PSI - Issue 78

Pasquale Bencivenga et al. / Procedia Structural Integrity 78 (2026) 1545–1552

1546

1. Introduction The assessment of the conservation state and safety level of existing bridges is currently a very important issue in the context of preservation of infrastructural heritage in Italy as well as in the rest of the world. The Italian bridge stock is significantly aging, as many structures were built over 50 years ago using outdated design criteria and thus potentially inadequate to withstand the traffic load models prescribed by current standards (Bencivenga et al. 2022). Moreover, recent studies have shown that these bridges may be in poor states of preservation (Salvatore et al. 2024, Rossi et al. 2023, Palmisano et al. 2023). Moreover, the original design documents from which the geometric and material properties of each structure could be derived are often unavailable. In this scenario, it results very difficult implementing control, management, and prioritization policies for interventions on all the structures (Zizi et al., 2023). Given this scenario, it is evident that the problem should be faced by adopting multi-level and holistic (i.e. including different source of risk) methodologies, such as the one introduced in the Italian Guidelines for existing bridges by Ministry of Infrastructures and Transport (2020). Within the framework therein proposed, bridges should be subjected to different measures (including accurate or fast assessment, monitoring activities, etc.) according to a synthetic judgment of the current state (so-called Attention Class). This helps reduce the financial and time-related efforts that could result from an extensive program of detailed assessments. Level 3 of this procedure involves conducting rapid structural checks primarily based on the construction age and the geometric characteristics of the structure. In particular, by comparing the traffic model of the technical code in force at the time of construction and the one of the current standards, it is possible to evaluate a synthetic safety index, defined as the past-to-current code demand ratio (Bozza et al. 2023). However, it is worth noting that this procedure, based solely on a comparison in terms of internal forces, does not take into account the capacity requirements of the original design. In fact, while the currently enforced traffic load model, together with load partial safety factors, results in significantly higher demands compared to past codes, the introduction of ultimate limit state approaches tends to overestimate the structural capacity when compared to the values derived from the admissible stress approach. It is therefore clear that a more refined approach at this assessment level may need to take into account the evolution of standards not only in terms of demand but also regarding the evaluation of structural capacity. Based on these premises, this work offers considerations for refining the fast assessment procedure proposed in the Italian Guidelines. In particular, by analysing the historical evolution of capacity assessment methods for reinforced concrete structures in past Italian codes, preliminary past-to-current capacity ratios are established. The research preliminary focuses on the bending problem, by assessing the capacity of randomly defined beam section according to different standards. 2. Evolution of Italian codes for the structural capacity assessment Since the early 20th century, a number of technical standards for the design of reinforced concrete structures had been progressively introduced, with some specifically dedicated to bridge construction. The regulatory framework originated with Royal Decree No. 2229 of 1939 (Ministry of Public Works 1939) and has since evolved, culminating in the current standards established by the 2018 Technical Standards for Construction (so-called NTC2018 by Ministry of Public Works 2018). Over the course of nearly a century, the design philosophy has undergone profound transformations, most notably with the 1996 regulation (Ministry of Public Works 1996), which definitively marked the transition from an allowable stress design approach to a limit state design methodology. Notably, the main difference between the two approaches lies in the stage of structural behavior they consider. The former methodology accounts for the second stage, where both concrete and steel are assumed to respond elastically, and the stress levels remain significantly lower than their ultimate capacities. In contrast, the limit state method focuses on the third stage of behavior, characterized by concrete failure due to the exceedance of its ultimate compressive strain ( ε cu = 0.35%). In both approaches, concrete is assumed to have no tensile strength. Between 1939 and 1996, the allowable stress design approach itself experienced significant developments, primarily influenced by advancements in the understanding of in-situ material behavior, refinements in the estimation of allowable design strengths, and improvements in specific analytical aspects, such as the homogenization coefficient n used to relate the elastic moduli of steel and concrete.