Assessment of the rigid behaviour of diaphragms from ambient vibration measurements: application to masonry buildings in pre and postearthquake conditions

The paper presents an inverse procedure, recently proposed by the Authors, aimed at assessing the possible in-plane rigid behaviour of horizontal diaphragms in the global response of existing buildings. The procedure is based on the estimation of floors rigid rotations and angular deformations from ambient vibration data. When the estimation is carried in pre and post-earthquake conditions, it can provide useful information about the loss of rigid behaviour of the diaphragms, possibly linked to a reduction in their stiffness caused by seismic damage. The procedure is tested through numerical simulations assuming different hypotheses on the diaphragms rigidity, with the purpose of simulating increasing levels of damage in the floors. The computational model employed adopts the equivalent-frame approach and is inspired by the geometry and by the structural configuration of the "Pietro Capuzi" school in Visso. The building, continuously monitored by the Italian Seismic Observatory of Structures led by the Department of Civil Protection, has been heavily damaged by the 2016-2017 Central Italy earthquake sequence and was recently demolished. The application to pseudo-experimental response data allowed to identify a variation in the dynamic response of diaphragms due to seismic damage, highlighted by a significant increase in their angular deformations.