Application of Structural Health Monitoring to the historical Monument to Four Days of Naples

Heritage structures have a significant role in the nation's history: the preservation, protection and if needed restoration of the heritage structures is a concern of the governing authorities. To understand the change in geometry, material, boundary conditions and loading, compared to its original state, is known as Structural Health Assessment (SHA), which becomes Structural Health Monitoring (SHM) if the damage assessment is continued to observe day by day variations. The main goal of SHM is to estimate the dynamic response of the monitored buildings in urban areas to daily stresses produced by environmental and anthropogenic factors (variations in ambient temperature and humidity, wind velocity, vibrations produced by vehicular or railway traffic, or other anthropogenic noise sources including visitors, service staff, etc.) to distinguish ordinary fluctuations in the buildings' response from other anomalous behaviour. Continuous monitoring also makes it possible to assess the impact of extraordinary events such as extreme weather events that are increasingly frequent due to climate changes, earthquakes, excavations, cultural events involving many people nearby the monitored buildings. The above considerations become highly significant when referring to historical heritage: information on the constituent materials and the construction phases is scarcely available for these buildings; in addition, avoiding invasive investigation practices is of primary importance for these ancient structures, which often host museums and other cultural assets. Moreover, in this case long-term continuous structural health monitoring represents an effective non-destructive investigation technique. An application of SHM is the “Monumento alle Quattro Giornate di Napoli” (Monument to Four Days of Naples), built by Marino Mazzacurati in 1964 and consisting in a wide basement and four stelas made of ignimbrite, which represent some scenes of the insurrection occurred in 1943 to release Naples. In order to provide stability against seismic excitation, a base isolation system has been properly designed and mounted to prevent the stelas' overturning: four elastomeric isolators coupled to four multidirectional isolators have been installed between the basement and the foundation. Due to the historical and artistical importance of the monument, a Structural Health Monitoring System has been activated to check the dynamic response of the structure under environmental vibrations produced by the vehicular traffic and the nearby Metro Line 6, and under a seismic event. The recorded signals can be processed by the SHM's software on the base of the Italian Standard UNI 9614 (2017 - Misura delle vibrazioni negli edifici e criteri di valutazione del disturbo), the Italian Standard UNI 9916 (2014 - Criteri di misura e valutazione degli effetti delle vibrazioni sugli edifici) and the German Code DIN 4150-3 (2016 - Le vibrazioni nelle costruzioni Parte 3: Effetti sui manufatti). Moreover, the proposed paper is within the framework of the emerging research field designated as urban seismology, that has both seismological objectives, as obtaining better microzonation maps and seismic risk management in highly populated areas, and engineering objectives, as the monitoring of traffic and train circulation or the surveying of historical buildings.