Dynamic behaviour of shallow founded historic towers: validation of simplified approaches for seismic analyses

Soil-structure interaction quite often plays a major role in the seismic response of masonry towers and this paper presents a simple but consistent framework for this kind of analysis. Using experimental data from a monitoring system installed on the Ghirlandina tower, it is shown that simple models can provide reliable estimates of the input motion and of the dynamic stiffness of the soil-foundation system. In this specific case, it is proved that the foundation input motion can be evaluated by neglecting the kinematic interaction between the foundation and the surrounding soil. Moreover, the classical computation approaches of the foundation dynamic stiffness leads to results in agreement with the response of the tower, if the effects of soil non-linearity are accounted for. The validation is provided considering three seismic events, which have been recorded by the monitoring system and by a free field accelerometric station in the nearby of the tower.