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The paper summarizes the conceptual development of a vibration-based strategy suitable to the structural health monitoring of ancient masonry towers and exemplifies its application in the continuous dynamic monitoring of the tallest historic tower in Mantua, Italy. The presented approach is based on the installation of low-cost monitoring systems (consisting of few accelerometers and temperature sensors) and on the combined use of automated operational modal analysis, regression models to mitigate the environmental effects on identified natural frequencies and multivariate statistical tools to detect the occurrence of abnormal structural changes. The application of the adopted strategy to 15 months of continuously collected experimental data: (1) highlighted the effect of temperature on the automatically identified natural frequencies; (2) demonstrated the practical feasibility of damage detection methods based on natural frequency shifts; (3) provided a clear evidence of the possible key role of continuous dynamic monitoring in the preventive conservation of historic towers

Abstract:The paper describes the strategy adopted to assess the structural condition of the tallest historic tower in Mantua (Italy) after the Italian seismic sequence of May-June 2012 and exemplifies the application of health monitoring using (automated) operational modal analysis. The post-earthquake survey (including extensive visual inspection, historic and documentary research, non-destructive (ND) material testing, and ambient vibration tests) highlighted the poor state of preservation of the upper part of the tower; subsequently, a dynamic monitoring system (consisting of a few accelerometers and one temperature sensor) was installed in the building to address the preservation of the historic structure, and automated modal identification was continuously performed. Despite the low levels of vibration that existed in operational conditions, the analysis of data collected over a period of about 15 months allowed to assess and model the effects of changing temperature on modal frequencies and to detect the occurrence of abnormal behavior and damage under the changing environment. The monitoring results demonstrate the potential key role of vibration-based structural health monitoring, implemented through low-cost hardware solutions and appropriate software tools, in the preventive conservation and the condition-based maintenance of historic towers.

After the Italian earthquakes of May 2012, an extensive research program had been performed to assess the state of preservation of the tallest historic tower in Mantua, Italy. Subsequently, a continuous dynamic monitoring system was installed in the tower, with seismic and structural health monitoring purposes. After a brief description of the investigated tower and its state of preservation, the paper summarizes some results of the long-term dynamic monitoring.

The paper summarizes the results of the first 15 months of continuous dynamic monitoring of the Gabbia tower in Mantua. After the Italian seismic events of May 2012 a wide investigation program was performed on the tower, suggesting the installation of a dynamic monitoring system in the upper region of the building. The continuously recorded signals allowed the automated identification of the natural frequencies of the key vibration modes of the structure and the clear detection of the effects of temperature on the natural frequencies. The study of the correlation between frequencies and temperature also revealed the non-reversible structural effects determined by a far-field earthquake, proving the effectiveness of damage detection techniques based on frequency shifts