Intervention criteria for historic masonry constructions subjected to seismic actions / Kryteria interwencji dla historycznych konstrukcji murowanych podlegających oddziaływaniom sejsmicznym

The acknowledgment of the scientific community that the structural safety aspects of constructions belonging to the cultural heritage can not be treated according to standard procedures that are fitted for new constructions, is leading to new, specific approaches for assessing their actual structural performances and then for developing new, more appropriate methodologies and design criteria for their repair and strengthening. The basic idea is that the usual, even the most sophisticated, structural design approaches naturally imply a certain level of “over-design”, and this can lead to unacceptable solutions when dealing with “existing structures” (ISO 13822). Over-designing interventions on existing structures could in fact imply in general unacceptable costs and, in the case of the historical heritage, unacceptable losses of cultural (historic, artistic) values. On this respect, the “seismic condition” presents the most delicate implications, as the very extreme resources of the structure , i.e. those connected with resistance mechanisms that are normally neglected and in any case almost impossible to include in structural models, are mobilized. Attempts to offer comprehensive methodologies to tackle such difficulties have been recently made at an international level by the ISCARSAH committee of ICOMOS and, for constructions in seismic areas, by the Italian codes and guidelines issued by Ministry of Culture and Ministry of Public Works. The consciousness that any calculation imply over-design has as relevant consequence that the formal match between “code requirements” and “formally demonstrated structural performances” can be misleading, and not necessarily required. Assessment is no more considered a more or less sophisticated exercise of analytical/numerical structural models, but an articulated and combined use of structural models, historical informations, surveys and investigations, personal, qualitative judgments (based on past experiences, specific training, comparisons, evaluations of past performances of a constructions) that in a “multidisciplinary” context , ensuring that all the values (economical, social, historic, artistic) of the constructions are taken into account, leads to a final decisions on if and how to intervene. Moreover, safeguarding “cultural values” implies also appropriated selection and design of the intervention materials and technologies. Attention is than paid to the possibilities offered by the traditional solutions and to their possible combinations with innovative ones.