PSI - Issue 78

Valentina Picciano et al. / Procedia Structural Integrity 78 (2026) 1167–1174

1168

1. Introduction In the context of transportation infrastructure, bridge structures represent critical elements whose safety, functionality, and operation are essential to ensuring the continuity of the flow of people and goods—a prerequisite for the economic and social development of a country (Di Prisco 2019). Following the end of the Second World War, Italy underwent significant infrastructural development, leading to the construction of the country’s main motorways between the 1950s and 1970s, and consequently, of most existing bridges and viaducts still in service today. Structural ageing—exacerbated by the onset of more or less aggressive deterioration mechanisms—combined with increased traffic loads and the resulting reduction in structural safety levels, characterises the current condition of the majority of bridge structures across the country. As such, extraordinary and targeted assessment, maintenance, and intervention plans are urgently required. Among the various structural typologies employed in the construction of bridges and viaducts, the Gerber system, characterised by the combination of simply supported spans and adjacent cantilever spans, saw widespread adoption. This was largely due to the theoretical and practical advances in reinforced concrete and prestressed reinforced concrete technologies during that period (Buratti et al. 2019). While the Gerber system offered advantages in terms of construction speed and the possibility of using isostatic rather than hyperstatic schemes, it has also revealed vulnerabilities over time. Specifically, Gerber saddles—the structural components that connect the simply supported spans to the cantilever arms—have proven to be particularly critical from a durability standpoint due to their geometric configuration. Degradation phenomena often concentrate near these joints, primarily due to continuous percolation of water from the deck. Moreover, from a structural point of view, the stress concentration typical of Gerber saddles, coupled with their inherently brittle behaviour, can lead to the failure of individual components or even the entire bridge, as occurred in the case of the Annone overpass collapse in 2016 (Di Prisco et al. 2023). Therefore, these elements demand special attention, as the structural safety and operational continuity of many existing reinforced concrete bridges depend on them. The critical issues associated with Gerber saddles have also been highlighted in the Italian Guidelines for the classification and risk management, safety assessment, and monitoring of existing bridges (MIT 2020), issued in response to the Morandi Bridge (Genoa, Italy) collapse, with the objective of defining standardised procedures for comprehensive management and maintenance of the national infrastructure network, and in particular, critical infrastructures. Within this context, the present paper investigates the structural behaviour of Gerber saddles in reinforced concrete bridges, with the aim of proposing optimised intervention strategies to enhance both structural capacity and the overall resilience of these infrastructures. The research adopts a multi-step methodological approach comprising: (i) a comprehensive review of the state of the art, supported by the development and analysis of a database of experimental tests on Gerber saddles from the literature; (ii) the application of machine learning techniques to develop regression models capable of predicting the load-bearing capacity of half-joints based on the collected data; (iii) the critical analysis of strengthening techniques; and (iv) the use of advanced nonlinear numerical modeling applied to case studies, leading to the proposal of a design methodology aimed at optimizing post-tensioning interventions and assessing their performance improvements. 2. Structural behaviour and experimental database of Gerber Saddles Gerber saddles (aka dapped-end beams or half-joints), represent a critical structural detail commonly found in both precast and cast in place concrete bridges and viaducts. These elements are characterised by abrupt geometric discontinuities at the ends of beams, creating localised regions with non-uniform stress distributions. As such, they are classified as disturbed regions (D-regions), where the assumptions of linear stress distribution based on Bernoulli’s principle are no longer valid. Consequently, their design and assessment require specific methodologies capable of accurately capturing their complex mechanical behaviour and failure mechanisms. Traditional design and assessment methods include analytical lower-bound approaches such as strut-and-tie models (STM), as codified in Eurocode 2 (EN 1992-2 2005) and the PCI Design Handbook (PCI 2010). STM idealises the internal force flow using equivalent truss mechanisms, providing conservative estimates of strength while guiding the design of effective reinforcement layouts (Picciano and Santarsiero 2024).