Non-destructive characterization and identification of the modal parameters of an old masonry tower

The aim of the paper is to present a structural characterization of the bell tower of the Cathedral of Trani (Bari, Italy) collecting the data of two different non-destructive techniques. The tower, built in 1200, is about 60 meters high and has a square plan with a side of about 7.50 meters; moreover, it is connected to the church through a step supported by a pointed arch. Firstly, an Operational Modal Analysis was performed on the tower by using the data taken by an experimental campaign of measurements of specific accelerometers. The tower vibrations due to ambient actions have been recorded and analyzed by means of two different algorithms in such a way as to estimate the modal parameters of the tower. Moreover, a recent procedure based on the data acquired by means of a ground penetrating radar (GPR) tests for estimating the thicknesses of the internal and external layers at each level of the tower was used. The results of the identification procedure have characterized the dynamic behavior of the masonry tower and were utilized also to evaluate the dynamic interaction between the tower and the Cathedral walls. GPR technology has permitted of foreseeing the internal composition of the building walls. Finally, the combination of the results of the two techniques, has allowed realizing a complete 3D finite element model.