PSI - Issue 84

Alessandro De Iasio et al. / Procedia Structural Integrity 84 (2026) 952–958

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1. Introduction Extreme flash floods are increasing in frequency and intensity due to climate change (Hirabayashi et al., 2013; Otto et al., 2018). Post-disaster surveys show that extreme floods cause extensive structural damage to residential houses (Wüthrich et al., 2024) and bridges (Pucci et al., 2023). In addition to hydrostatic and hydrodynamic pressure loads, flood actions include waterborne debris impacts, which create high-strain-rate loading conditions in structures (De Iasio et al., 2023). The impact force-time (F-t) diagram is governed by the debris-structure interaction, which depends on debris mass, stiffness, and velocity, and on structural mass and stiffness (De Iasio et al., 2025b), including structural nonlinear behaviour (De Iasio et al., 2025a). Bridges and viaducts are critically exposed to flood actions when they cross floodplains (Kazantzi et al., 2025). As such, their piers are directly exposed to flood actions. Recent studies have highlighted that waterborne debris impacts can cause both local and global damage to masonry bridges (Majtan et al., 2023) and the dynamic serviceability behaviour of short-span bridges (Ma and Zhang, 2022). Nevertheless, the effects of debris impacts on reinforced concrete (RC) viaduct piers have not been studied to date. These structures are peculiar since their pier can have significantly large dimensions in height and cross-section. Moreover, these cross-sections commonly have box geometries with one or more cells, which likely lead to non-negligible local behaviours under concentrated forces, such as debris impacts. This study aims to investigate the local and global behaviour of an RC viaduct pier under waterborne debris impacts. In particular, these investigations focus on the accuracy of an equivalent wall model representing the portion of a rectangular multi-cell box cross-section impacted by a debris, in place of a model representing the entire pier geometry. High-fidelity simulations, directly representing the debris-structure interaction with debris and structure explicitly represented and interacting with each other, are used following the modelling strategy developed and validated in De Iasio et al. (2024). An initial reference simulation of the entire pier is performed. Successively, an equivalent wall model is proposed to represent the pier portion impacted by the debris. Different boundary conditions are analysed. 2. Methodology 2.1. RC viaduct pier The model of the studied RC viaduct pier is defined using data from an existing RC viaduct. In particular, the viaduct “Pesco di Faggio” in southern Italy (Riccia, Molise region, see Fig. 1-a) is selected and modelled using data collected during previous survey studies (Rocco, 2013). The viaduct, built around 1975, is straight with a total length of 284.5 m. It has 8 spans of approximately equal length (36 m), each consisting of 5 simply supported prestressed concrete beams and a top concrete slab forming the roadway, with a width of 11.50 m (Fig. 1-b). Pier heights vary with the ground profile, with a maximum height of 45 m for pier n.4 (Fig. 1-b). The viaduct crosses the basin of the River Succida (Fig. 1-b). Piers are made of reinforced concrete with a multi-cell rectangular cross-section, with global dimensions of 10.6 x 3.8 m 2 and a wall thickness of 0.3 m (Fig. 1-c). Beams are directly supported by pier tops, as no pier cap is present. A standard neoprene layer is placed at the interface between the pier and the beams. Assumptions regarding material properties, particularly concrete, are as follows: cylindrical concrete compressive strength = 28.23 and Young’s modulus = 30 . This study focuses on pier n.4 (see Fig. 1-b), which is at bridge mid length, close to the riverbed. This pier is 45 m tall. Based on a rational, simplified load analysis, the total permanent load for these two spans is 225 kN/m. Assuming a tributary area equal to half of each span, the total load acting on the pier top is = 8100 , which corresponds to a mass = 825688 .