Experimental Ductility and Probabilistic Robustness Contribution of Punched Metal Plate Fasteners in Prefabricated Timber Truss Systems

Punched metal plate (PMP) fasteners are often a used connection type in modern prefabricated timber truss systems, used extensively in sustainable timber building design primarily for prefabricated houses. Their mechanical behaviour—particularly ductility and slip modulus—directly affects internal force distribution, system reliability, and structural robustness. This paper presents experimental results from 16 PMP specimens (Wolf 15N and Mitek GNA20, tested in tension and compression) and eight rotational stiffness specimens (Wolf 15N), alongside a probabilistic robustness assessment of six timber truss configurations. The experiments demonstrate that both plate types exhibit medium-to-very-high ductility (Cf = 0.60–0.89), with classification depending on plate type and loading direction: Wolf 15N shows very high ductility in tension (mean Cf = 0.86) and medium ductility in compression (mean Cf = 0.67). The longitudinal slip modulus of Wolf 15N (mean 2.59 N/mm3) agrees with the producer’s declaration (2.33 N/mm3, DIBt 2009); rotational stiffness (5.02–5.53 N/(rad·mm)) is reported for the first time on an experimental basis. Robustness analysis reveals a counter-intuitive but physically traceable finding: for the X-truss (RN2) with parallel compression subsystems, the nail plate spring model yields lower robustness factors than the brittle pin-jointed model for certain failure scenarios—meaning that the brittle assumption is non-conservative for that configuration. The difference in Frob reaches 1.0 unit for the most critical scenario, corresponding to one order of magnitude in failure probability. For all other configurations (N-truss, V-truss), the brittle model is conservative or equivalent. Importantly, the experimental ductility of PMP fasteners does not translate to improved system robustness unless structural redundancy is also present. The findings have direct implications for sustainable timber building design, where prefabricated truss systems form a key structural component.