PSI - Issue 78

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

1187

4.1. Cyclic Behavior of the Dowel-Nut Connection The dowel – nut connection exhibited good performance under both tensile and compressive loading. The application of an initial tightening torque introduced a distinct behavioural difference between the two load directions. Under tension, the load is transferred to the timber element through direct contact with the cylindrical connector. In compression, however, the load is primarily transmitted to the specimen head, effectively unloading the connector. This mechanism helps prevent buckling of the threaded rod and enables a more uniform load distribution over a larger area, resulting in increased compressive strength. As shown in the F – δ curves in Fig. 2, the configurations with edge distances of 5 d and 7 d demonstrated favourable ductility. This behaviour is mainly attributed to localized plastic deformation in the contact region between the nut and the timber surface. In previous studies (see Fabbri et al., 2022), reducing the yield strength of the connector did not consistently lead to a significant increase in plastic deformation. However, in certain tests, greater plastic strains in the connector were observed, potentially improving the global behaviour of the connection. For this reason, the use of steel with even lower yield strength may be worth exploring, provided that an appropriate experimental campaign is conducted. Nevertheless, particular attention must be paid to the potential reduction in overall load-carrying capacity. An alternative approach could involve locally machining the threaded rod to introduce a region with properly reduced yield capacity. This controlled weakening may enhance the dissipative capacity in tension by localizing deformations. However, it would have no significant effect under compressive loading, where the rod remains unstressed. In this case, it is also essential to consider the interaction between localized deformation and potential loss of pretension. Such a scenario may activate a secondary load-transfer mechanism, whereby the force is transmitted directly from the nut to the timber. This increases the risk of longitudinal threaded rod instability. All observed failures were brittle, attributed to the limited connector-to-edge distances. Specifically, the transverse edge distance was only 1.25 d . Increasing this distance could promote a more ductile failure mode, potentially delaying — or in some cases preventing — premature rupture. Finally, it should be noted that increased ductility does not necessarily translate into higher energy dissipation capacity. The connection exhibited limited hysteretic cycles, primarily penalized by significant residual displacements upon unloading. 4.2. Cyclic Behavior of the Threaded Insert Connection The threaded insert connection, unlike the dowel-nut system, exhibits a more consistent response between tension and compression. The pre-drilling required for installation reduces the contact area at the base of the connector. Although modest, this reduction appears sufficient to equalize the stiffness in both loading phases. Nevertheless, some overstrength in compression was still observed. The connection displays high initial stiffness, but brittle failure — promoted by the limited edge distances — prevents the development of a ductile response. As a result, the hysteresis loops are narrow, and energy dissipation remains limited. The use of larger cross-sections, with increased edge distances, could enhance the ductility of the connection by reducing the risk of premature failure. Based on these observations, this type of connection may be effectively employed in combination with a seismic energy dissipation device. 4.3. Incorporation of Dissipative Device at one Bracing End The integration of a seismic energy dissipation device (Hashemi et al., 2021) can significantly enhance the hysteretic performance of the system. Acting as a “controlled fuse,” the damper ensures wide and stable hysteresis loops, thereby improving the overall seismic response. Deformations are concentrated in replaceable components, preserving the integrity of the primary connections and promoting recentering and elastic recovery after the event.