Aging of Historical Masonry Towers: The Garisenda Case Study

The monitoring of ancient masonry towers highlights the daily and seasonal cyclic deformation due to the differential heating of the faces and core of the structure. For medieval towers, built around the XII Century, approximately 1000 annual and 300,000 daily cycles must be considered. In terms of fatigue, the seasonal cyclic phenomenon is at the upper limit of the so-called low-cycle fatigue (LCF), while the phenomenon due to daily cycles ascribes to high-cycle fatigue (HCF). Some authors have conjectured that these phenomena may have been the cause of particularly sudden collapses in combination with time-dependent aspects such as creeps and diffusion phenomena [1, 2]. In this work we analyse the Garisenda Tower in Bologna (Italy). The evolution of the tower eccentricity is monitored by means of a pendulum, while the deformations of the selenite basement walls are measured by means of optical chords deformometers. In addition, the slow decay and microcracking progression in mortars and stone blocks is detected by monitoring Acoustic Emission (AE). The information acquired by the monitoring is combined with the measured temperatures to distinguish between reversible and permanent deformations, the latter due to progressive damage accumulation [3, 4-5]. The comparison is carried out with FEM numerical simulations of the tower evolution and experimental tests on Selenite stone specimens characterizing the block masonry of the tower basement. This analysis allows to improve the understanding of the phenomenon and helps the development of more reliable warning procedures for unstable damage progression and possible collapse.