Ambient vibration tests on a reinforced concrete school building before and after retrofitting works with external steel "dissipative towers”

The paper deals with the ambient vibration tests carried out on an existing reinforced concrete school building sited in Camerino, a small town in central Italy near the epicentre of Umbria-Marche 1996 earthquake. Tests were performed before and after the seismic retrofitting of the building achieved with of an innovative system that uses external steel dissipative towers. Before the retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic behaviour of the building including the contribution of non-structural components (e.g. external and internal walls); modal parameters, i.e. natural frequencies and mode shapes determined by means of experimental modal analysis, allowed the calibration of a predictive f.e. model of the building which was crucial for the final design of the retrofitting system. Tests were repeated after the retrofitting works, with the main objective to verify that changes of modal parameters are in agreement with those predicted by the f.e. model. Tests were carried out by positioning low noise piezoelectric accelerometers opportunely so as to survey the rigid motion of each storey. The modal parameters were identified with two identification techniques, the Enhanced Frequency Domain Decomposition and the Covariance-driven Stochastic Subspace Identification, in order to assess whether a method is more suitable in such cases. Results and comparisons between the modal parameters obtained before and after the retrofitting works demonstrate that the ambient vibration tests and the data processing techniques used allow identifying the dynamic behaviour of r.c. few-storeys buildings under operating conditions and catching the changes due to seismic retrofitting works performed using external steel dissipative towers.