Ambient Vibration Tests on a Building Before and After the 2012 Emilia (Italy) Earthquake, and After Seismic Retrofitting

Structural monitoring of strategic buildings is an important tool for the identification of dynamic characteristics changes caused by an earthquake, of primary interest to prevent potential damage due to future seismic events or even to assess the capability of a seismic retrofit to the damaged structure. Recent studies have shown how these variations can be assessed with special focus on the permanent and transient decrease of the main frequency during ground shaking. In this work, we analyzed three data set acquired on one building of the University of Ferrara (Emilia Romagna Region, Northern Italy). Ambient noise surveys were performed at each floor of the building: the first data set was acquired few months before the earthquake that struck the Emilia region on May 20, 2012; the second was acquired right after the earthquake, when the building showed slight damage; finally, the third data set was acquired in 2016, after the repair of seismic damage occurred to non-structural components. The analysis of those data sets highlighted the permanent drop of the building main frequency after the earthquake due to damage, with its partial recovery after the repair of seismic damage. This study demonstrates that building monitoring, even with low-cost instruments, allows understanding if and how the building main frequency changes due to an earthquake, providing a preliminary assessment of possible damage. Low-cost monitoring systems can therefore be considered a valuable prevention and monitoring tool for structures.