New insights in the modal identification of a monumental structure from long-term seismic structural monitoring

The paper summarizes the main findings obtained during the development and management of a longterm seismic structural monitoring system installed in a historic monumental church, the Basilica S. Maria di Collemaggio, which was heavily damaged in the 2009 L'Aquila earthquake. Critical issues and potential benefits in the use of wireless sensor network technology for the long-term monitoring of a monumental masonry structure in a seismic area are analyzed and discussed. The specific features of the implemented system, purposely designed to detect earthquake-induced vibrations, are illustrated. The information collected during preliminary tests and three years of data acquisition is presented, together with careful evaluations on the operating system performance. Some advancements with respect to the traditional procedures of the operational modal analysis are introduced, in order to check whether the natural frequencies and the modal damping ratios may depend on the vibration amplitude of the structural response to a non-stationary seismic excitation. In this respect, the identification results obtained from different techniques commonly used to estimate the modal parameters of time-variant systems, such as the Wavelet Transform (WT) or the Short-Time Fourier Transform (STFT), are compared. Moreover, the seismic data may suffer from severe noise contamination, that could compromise the data processing and interpretation. In this respect, denoising procedures of the signals are applied to determine the real influence of the instrumental noise on the identification results obtained using data registered during different seismic events. Finally, optimization solutions for a rapid assessment of the main modal characteristics in the case of the seismic-induced vibrations are discussed to define possible strategies of damage identification.