Integrating modal analysis and seismic interferometry for structural dynamic response: The case study of giotto's bell tower in Florence (Italy)

The preservation of historical building requires monitoring their long-term behaviour as well as to quickly detect possible damage. In this respect, dynamic characteristics (frequency, shapes mode and seismic wave velocity) can be identified and tracked in real-time combining modal analysis with seismic interferometry, as it is shown in this paper for the Giotto's Bell-Tower in Firenze (Italy) by using a temporary seismic network. Natural frequencies and modal shapes of the structure are calculated by classic Enhanced Frequency Domain Decomposition (EFDD). Changes of the modal frequencies may reflect changes in the dynamic properties of the building and/or in the soil-structure interaction effects. The effects of soil-structure interaction are analysed with the seismic velocity and transfer function of the structure estimated using seismic interferometry technique. Seismic wave velocity inside the building depends only on the structural properties of the building and it is uncoupled from the soil-structure interaction system. The results of seismic interferometry (i.e fixed-base frequency, pseudo-flexible body frequency) combined with the first modal frequency allow to define the degree of the soil-structure interactions on the dynamic behavior of the Giotto's bell tower. The full structural dynamic identification shows a weak contribution of the soil-structure interaction, that is supported by the high values of the horizontal and rocking stiffness, evaluated from the geotechnical properties of the soil. It is suggested that integrating the Operational Modal analysis and the seismic interferometry of the same ambient noise recording provides a unique and complete dynamic response of one of the worldwide important monument like the Giotto's Bell-Tower.