13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- AE monitoring and structural modeling of the Asinelli Tower in Bologna Alberto Carpinteri 1 , Giuseppe Lacidogna 1,* , Stefano Invernizzi 1 , Amedeo Manuello 1 1 Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, 10129, Torino, Italy. * Corresponding author: Giuseppe.lacidogna@polito.it Abstract The Acoustic Emission (AE) technique was used to assess the structural stability of the Asinelli Tower, the tallest building in the city of Bologna, which, together with the nearby tower, named Garisenda, is the renowned symbol of the city. AE is a passive, non-destructive structural evaluation technique based on the spontaneous emission of pressure waves by evolving fracture processes. The monitoring program was carried out with the aid of a USAM tool, which is part of the equipment used at the Fracture Mechanics Laboratory of the Department of Structural Engineering at the Politecnico di Torino. This tool makes it possible to conduct a complete analysis of AE signals, acquire a huge quantity of data from on site monitoring, and identify the microcracks triggering the damage processes in a structure. In the second part of the paper, the results from a preliminary linear analysis are presented, in order to assess the structural behavior of the tower. The cracking and crushing strengths of the masonry have both been compared with the calculated stresses. The numerical analysis gives a valuable picture of the modal response of the tower, providing useful hints for the prosecution of structural monitoring. Keywords Acoustic emission, monitoring, finite element method, modal analysis, structural assessment. 1. Introduction For some years, the authors have been conducting research through the application of a material and structure control method based on the spontaneous emission of pressure waves from evolving defects. With the Acoustic Emission (AE) monitoring technique, the ultrasound signals emitted by damage phenomena are received by wide-band piezoelectric (PZT) sensors, i.e., sensors calibrated for a frequency range of between 50 and 800 kHz. The AE technique is non-invasive and non-destructive and therefore is ideally suited for use in the assessment of historic and monumental structures that are subjected to high, long-term loads or cyclic loads, or, more generally, are exposed to seismic risk. Having to identify the fractured or damaged portion of a structure, it is possible to evaluate its stability from the evolution of damage, which may either gradually come to a halt or propagate at an increasingly fast rate. Moreover, if the position of the defects is not known to begin with, it can be located by making use of a multiplicity of sensors and by triangulation, prior to assessing the stability of a structure based on the evolution of damage phenomena. Using the AE technique, the authors have acquired considerable experience in the monitoring of medieval masonry towers and other significant historic buildings [1-5]. The AE technique is ideally suited to control the evolution of structural damage caused by pulse phenomena, such as wind and seismic actions. In this study − a significant zone of the masonry structure to be analysed having been identified − the AE count obtained during the monitoring period was correlated with the frequency of seismic events in the areas around the city of Bologna. The data were used to assess the critical phenomena taking place in the structure with the aim to predict the evolution of damage over time. 2. Structural description of the tower The authors of the various histories of the city of Bologna all agree in dating the Asinelli and Garisenda Towers to the early twelfth century, but with some minor discrepancies as to the year of construction, as pointed out by Ludovico Savioli in discussing the Asinelli Tower in his Annali