Which Earthquakes Are Expected to Exceed the Design Spectra?

Extended recording coverage of contemporary seismic events allows a comparison of observed seismic actions with their counterparts used for design. Said comparison shows actions systematically exceeding design spectra. This paper discusses: (1) that considered exceedances can be anticipated by the probabilistic seismic hazard on the basis of which design actions are determined, (2) exceedances of elastic design actions are expected for earthquakes occurring close to the site even if their magnitude is far from the maximum magnitude considered in the hazard assessment, and (3) design spectra are likely to be exceeded in epicentral areas of earthquakes that occur frequently in the region where the code is enforced, but rarely occur close to the site under consideration. In fact, code-mandated protection against these earthquakes is factually warranted by the rarity with which they are expected to occur near the structure and other safety margins implicit to earthquake-resistant design. All these issues, addressed with reference to Italy, are discussed with the intent not to criticize the way spectra are determined, but rather to raise awareness and give a probabilistic measure about what to factually expect from state-of-the-art design at a national level.