La valutazione della sicurezza sismica delle chiese a scala territoriale

Churches appear to be one of the most vulnerable types of building to earthquakes. The high seismic vulnerability is generally due to their structural arrangement and geometric proportions, material composition, and potentially deteriorated condition due to their age. More specifically, some intrinsic vulnerabilities are represented by wide halls, long thin span vaults, slender towering, slender walls with large openings, different constructive phases, etc. However, despite the great variability of structural weaknesses, the systematic observation and documentation of damage sustained by churches has led to definition of the macroelement concept, where the collapse mechanisms of church are based upon the failure of individual structural components (such as façades, side chapels, bell towers, and presbyteries) that are assumed to effectively behave autonomously. For each macroelement, by considering its typology and connection to the rest of the church, it is possible to identify the damage modes and the collapse mechanisms. Since the Umbria-Marche earthquake in 1997, a specific survey form, based on this macroelement approach and considering up to 28 possible collapse mechanisms, has been used in Italy for the assessment of Italian churches damaged by earthquakes. Along with its usefulness in emergency management, this approach is also very useful for seismic prevention, as it allows one to single out the most vulnerable structures of homogeneous groups of buildings and to plan retrofitting priorities in risk reduction programs. In this direction, the macroelement approach has herein been applied to a sample group of churches located within the historical city centre of Naples, as a first step in a systematic program of assessment at territorial scale. Through the indexing of the damage and vulnerability, also including non-seismic damage, a qualitative judgment of the functioning of each macroelement is supplied by pointing out the weaknesses of the fabric due to the absence of some structural details, usually present with the aim of preventing seismic instability. Of the churches examined - S. Andrea a Capuana, S. Nicola of Caserti, S. Pietro in Vinculis, S. Potito - the macroelements most affected by damage were the façade (shear mechanisms, overturning of gables) and the nave (cracking of vaults, seismic transversal response), though involving rather limited global scores. Instead, the predisposition of these churches to be damaged by an earthquake, indexed by the vulnerability score, appeared to be relatively higher; the main weaknesses can be recognized in structural deficiencies, presence of pushing elements, general decay, etc.. However, the calculation of the global seismic safety index allows to identify safe and partially safe churches, certainly all with the need for local improvements. The prediction obtained from this methodology is, at best, approximate and can only be used as a first assessment, while more detailed information is required to identify weaknesses in individual churches. Also, for a correct interpretation of damage and vulnerability, it is necessary a deep knowledge of local construction techniques and of the historic transformation sequence. For a large population of churches, however, a statistical derivation of expected damage is very important, for example in planning systematic retrofitting interventions at regional level.