PSI - Issue 78

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

1186

3. Threaded insert connection Steel-to-timber connections commonly employ metal plates, which allow for the use of multiple fasteners and enable quick assembly. The plates can be placed either externally or internally within the timber member, with the choice often driven by architectural considerations. The connection to the timber element is typically achieved using mechanical fasteners such as bolts, dowels, or screws. Screws offer the advantage of providing both shear and withdrawal resistance, although they generally come at a higher production cost. They are typically used for loading perpendicular to their axis, as their axial tensile capacity is limited — particularly when the angle between the screw axis and the grain direction is less than 30° (European Committee for Standardization, 2014). As an alternative to conventional solutions, for elements primarily subjected to axial tension or compression — such as bracing members — threaded metal inserts installed parallel to the veneer layers are of particular interest. This type of connection is simple, quick to install, and only requires a pre-drilled hole. A schematic view of the connection is shown in Fig. 1b. To evaluate the monotonic and cyclic behavior of these connections, experimental tests were conducted using the same setup as in Fabbri et al. (2022). The results are shown in the Fig. 3. The tests were performed on European beech LVL members with a cross-section of 60×60 mm, fitted with threaded inserts having an outer diameter of 25 mm. Two connector lengths ( L ) were tested: 60 mm and 80 mm. For each geometric configuration, three monotonic tests and one cyclic test were carried out. The monotonic F – δ curves showed an initially very stiff response (approximately 170 kN/mm) up to the peak load, followed by a rapid drop and then a gradual degradation. Failure consistently occurred by pullout of the insert, accompanied by splitting cracks either parallel or orthogonal to the veneer layers, partially or fully extending across the cross-section. The cyclic test exhibited similar behavior in tension and compression; however, failure always occurred under tensile loading. For the 60 mm long connector, a compressive load of 48 kN was reached an increase of 35% compared to the tensile capacity. Under cyclic loading, the tensile response was similar to the monotonic one, with failure load variations of +2.9% (60 mm) and – 18.4% (80 mm). 4. Cyclic Behaviour of Connections: Assessment and Improvement Perspectives Timber bracing elements are typically integrated into timber frames to ensure adequate resistance to horizontal actions. They can also represent an effective strategy for the seismic retrofitting of existing steel or reinforced concrete structures, offering both structural and environmental benefits. The connection between the brace and the frame structure can be realised through the types of joints previously described. To be effective under seismic conditions, such joints must be capable of dissipating energy and exhibit a stable and repeatable hysteretic response. Although not specifically designed for seismic applications, the connections investigated in this study demonstrated promising mechanical behaviour, deserving further investigation. This section discusses the main experimental findings and outlines strategies to improve their seismic performance.

Fig. 3 Force – displacement diagrams from tests on specimens with threaded insert connections of length (a) 60 mm and (b) 80 mm. Grey curves refer to monotonic tests; blue curves refer to cyclic tests.