Conservazione dei centri storici: studio della vulnerabilità sismica e interventi di rinforzo innovativi e compatibili con i principi del restauro. Il caso di studio dell’area UNESCO di Firenze

The seismic events that have affected Italy in recent decades have highlighted the high vulnerability of historic urban centres. These areas consist of masonry building aggregates shaped by centuries of construction practices and typological transformations that have significantly altered their geometric configuration and structural behaviour. The unique environmental, cultural, and historical characteristics of each historic centre prevent the adoption of standardized methodologies for assessing the seismic vulnerability of masonry buildings. Therefore, a comprehensive understanding of traditional construction techniques and the evolutionary processes that have taken place over time represents a fundamental prerequisite for any vulnerability assessment. With the aim of performing a qualitative urban-scale analysis of the seismic vulnerability of aggregated masonry buildings in the historic centre of Florence, a UNESCO World Heritage Site since 1982, studies on the city's main architectural typologies and their evolutionary processes were used to identify the principal vulnerability factors arising from transformations that are inconsistent with the original structural configuration. In particular, a block-scale survey of 643 aggregated buildings, selected as representative of the historic urban fabric, identified three recurring sources of vulnerability associated with the processes of densification and infill that have occurred over the centuries: the infilling of internal courtyards through the incorporation of additional volumes into pre-existing building units; the construction of extra residential storeys on the original structures using materials, technologies, and morphological solutions incompatible with the existing fabric; and the alteration of openings on the main façades in terms of both size and location. Using the second-level GNDT approach, integrated with the results obtained through the macroseismic methodology, and with the objective of effectively representing the vulnerability factors identified within the historic built environment, a modification of the second-level GNDT assessment form was proposed through the introduction of three new parameters. Their corresponding scores and classification ranges were calibrated on the basis of the survey results and the conducted studies, according to engineering judgement. The comparison between vulnerability indices obtained from a sample of 40 buildings, selected according to their typological characteristics and degree of transformation and evaluated using the second-level GNDT form with and without the proposed parameters, demonstrated that the modified methodology is capable of distinguishing, within the established GNDT Level II framework, those buildings whose inappropriate transformations have significantly increased their seismic vulnerability. The application of the method also enabled the derivation of vulnerability and fragility curves and, consequently, the development of damage and loss scenarios for different seismic intensities. At the scale of the individual structural unit, the analysed case studies revealed a marked susceptibility to simple overturning mechanisms involving the upper portions of buildings, primarily associated with incompatible vertical extensions. Such failure mechanisms can be effectively mitigated through innovative confinement techniques employing fibre-reinforced composite systems with lime-based matrices. Considering the critical role of the composite matrix in ensuring material compatibility, the final part of the research focused on the development of a methodology for identifying a repair mortar consistent with both conservation principles and the characteristics of the analysed historic masonry. The methodology, applicable to a broad construction context such as that of Renaissance Florence, was based on the investigation of 14 historic bedding mortar samples collected from nine monumental and residential buildings representative of the fourteenth and fifteenth centuries AD within the historic centre, leading to the proposal of a new mix design compatible with the analysed historical materials. To achieve this objective, the results of Digital Image Analysis (DIA) performed on thin sections extracted from the historic mortar samples and examined under polarized light optical microscopy were integrated with data obtained from in situ penetrometric tests and laboratory chemical and physical characterization tests conducted on the same specimens. Statistical analysis based on different measures of central tendency made it possible to define compatibility standards for the repair mortar by establishing threshold values for water-accessible porosity and compressive strength. The acceptable ranges for these parameters were quantified to assess the extent to which the proposed mortar reproduces and is therefore compatible with the original historic material. The same chemical, physical, petrographic, and mechanical laboratory investigations were subsequently carried out on specimens produced using the new mortar mix. The comparison of the results confirmed its compatibility not only in terms of mechanical and physical performance but also with respect to the original manufacturing techniques and the provenance of the constituent materials. According to this innovative procedure, once quantitative compatibility criteria based on physical and mechanical properties have been established, the mortar mix design can be progressively adjusted and refined through an iterative process.