The Operational Modal Analysis for the identification of historical structures: the Tower of the Nations in Naples

Relevant seismic risk affects a large part of some European regions and requires effective measures in order to protect constructions and infrastructures and to mitigate losses due to seismic events. This objective can be reached increasing the knowledge of structural response of existing constructions and appropriate guidelines for the definition of security measures. This circumstance points out the need to monitor the performance of civil structures over their operational lives. These experiments allow a deeper knowledge of effects of earthquakes on structures and of their behaviour in operational conditions. Combination between numerical models and experimental measures offers interesting opportunities in the fields of vibration and seismic protection of strategic or historical constructions. Various methods for the evaluation, with unknown input, of structural dynamic parameters are available: some of them works in the time domain (Least Square Complex Exponential, Ibrahim Time Domain, etc.) while others in the frequency domain (Frequency Domain Decomposition, etc.). In this paper the use of Operational Modal Analysis for the evaluation of the modal parameters of relevant historical reinforced concrete building such as the Tower of the Nations, located within the Mostra D'Oltremare of Naples, is described and role of OMA techniques is analyzed.