Frequency splitting in the tie-rods of the Cathedral of Milan

During experimental investigations on the dynamic behavior of the tie-rods of the Cathedral of Milan, a frequency splitting phenomenon, also known as "dispersive phenomenon", was detected in the structural response of 4 ties. The present paper reports on the techniques adopted to investigate the observed behavior. The recorded data were first analyzed by estimating the Frequency Response Functions, then by applying classic Operational Modal Analysis techniques to confirm the presence of the phenomenon. Subsequently, the possibility of reproducing the frequency splitting was verified by implementing a linear model which takes into account an imaginary dispersion matrix D. Since, according to the literature, dispersive phenomena are supposed to be associated to the response of damaged structural elements, accurate visual inspections and further dynamic tests (hammer and ambient vibration tests) of the ties exhibiting the mentioned behavior have been scheduled in the near future.