Seismic capacity of the Ghirlandina Tower in Modena, Italy

This paper presents a simplified vulnerability assessment method for masonry towers taking into account the soil-structure interaction and masonry limit analysis method. A model to determine the plane of fracture that defines the kinematic blocks of an overturning mechanism is proposed, based on simple equilibrium conditions and is applied to the Ghirlandina Tower in Modena. According to the Italian codes, towers are classified as a macroelement characterized by peculiar collapse mechanisms. For towers a slight variation in mechanism geometry implies relevant variations in collapse multiplier values; this is mainly due to the importance of mass and height in these structures. Hence a correct definition of mechanism geometry is very important. The assessment is based on a in situ investigation defining a detailed soil profile and the relevant mechanical parameters, aimed at modelling the soil-structure interaction during both static long-term life and during expected seismic events. Parameters were used to run a frequency analysis (to determine seismic demand), in which different stiffness values for soil restraint were used, corresponding to different hypothesis about soil behaviour. Limit analysis method was then applied to identify the collapse mechanisms geometry, under the assumption that masonry is a no-tension material, hence at the limit of overturning, a part of the masonry will remain attached to the base and a stress-free diagonal surface of fracture will form. These analyses allowed for comparing of the limit loads related to the collapse of the fractured masonry with those related to the overturning of the tower as a whole, the horizontal rotation axis being located at the interface foundation-subsoil, establishing a hierarchy of mechanisms.