Reliability-Based Structural Assessment of Monumental Heritage Structures: Application to St. Peter’s Basilica in Vatican City

The structural assessment of monumental masonry buildings is challenging because of their complex geometries, heterogeneous materials, multi-leaf walls, large-span vaulted systems, and significant uncertainties in mechanical properties and construction details. Although probabilistic methods and reliability-based safety formats are increasingly applied to modern and existing structures, their use in monumental masonry heritage remains limited. This thesis develops an integrated framework for the reliability-based structural assessment of complex heritage buildings and applies it to the 17th-century body of St. Peter’s Basilica in Vatican City.The proposed methodology combines historical and construction knowledge, experimental diagnostics, multiscale numerical modeling, probabilistic material characterization, Bayesian updating, and global non-linear finite element analysis. A particular focus on the Opus Caementicium walls of the Basilica, consisting of an Ancient Roman Concrete core enclosed by brickwork leaves and, in some areas, external travertine masonry. Data obtained from ultrasonic tests, dynamic identification, endoscopic surveys, flat-jack tests, and laboratory tests on extracted cores are interpreted through three-dimensional numerical analyses. Drucker–Prager models are first calibrated to reproduce the compressive response of brickwork masonry and Ancient Roman Concrete, supporting the homogenization of the multi-leaf walls into equivalent materials suitable for global analyses.Material uncertainty is subsequently represented through a probabilistic Concrete Damage Plasticity model. The main parameters governing the compressive and tensile stress–strain responses are treated as random variables. Prior distributions derived from literature and standards are updated using experimental evidence through Bayesian inference, while stochastic non-linear analyses are used to investigate the resulting variability in the homogenized properties.A detailed three-dimensional non-linear finite element model of the Basilica is then developed, including the main walls, piers, vaults, secondary domes, and foundations. The model is calibrated using dynamic identification results and employed to investigate stress distributions, damage patterns, critical structural regions, and global safety under gravity loading.Finally, the probabilistic material models and global simulations are incorporated into a semi-probabilistic safety format in which partial factors for material properties and action effects are calibrated as functions of target reliability indices. The framework takes into account both aleatory and epistemic uncertainties, allowing reliability indices and failure probabilities to be estimated. The proposed method can be applied to other monumental masonry structures and provides a straightforward basis for long-term structural monitoring, conservation planning, and reliability-based assessment.