Sviluppo di procedure per valutazioni sistematiche di vulnerabilità sismica di edifici esistenti in muratura

An important element for the development of strategies for the prevention and the reduction of the seismic risk is the evaluation of the structural behaviour under seismic actions with destructive intensity: for existing buildings, the problem arises mainly in the assessment of their current susceptibility to damage, namely of their vulnerability. The procedures used for these assessments can have different levels of detail, depending on the quality and quantity of information that are acquired for each individual building. Considering the mitigation of seismic risk, these analysis should be performed on whole territories or municipalities, or on a significant number of buildings: this leads to the research of procedures for the vulnerability assessment that, starting from a rapid acquisition of information, allow to realize a reliable estimate of the level of seismic damage, without resorting to overly refined models. In addition, the architectural heritage and, in particular, the historical urban tissue of Italy and of many other countries with relevant seismic risk, are mostly made of stone masonry load-bearing buildings, mostly built in ancient times without any seismic standard. The traditional buildings can be considered as assemblages of statically determined elements mono-laterally constrained and essentially based on the indications of the so called "rule of the art": such arrangements are actually present only in important buildings, while are quite rare in ordinary buildings, with a consequent increase of their static and seismic weakness. These conditions imply a preference, for the purposes of a structural analysis for the evaluation of the seismic vulnerability, detailed verifications of elementary collapse mechanisms of individual elements, seeking possible weak points of the building, rather than studies that address the entire building considering the ideal behaviour of the elastic or elastoplastic multi-connected box. Finally, a territorial analysis, that moves its investigation scale from the single structural unit to the entire town centre and beyond, cannot ignore the fact that the masonry buildings are often organized into nuclei of buildings that interact in complex ways: this makes even more difficult the approach, the development of analysis and the interpretation of results. In recent years, different studies have been carried out (the most recent in the ReLUIS - Laboratories University Network of Seismic Engineering - projects), which led to propose a methodology that can be considered innovative towards the complexity of historical buildings and versatile both for predictive analysis and post-earthquake monitoring, both for the planning of mitigation actions. The method, already applied in several city centres, where significant specimens of buildings (structural units), belonging to different structural types, were found, considers an essential and preliminary knowledge phase, carried out with a strong interdisciplinary approach and based on the on-site data collection, by the use of special survey forms and implementation of minimum campaigns of experimental investigations. Starting from the available information historical, typological and structural analysis and studies of damage mechanisms and past interventions were developed and assessments of the seismic vulnerability of aggregated building systems through the application of automatic procedures, based on an estimate of the in-plane resistance and on the study of out-of-plane local mechanisms of structural macro-elements, were carried out: these procedures of limit analysis depends on few geometric and mechanical parameters, and therefore do not require an extremely accurate survey and a heavy computational burden. In this thesis, the latest versions of these procedures, updated to the codes developments, are outlined: by the application to three historical towns located in the central part of Italy, chosen as case studies (Campi Alto di Norcia, Castelluccio di Norcia and Sulmona), they showed their completeness and reliability, by the comparison of the results with the actual behaviour of the classes of existing masonry buildings towards recent seismic events, with the statistical forecasts given by the macroseismic scales and with mechanical-capacitive detailed models of the in-plane and out-of-plane behaviour. These models have allowed the calibration of the procedures and the development of computational and structural verification methods: in this sense, the capacitive approach developed for the study of in-plan behaviour was particularly innovative and, for this reason, particularized on the single case study of Sulmona. The general methodology systematize the vulnerability assessments for masonry buildings aggregates within city centres and lends itself to interesting applications in codes indications, related to the management and protection of the architectural heritage; the results obtained will also have noticeable exploitations in research activities and in concrete applications that are developing in Abruzzo as a result of the events related to the earthquake of April 6th 2009.