Local mechanism analysis in unreinforced masonry buildings according to a new procedure based on floor spectra evaluation

Local rather than global seismic behaviour is a well-known feature in both masonry aggregates and monumental buildings with large internal spans, such as churches or palaces. In such cases safety assessment through local mechanisms kinematic analysis is generally considered a viable solution. However, the definition of the seismic forces acting on secondary architectural elements (pinnacles, upper portions of facades, turrets, etc.) or masonry macroblocks, which may interact dynamically with the main structure, is not banal. Therefore, the recent update of Italian seismic code stresses the role of a building’s global dynamic response and floors’ stiffness in the evaluation of seismic actions on the macroblocks in which it can be subdivided. This result in a complete new definition of floor spectra which are strongly dependent on the dynamic parameters (damping, frequencies) of both, the building and the local mechanism, which also change at the different limit states. The paper aims at the implementation of the new procedures in an existing unreinforced masonry building (Palazzo Carraro in Noale – Venice) with flexible horizontal diaphragms. Modal analysis is used to detect the possible local mechanism and its results are compared to the evidence of the visual inspection of vulnerability factors, showing some correspondence. Safety evaluations, in linear and non-linear field, according to the previous and the current Italian seismic codes are carried out and compared. For the case study, the new procedure is much more pejorative, since acceleration and displacement demands are more than twice the ones obtained in the old one.