Operational issues in the dynamic identification of URM buildings targeted to seismic assessment

Ambient vibration measurements are useful tools in many engineering applications requiring the fine calibration of reliable dynamic models. Particularly, the employment of experimental modal analyses is rapidly growing within the complementary fields of structural health monitoring and seismic assessment of existing buildings. Simplified procedures based on modal parameters identified by ambient vibration test (AVTs) have been recently developed to support seismic mitigation policies at the urban scale. Among the others, the SMAV procedure has been developed by the Italian Department of Civil Protection to evaluate the operational level of strategic buildings in the post-earthquake scenario characterized by an assigned intensity of the seismic event. Being conceived for large scale applications, the procedure presupposes some simplifying hypotheses regarding the mechanical behaviour of the structures under investigation. For instance, the linear dynamics of each floor in multi-storey buildings is approximated by the planar motion of one or more polygonal rigid bodies. Despite the efficiency of the procedure and its limited computational effort if compared to more refined numerical models, different recent applications highlighted that some improvements are required in the use of AVTs for the seismic assessment of unreinforced masonry buildings. Indeed, the unique features of this structural typology such as the flexibility of the diaphragms (e.g. timber floors or vaults) tend to challenge the rigid body assumption even for simple planar geometries. This issue calls for more accurate discretization schemes and a higher number of sensors. Within this context, the paper provides a first contribution in the development of rapid but effective checks to verify the rigid body assumption for the diaphragms, supported by a preliminary application to a case study.