ABSTRACT
At present, seismic design is made with reference to an earthquake level which has a given probability of being exceeded over a certain period, or, which is equivalent, to a level of earthquake which on average should take place after a certain period of time (return period). In Eurocode 8, for instance, the typical value of reference peak ground acceleration for each seismic zone and for the no collapse requirement corresponds to a reference probability of exceedance PNCR in 50 years equal to 10% or to a reference return period TNCR equal to 475 years. Implicitly, this definition contains the fact that greater values of accelerations can take place: those greater values correspond to what is called “exceptional” or “catastrophic” earthquakes: those which impose abnormally large inelastic deformation demand to structures. When inelastic deformation demand is very large, complex non-linear phenomena occur: buckling and/or fracture, in case of steel structures; material crushing and/or loss of bond between concrete and steel, in case of reinforced concrete structures; crushing and/or loss of integrity, in case of masonry constructions. The effect of such phenomena is a degradation of mechanical response, such as strength and/or stiffness degradation of plastic zones in engineered frames, which ultimately may lead to complete collapse of the structure.
