The role of the base isolation in the reduction of seismic effects on monolithic objects of art

Both the model of rigid block and the base isolation technique have been widely discussed in literature, but only in the last decades models concerning isolated works of art, modeled as rigid bodies have been analyzed. The general bi-dimensional well-known formulation of slide-rocking rigid block has been used to analyze the behaviour of rigid blocks under different kind of base excitations such as earthquake excitations. Recently, in some works, a base isolation system has been used to protect statues from seismic effects; the model consists in a non-symmetric rigid block supported on an oscillating base connected to the ground by a linear visco-elastic device. The authors included in the model security stops able to limit the displacement of the oscillating base to a safety value. Later the model has been enriched introducing the slip effects and focusing the analysis on the effects of the friction coefficient. In the present paper, this model is improved considering special sliding constrains that prevent the rigid body from falling off the base. Alternatively, the possibility for the rigid block to be partially out of the oscillating base is considered. Original exact nonlinear equations of motion, describing the slide-rocking motion of the rigid block around a point different from one of the corners of its base, are obtained, transition conditions among the different phases of motion are also written. Seismic excitation is considered by using two Italian registered earthquakes. The effects of the slenderness, of the friction coefficient and of the sliding constraints have been investigated. The analyses pointed out that, even if these sliding constraints exclude the possibility of collapse due to falling off the base, they induce a general worsening of the rocking behaviour above all for bodies with higher slenderness. Instead, when sliding stops are not considered the system shows an improvement of behaviour compared to previous models. Finally the paper confirms the effectiveness of the base isolation applied to rigid blocks representing real monolithic objects of art.