Experimental Damage Assessment of the Garisenda Tower Combining Measurements of Optical Fiber Strains, Pendulum Displacements, and Acoustic Emission

One of the most important medieval remnants and a municipal icon in Bologna (Italy) is the Garisenda Tower. It was built in the early-twelfth century at the same time as the Asinelli Tower. The towers, known as the “Due Torri” of Bologna, were initially intended to be the same height, but in 1351, the Garisenda started to tilt because of foundation subsidence. As a result, it was reduced in height from 61 m to around 13 m for safety, today sitting at 48 m (about half of the 97 m of the Asinelli). The Garisenda Tower is wide 7.5 m on its sides, with walls up to 2 m thick, and it leans about 3.22 m to the East side. Since the beginning of the last century, the structural behavior of the tower has been studied and, in more recent years, the topic of the tower’s stability started to gain traction. This sparked the interest of assessing structural damage and cracking evolution by means of different non-destructive testing (NDT) techniques. There is currently a multitude of sensors installed in the Garisenda Tower. More precisely, 16 Acoustic Emission (AE) sensors, six thermometers, five Fiber Optic Sensors (FOSs), four extensometers, two accelerometers, one seismometer, and (impressively) a 30m long pendulum are located inside the tower. The accelerometers and the thermometers are present in the Garisenda since 2009. The optical strands were installed on March 2019 and were followed by the AE sensors on May 2019. The pendulum was installed on March 2021 and, lastly, the extensometers on June 2021. Together, these instruments compose a database capable of analyzing and relating various aspects of the mechanical behavior of the tower: with the aid of the FOS, the extensometers, and the pendulum, the steady-state displacement regime of the tower can be described by means of displacements and strains. This data can be confronted with AE and temperature data to quantify damage, for example. Short-term events, such as seismic activity, can also be analyzed via AE and the seismometers. In these regards, the present work intends to showcase a methodology for the damage assessment of the Garisenda Tower in Bologna by the combined analysis of some of these sensors. In particular, the pendulum displacements, the FOS strains, and AE data will be analyzed. These highlight how the correlation of different SHM equipment datasets can provide valuable insights about the damage state of the tower (particularly by means of the b-value parameter), and the mechanical behavior—in particular, its in-plane movements.