Development of innovative methods for the seismic risk classification of masonry aggregates

Historical centres are the result of a process occurred during the centuries: masonry buildings constituting the urban environment are usually organized within Structural Aggregates (SAs) made up of Structural Units (SUs), connected in a variety of different ways, characterized by heterogeneous structural features, materials and architectures. The morphological variety of the resulting urban settings enriches the cultural heritage of a place, meanwhile, increases local and global vulnerabilities towards static and seismic actions. Consequently, the study of transformation processes of a historic centre is the first step for the prevention and mitigation of seismic risk. Another important assumption, as result of the first one, is that the protection of historical centres requires many resources in terms of assessing the seismic vulnerability and implementing retrofit strategies considering each individual building and their mutual interaction. In this sense, the mitigation of the seismic risk of historical built heritage has to be performed through successive in-depth process: firstly, the identification of the most vulnerable aggregates by means of urban scale screening; secondly detailed analyses on these aggregates to identify specific critical issues. The aim of this research activity is to provide new tools for the analyses of seismic risk on urban scale, in order to forecast damage scenarios and economic impact of seismic events on masonry aggregates: four clear objectives are accomplished through the development of four research tasks structured along the eight chapters that compose this document. The first part of the thesis provides a general overview of all those necessary procedures for a correct identification of the aggregate and its corresponding seismic vulnerability assessment. The literature review and on-site inspection, carried out in the case study of Campi Alto di Norcia in Valnerina (Italy) stroke by the earthquakes of 24 August and 30 October 2016, highlight advantages and disadvantages of current approaches and tools used for the vulnerability assessment of masonry aggregates. The second part deals with the development of an improved Empirical /Expert base method for the seismic vulnerability assessment of SAs on urban scale. Indeed, different statistical methods are selected and applied to the case study with the aim of determining the corresponding vulnerability index (Iv) and comparing their accuracy in predicting damage scenarios. Since recent earthquakes have shown that local mechanisms often cause failures of masonry aggregates, new parameters are introduced in the Improved Method, to take into account the out of plane behaviour of structural units in aggregates. The third part of the thesis addresses the calibration of the Improved method, by comparing the ‘observed’ damage caused by the 2016 earthquake in the case study with the ‘analytical’ ones predicted. To overcome the limits of the Empirical /Expert base methods, analysed in the previous phases, an additional Information Quality (IQ) index is, then, introduced in the calibrated procedure accounting for uncertainties related to the lack of information due to limited access or inspections. Finally, the work ends with the elaboration of codified procedure, identified with the acronym “MARS” (Masonry Aggregate Risk Scenarios), that takes into account damage, loss and economic impact of seismic events on masonry aggregates and provide further information for the seismic risk management of historic centres.