Simple model for predicting the vibration transmission of a squat masonry tower by base forced vibrations

This paper presents the analysis of the dynamic performances of a simple model related to a squat masonry tower situated in the Swabian Castle of Trani (Italy). The main objective of this paper is to introduce a novel strategy based on a simple model validated by experimental data for defining the influence of the excitation frequency on the structural damping dynamic transmission. To this aim, first, the accelerations have been acquired simultaneously in 23 points of the tower at different levels, both due to environmental vibrations and due to a series of sinusoidal forced vibrations applied at the base by using an electro-hydraulic shaker device specifically designed for the tests. Four different excitation frequencies have been then selected for exciting the structure. An operational modal analysis has been carried out by the environmental recordings and with the different forcing loads obtaining a very good correlation of the identified frequencies in all the cases. Then a digital filtering process has been applied over all the recorded signals to evaluate the specific contribution for each frequency generated by the shaking device at each level of the tower. Increments of damping ratio have been detected with these forced vibrations at the base. Finally, a simple frame numerical model has been developed to reproduce the dynamic amplification at the most significant locations of the tower. It has been updated not only to have the same main frequencies and modal shapes but also to get a similar response under forced vibrations at the base. A good correlation has been obtained between the model and the real structure for the base forced vibrations at different excitation frequencies in order to correctly predict the dynamic behaviour of the structure.