PSI - Issue 78

Federica Rauseo et al. / Procedia Structural Integrity 78 (2026) 473–480

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1. Introduction Reinforced concrete (RC) buildings constructed before the enforcement of modern seismic codes are widespread across Europe and are typically characterized by limited ductility, non-seismic detailing, and gravity-load-based design. In Italy, a large portion of the existing building stock was designed in the 1950s and 1960s following the provisions of Royal Decree RD 16/11/1939, which did not include any seismic criteria. Those buildings often rely on unidirectional RC frames, flat beams and joist slabs, sometimes lacking structural elements such as foundation beams or transverse ties that could enhance global redundancy. Vulnerability of those RC buildings can be further exacerbated by cumulative effects of ageing and environmental degradation. Among the main degradation processes affecting ageing RC buildings, reinforcement corrosion plays a critical role, progressively reducing cross-sectional areas, mechanical properties of materials and bond between concrete and steel. The behaviour of heavily corroded RC structures subjected to seismic action is generally more brittle than the as built condition, and it is characterized by lower maximum displacements and reduced peak strength (Michelini et al., 2023; Liu et al., 2025). Moreover, corrosion can alter the hierarchy of failure, as well as the failure mechanism, which can shift from flexure to shear (Pantazopolou et al., 2025). These adverse effects are incorporated into numerical models in a variety of ways, depending on the specific approach that is adopted. When a lumped plasticity approach is followed, corroded primary elements are modelled as elastic elements in series with nonlinear hinges, whose behaviour is described through a properly modified backbone curve calibrated on the basis of experimental results, with reduced initial stiffness, peak load capacity, peak displacement, and ultimate load capacity (Dai et al., 2020; Pantazopolou et al., 2025). In fibre-based approaches, the effects of corrosion are instead incorporated by modifying the constitutive relations attributed to steel and concrete fibres (Afsar Dizaj & Kashani, 2022) and reducing the rebars’ cross-section over time. In addition to material degradation, existing RC buildings may suffer from pre-existing structural damage due to phenomena like differential soil settlements. Those settlements ―which can be caused by heterogeneous soil conditions, excavations, or long-term subsidence ― do not degrade materials directly while producing non-uniform displacements at the base, which in turn induce additional internal forces, cracking, and second-order effects. Differential settlements under RC buildings have been investigated through various modelling strategies. Simplified approaches, such as the Limiting Tensile Strain Method (Burland and Wroth, 1974), offer quick damage estimates while overlooking the role of structural configuration. More refined studies developed finite element (FE) models assigning settlements to either single columns (Lin et al., 2017) or all supports evenly (Fotopoulou and Pitilakis, 2021). These methods have been used to develop fragility curves (Parisi and Acconcia, 2021), while fully coupled soil– structure interaction models are reserved for detailed case studies (Karafagka et al., 2021). Although corrosion and differential settlements have been investigated as separate phenomena, their impact as sources of pre-existing damage in seismic assessment has received comparatively less attention. In real contexts, especially in older RC buildings, these forms of degradation may already compromise the structure before any seismic event occurs. As a result, the residual capacity is affected not only by the severity of seismic action, but also by the extent and distribution of prior damage, which is often uneven and hard to detect. In this study two numerical models ―which were developed via different software packages while sharing the same structural layout and modelling approach ― are subjected to nonlinear static analyses under a range of degradation scenarios. By isolating and combining the effects of corrosion and settlements, the purpose of this study is to evaluate how different levels of pre-existing damage ― resulting from reinforcement corrosion and/or differential settlements ― affects the seismic capacity of an existing RC building representative of pre-code construction. 2. Case study and modelling approach 2.1. Case study building The building selected as case study for the numerical analyses is representative of typical Italian structures from the 1950s–1960s, designed for gravity loads only (GLD) (Belletti et al. 2023, De Risi et al. 2023). The ground-floor plan of the prototype, shown in Fig. 1, qualitatively reflects the configuration reported by De Risi et al. (2023). The structure is a three-story reinforced concrete building with five by three bays, featuring unidirectional frames along