Vibration Testing and Monitoring of Historical Towers

Ancient masonry towers - built in different ages for different purposes and using different construction practices - are generally sensitive to dynamic actions such as bell swinging and earthquakes. Particularly, the sensitivity to ambient vibrations like micro-tremors and wind can be leveraged to apply Operational Modal Analysis (OMA) effectively for the structural assessment of masonry towers. In the present paper, data collected on 3 different towers are used to exemplify that: (a) a quite large number of vibration modes can be identified and correctly classified by using only a 4-sensor setup even if higher modes, involving the warping distortion of the cross-section, are involved; (b) ambient vibration tests, together with classical architectural research, are of utmost importance in the validation of numerical models of masonry towers; (c) despite the mask effects exerted by the environmental variability, damage detection can be performed with unsupervised methods (i.e., methods not requiring the measurement of environmental data), such as the Principle Component Analysis together with control charts.