Effects of soil-foundation-structure interaction on the seismic behavior of monumental towers: the case study of the Carmine bell tower in Naples

The seismic safety of historical buildings may be affected by the interaction among soil, foundation and structure, usually neglected in the conventional assessment procedures. Focusing on numerous monumental towers of the Italian cultural heritage, the paper first examines the dynamic behavior of squat to very slender tower structures, through the simplified approach of the ‘equivalent oscillator’ proposed by Veletsos and Meek (1974). A method is then proposed for preliminary predictions on the variation of natural period and damping ratio due to the interaction, as a function of structure/soil stiffness ratio. Deeper evaluations of the interaction effects have been carried out considering the case study of the Carmine Bell Tower in Naples, a slender brickwork structure 68 m tall and about 10 m wide, featured by a high artistic value and representing a popular historical symbol of the city. Geotechnical and geophysical investigations have been performed to reconstruct the geometry of the shallow foundations and the subsoil profile, constituted by a deformable deposit of man-made ground, marine and pyroclastic sands overlying volcanic tuff. The dynamic behavior of the Bell Tower has been simulated both by the ‘equivalent oscillator’ method and by linear dynamic analysis carried out on a more complex and detailed model. Both approaches have been adopted considering the seismic actions suggested by the Italian Code and those determined through seismic response analyses. Comparing the results of the analyses on the fixed based structure with those of the structure on a compliant base, the increase in the period and the reduction in the damping ratio were recognized as compatible with those expected on the basis of the simplified preliminary predictions.