PSI - Issue 84

Alessandro Scala et al. / Procedia Structural Integrity 84 (2026) 497–504

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critical drivers of severe damage and collapse. In this context, anthropogenic factors such as neglect or inadequate maintenance primarily act as predisposing conditions, which may exacerbate the effects of natural hazards rather than constitute the direct triggering mechanism. The Italian Guidelines for bridge risk classification and management aim to establish standardized and repeated procedures for surveillance and assessment of infrastructure condition. The systematic implementation of such procedures is expected to significantly reduce future distress associated with anthropogenic causes, particularly those related to negligence and insufficient maintenance. By contrast, natural hazards, and especially extreme events, remain inherently more challenging to address, both during the risk assessment phase and in the subsequent management of infrastructure networks, due to their sudden onset, high intensity, and limited predictability. Building on the evidence highlighted by Scala et al. (2025), the present study investigates how extreme events influence bridge collapse mechanisms by considering structural typology and construction material. To support this analysis, a synthesis table is introduced to enable a direct comparison of distress and collapse mechanisms associated with hydraulic and landslide-related events, under both ordinary and extreme conditions, highlighting the prevailing triggering cause and the dominant damage level among the three defined categories. 3. Results 3.1. Extreme events and structural type The influence of extreme weather events on damage severity was first investigated with reference to structural typology, focusing on arch bridges and girder structures, which represent the two most prevalent categories within the database. For both structural categories, the distribution of damage levels was analysed by distinguishing cases that occurred in the absence and presence of extreme weather events (Fig. 5). It should be noted that the sample size differs between the two conditions. In the absence of extreme events (Fig. 5a), the dataset includes 83 cases, comprising 28 arch bridges and 55 simply supported structures. When considering failures occurring in conjunction with extreme events (Fig. 5b), the sample is reduced to 31 cases, including 12 arch bridges and 19 simply supported structures.

a)

b)

Fig. 5. (a) Distribution of damage levels (DL1, DL2, DL3) for arch bridges and girder bridges in absence of extreme events; (b) Same classification for failures occurred during an extreme event.

Under ordinary conditions, the distribution of damage levels is broadly comparable between the two structural typologies. In particular, the relative frequency of partial collapses (DL2) is similar for arch bridges and simply supported structures. However, for girder bridges, an increase in total collapses (DL3) is observed at the expense of minor distress cases (DL1), suggesting a tendency toward more severe outcomes even in the absence of extreme meteorological forcing. When extreme events are considered, the overall damage scenario becomes markedly more severe for both structural categories. As shown in Fig. 5b, the proportion of DL1 cases decreases significantly for both arch and simply supported bridges, while total collapses (DL3) increase. This shift highlights the critical role of