PSI - Issue 78

Riccardo Raimondo Milanesi et al. / Procedia Structural Integrity 78 (2026) 1374–1381

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Focusing on the batch 1, the undamaged panels (T1 and T4) exhibited natural frequencies between 35.4 Hz and 39.3 Hz, variations attributable to material properties and construction details. In‑plane drift caused rapid stiffness loss, with all specimens sho wing over 20 % frequency reduction by 0.30 % drift . During subsequent out‑of‑plane excitation, T2 and T3 incurred an additional > 40 % degradation at only 0.20 g PFA; by contrast, the undamaged T4 reached the same degradation only at 0.75 g PFA. Furthermor e, a discussion about the frequencies of the infills and innovative techniques to capture them is reported by Bolognini et al. (2024). The degradation of the fundamental frequency of the infill T1 is, as an example, reported in Figure 8. By correlating the measured in‑plane drifts with the peak out‑of‑plane forces at the panel centre, it is possible to define the interaction between cyclic in‑plane displacements and dynamic transverse loading. Figure 8 presents the maximum out‑of‑plane force plotted against the mean drift achieved over the final three in‑plane cycles (positive and negative), also including the ultimate force for T5. Here, forces correspond to out‑of‑plane collapse for T2 – T4, to ultimate conditions for T5, and to incipient in‑plane collapse for T1. These results are drawn from batch 1, although the same procedure may be applied to all batches for direct and cross‑specimen comparisons.

Figure 8. Influence of the in-plane drift damage on the fundamental frequency of the infill T1 (left) and the out-of-plane force resistance of batch 1 (right). (Morandi et al., 2025). 4. Conclusions and Future Developments In response to the limited scientific literature on dynamic testing of existing masonry infills, especially studies that account for their interaction with in‑plane seismic behavior, the Eucentre Foundation has undertaken an extensive experimental program of nineteen full‑scale tests, organized into four homogeneous batches, on slender infills representative of Italian reinforced‑concrete frames constructed between the 1960s and 1980s. Steel – concrete composite frames were filled with single‑story, single‑ba y masonry panels and subjected to a range of boundary conditions (fully bonded perimeters or gaps at the top and sides), two unit thicknesses (12 cm and 8 cm) and both solid and with opening configurations. Each batch was evaluated by means of in‑plane pseudo‑static cyclic loading, dynamic out‑of‑plane shake‑table tests and combined sequences in which panels were first cycled in‑plane and then dynamic out‑of‑plane up to failure. This campaign has highlighted the progression of damage under varying support conditions, unit thickness (infill slenderness) and opening arrangements, enabled the development of force – drift response charts for out‑of‑plane loading, and facilitated mapping of damage and acceleration patterns as well as identification of predominant arching and bending failure mechanisms. For each batch, the evolution of damage patterns and limit states has been characterized, the in‑plane force – displacement response has been quantified, the out‑of‑plane deformed shapes during dynamic testing has been captured, the fundamental frequencies o f the panels have been measured and the reduction in out‑of‑plane resistance as a function of the maximum in‑plane drift attained has been assessed. The study is currently ongoing, with several future developments under investigation: the influence of openings and panel thickness on both in‑plane and out‑of‑plane seismic response; the development of a rapid assessment procedure using audio‑wave interrogation (Bolognini et al., 2024); experimental protocols for simultaneous in‑plane and out‑of‑plane loading; analysis of the energy‑content effects under varying loading histories (Gentile et al., 2025); refinement of design and assessment guidelines based on these experimental results according also to the procedure reported in Morandi et al. (2025); and complementary numerical studies (Kurukusaruriya et al., 2025).