European beech LVL represents a promising engineered wood product for structural applications. It combines high mechanical strength, good internal homogeneity, and compact aesthetics (Knorz and van de Kuilen, 2012). Its mechanical performance makes it suitable for both tension and compression applications (Dill-Langer and Aicher, 2014), provided that instability phenomena are properly managed. For equivalent structural demand, LVL elements can be smaller in cross-section compared to solid timber. While this offers practical advantages, it poses challenges for joint design. Reduced cross-sections are particularly sensitive to the edge distance requirements prescribed by design codes (European Committee for Standardization, 2014). Several recent studies have investigated steel-to-timber connections for slender beech LVL members (Fabbri et al., 2022; 2025b). These include traditional configurations with plates and bolts, as well as more innovative systems. Among the latter are dowel – nut connections, which use transversely inserted cylindrical connectors, and threaded insert connections, with steel inserts screwed parallel to the grain. Such connections can be used to link timber members directly to nodes, as in spatial truss structures with spherical steel joints (Fabbri et al., 2025a), or to connect bracing elements to steel, concrete, or timber frames. In seismic applications, the performance of these systems can be enhanced by introducing energy dissipators at the ends of the braces. Devices such as resilient slip friction joints (Hashemi et al., 2020; 2021) can improve the hysteretic response by dissipating cyclic energy. In these systems, the connection between the dissipator and the timber member becomes a critical design aspect, especially when the timber cross-section is small. This study investigates the cyclic performance of two steel-to-timber connections: a dowel – nut connection with a transverse cylindrical connector, and a threaded insert connection aligned with the grain. Experimental results are presented to evaluate stiffness, strength, and hysteretic behavior. The aim is to assess their suitability for seismic applications and propose strategies to enhance their performance under cyclic loading. 2. Previous studies on dowel-nut connection The dowel – nut connection employs a cylindrical steel connector inserted transversely into the timber member. The connector includes a threaded hole, orthogonal to its axis, into which a threaded rod is screwed. This solution combines constructional simplicity, good mechanical performance, and limited steel usage. A schematic view is shown in Fig. 1a. The connection has demonstrated good performance under both monotonic (Kobel et al., 2014) and cyclic loading. To enhance its compressive behavior under cyclic loading, the application of an initial tightening torque has been proposed by Fabbri et al. (2022). To investigate brittle failure mechanisms, monotonic pull – pull tests were carried out and complemented by numerical simulations using a regularized XFEM model (Benvenuti et al., 2024). Fig. 2 shows cyclic tests on dowel-nut connections. The specimens, made of beech LVL, have a constant cross section of 50×50 mm. The cylindrical connector, with diameter d =20 mm, is inserted transversely into the timber element. The longitudinal distance from the loaded end ( a 1 ) is set to 50 mm (2.5 d ), 100 mm (5 d ), or 150 mm (7.5 d ), while the edge distance in the transverse direction ( a 2 ) is constant and equal to 1.25 d . For a 1 =2.5 d , the connection exhibited an essentially elastic response, with failure occurring at relatively low loads. Higher failure loads and a plastic response were recorded for a 1 =5 d and a 1 =7.5 d , with localized deformation in the timber. In some tests with edge distances a ₁=5 d and 7.5 d , as shown in Fig. 2d,f, reducing the yield strength of the connector (S355) led to increased ductility. Significant plastic deformations in the connector were also observed. However, this behavior was not consistently reproduced across all tests. Further experimental investigations are therefore required to validate the observed trend.