On-site investigation and dynamic monitoring for the post-earthquake assessment of a masonry tower

On May 29th, 2012 a strong earthquake occurred in the region Emilia-Romagna but it was significantly felt also in Lombardy and damages were reported on several Cultural Heritage buildings placed in the city of Mantua. Therefore, the VIBLAB (Laboratory of Vibration and Dynamic Monitoring of Structures) of Politecnico di Milano was committed to assess the structural condition and the seismic behavior of the "Gabbia" tower, which is the tallest historic tower in Mantua. After a brief description of the masonry tower, about 52.0 m high and dating back to the XIII century, the paper focuses on the tasks involved to assess the structural safety:(a) historic and documentary research;(b) geometric survey and on-site survey of the crack pattern and structural discontinuities;(c) non-destructive and slightly destructive tests of materials on site;(d) dynamic tests in operational conditions;(e) F.E. modelling and vibration-based validation of the model; (f) use of the validated model to assess the structural safety and predict the seismic performance. Visual inspection of all main bearing walls clearly indicated that the upper part of the tower is characterized by the presence of several discontinuities due to the historic evolution of the building, local lack of connection and extensive masonry decay. The poor state of preservation of the same region was confirmed by the observed dynamic characteristics and local modes involving the upper part of the tower were clearly identified by applying different output-only techniques to the ambient response data collected for more than 24 hours. Furthermore, the continuous dynamic monitoring system, installed on the tower in December 2012, and the software developed for processing the collected data is described. Typical results of the dynamic monitoring are presented and discussed, along with the analysis of some earthquake records collected since the beginning of monitoring.