Identification of the modal properties of an historic masonry clock tower

The aim of the paper is to describe the non-destructive tests performed on the clock tower of the Castle of Trani (Bari, Italy). The tower, built in 1848, was realized in tufa masonry (Stone of Trani); it is about 9 meters high and has a square plan with a side of about 3.9 meters; it is built on the principal enter of the Castle and supported by a barrel vault reinforced with an arch. The non-destructive monitoring was performed by using specific accelerometers placed on the structure at different levels for measuring the acceleration in different points. The particular squat structure of the clock tower suggested the authors to make not only the traditional monitoring, with only environmental actions, but also forced tests by mean of a vibrodine adhoc designed and realized, for determining the building modal parameters. All the phases and the procedures of the experimental monitoring are described and the dynamic identification of the building modal parameters (the frequencies and their corresponding mode shapes) is presented discussing the results in both the operative conditions and the effects of the vibrodine excitation.