The carrying capacity for vertical loads of well-maintained masonry arch bridges is reasonably high. This might not be the case for horizontal loads, the effects of which have not been the subject of extensive research aside from seismic occurrences. Arch bridges crossing rivers are subjected to sudden horizontal loads, due to river torrents, carrying debris from higher grounds. The magnitude of these horizontal loads is similar to those of coastal waves and debris; however, their effect on these structures has yet to be explored in detail. The narrow and high Devil’s Bridge across the Arda River (BG) and the wide, low Candia Viaduct across the Sesia River (I) were chosen as examples. Both are strongly exposed to fast-washing flow in the river during spring. FE simulations show that the impact of the rapidly rising river water influences the general stability, while the effect of debris mainly causes local damage. The results exhibit that tall, slender masonry arch structures fail due to the brittle fracture of the material, followed by the shear failure of a pier body. In contrast, lower and wider viaducts fail due to exaggerated tensile cracking in the upstream parts of a pier and the associated increasing pressure at its downstream parts.
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