Post-earthquake assessment of the civic tower in L'Aquila: ambient vibration tests and structural health monitoring

The paper is focused on the post-earthquake assessment of the Civic Tower, an emblematic historic structure, part of ‘Palazzo Margherita’, the City Hall of l’Aquila, Italy, severely damaged by the strong earthquake occurred on the 6th of April 2009. The Civic Tower (6.5m x 7m base and 43m high) is a structural element particularly vulnerable due to its tall and slender shape and the scarce mechanical characteristics of the masonry walls. The earthquake induced a diffused and serious crack pattern along the entire shaft of the tower with a concentration of stresses at the basement. The precarious structural conditions of the building suggested implementing some temporary emergency interventions in combination with the installation of a permanent structural health monitoring system, able to evaluate quantitatively the progression of the assessed damage pattern. Ambient vibration tests were preliminarily performed to characterize the dynamic response of the structure and choose the optimal positions for the dynamic monitoring layout. Main results obtained from the application of various output-only modal identification techniques to the collected dynamic data are summarized. Subsequently a static and dynamic monitoring system was installed, including devices to measure set of displacements and strains at critical points of the structures (displacement transducers, strain gauges, inclinometer, temperature and humidity sensors) and to record ambient vibrations (accelerometers). The paper reports the outcomes of the first 2 years of monitoring, analysing the dependency of the monitored parameters on environmental factors. It presents also an automated procedure for static data processing and modal parameter estimation.