Insight into Floor Response Spectrum Assessment of Masonry Towers: Lessons Learnt from the Case Study of San Niccolò Tower (Florence, Italy)

The estimation of the Floor Response Spectrum (FRS) is fundamental for the characterization of the seismic demand at different heights of a structure, particularly for assessing seismic safety of both local mechanisms and non-structural elements. This paper aims to provide insight into the assessment of floor response spectra in masonry towers by presenting and discussing the results of an investigation carried out on the San Niccolò Tower in Florence, Italy. To determine the tower’s dynamic properties, Ambient Vibration Tests (AVT) and Operational Modal Analysis (OMA) were conducted, that revealed a complex structural behaviour influenced by geometric irregularities and material heterogeneity. The floor response spectra were evaluated using three complementary approaches: (i) a simplified analytical formulation provided by the latest Italian Standards, (ii) an experimental method based on seismic interferometry, and (iii) a numerical approach based on Finite Element (FE) linear time-history analyses. The comparison of results from these procedures shows that, while code-based simplified formulations offer a practical tool for preliminary estimation of floor response spectra, they can be highly sensitive to modal parameters, which may significantly influence the predicted seismic demand. In this respect, this study stresses the importance of integrating experimental and numerical methods for a more reliable evaluation of FRS in slender historic structures.