PSI - Issue 84

L. Fieno et al. / Procedia Structural Integrity 84 (2026) 1127–1135

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1. Introduction and historical background Although concrete was known and used as early as the nineteenth century, the systematic introduction of steel reinforcement occurred only toward the end of that century. In this initial phase, reinforced concrete was applied to arch bridges according to criteria directly derived from the masonry tradition. Early structures were therefore conceived as massive arches, in which the new material replaced stone or brick without a full understanding of its specific mechanical properties. The first reinforced concrete arch bridges consequently featured moderate spans and conservative static configurations. Nevertheless, they immediately demonstrated the potential of the new material, allowing for reduced cross-sections and greater formal freedom compared with masonry structures. These early experiences formed the basis for subsequent developments, paving the way for more daring structural solutions. In this context, arch bridges represented an ideal field of experimentation, combining the elegance of a traditional structural form with the potential offered by the new material. However, many of these bridges continued to reflect a conceptual approach derived from masonry construction, with relatively massive arches and extensive use of spandrel infill. Although their structural performance was high, the use of the material was not yet fully optimized, and the overall static behavior did not always reflect a design truly tailored to reinforced concrete. A fundamental turning point in the history of reinforced concrete arch bridges was introduced by Robert Maillart. His contribution was not limited to new formal solutions, but was primarily rooted in a deep understanding of the structural behavior of reinforced concrete. Maillart conceived the bridge as an integrated system, in which the arch and the deck act in structural collaboration. The adoption of extremely slender arches, the elimination of superfluous infill, and the systematic use of three-hinged arches made it possible to reduce hyperstatic stresses and effectively control imposed deformations. These principles led to the construction of works characterized by an exemplary balance between structural efficiency, material economy, and formal elegance, profoundly influencing the design of arch bridges throughout Europe. In parallel with the development of upper-deck arch bridges, the tied-arch typology also became established, in which the horizontal thrust of the arch is resisted by a tension element, the tie or chain. Originally developed for steel structures, this solution was later adapted to reinforced concrete, allowing the construction of arch bridges even under unfavorable geotechnical conditions or in urban contexts where the transfer of horizontal thrusts to the foundations is limited. In this configuration, the deck itself—comprising longitudinal girders and the slab—performs the function of the tie. Some examples adopt vertical reinforced concrete hangers arranged at constant spacing from which the deck is suspended; in other cases, the hangers are made of steel. A particular evolution of this typology is represented by arch bridges with inclined hangers, known as Nielsen-type bridges, which are characterized by increased stiffness and a reduction in bending stresses in both the arch and the deck. The construction methods of these works provide a clear example of how the engineering of the time, fully aware of the limitations of concrete in tension zones, sought to anticipate the principles underlying the adoption of pre-stressing. Indeed, numerous arch bridges—particularly tied-arch structures—employed construction techniques based on staged execution, the use of active anchors, or temporary weights to be removed during the construction process. These measures were aimed at inducing more favorable stress states in predominantly tensioned elements, such as the tie and the hangers, thereby reducing tensile stresses and improving the overall structural performance of the bridge. These structures are often located along primary arterial roads, frequently adjacent to industrial development areas, which have experienced a significant increase in heavy traffic since the post-World War II period. In many cases, the evolution of operational requirements has necessitated repeated strengthening interventions, of varying effectiveness, which today require conservative rehabilitation measures. Only in relatively recent times, however, has the growing