## 2 Historical Masonry Structures
### Cathedrals and Vautled Structures  
A monitoring campaign and numerical analysis has been performed on the ancient Cathedral of Syracuse in Sicily [24] (Figure 6a). The acoustic emission (AE) technique is adopted to assess the damage pattern evolution. The localization of the propagating cracks is obtained using six synchronized acoustic emission sensors. A clear correlation between the regional seismic activity and the AE acquisition data has been obtained. In fact the AE count rate presents peaks corresponding to the main seismic events. During the observation period (Figure 6b), the number of AE counts was \(\cong\) 4300.  
In order to assess the rate of the damage propagation, as given in Equation (1), the AE technique data were reported in the bilogarithmic plane to obtain the best-fitting slope, providing \(\beta_{t}\)\(\approx\) 0.98. The quasi-linear progression of damage over time confirms that the process in the pillar is in metastable conditions. A detailed geometrical survey of the most damaged pillar allowed for the definition of an accurate 3D model.  
Figure 6: (**a**) Plan of the Syracuse Cathedral and most critical pillar; (**b**) cracking pattern and localization of AE sources; (**c**) deformed mesh due to the seismic load; (**d**) cracking in the pillar due to to the seismic acceleration.  
The geometry of each block, as well as the presence of masonry inserts, have been considered. Two main loads were considered: the dead load (of the pillar and of the surrounding structure), and a horizontal seismic load provided as horizontal ground acceleration. Due to the horizontal acceleration, cracking can take place in the pillar. A detail of crack nucleation is shown in Figure 6d. The crack occurrence provided by the analysis agrees quite well with the crack localization provided by the AE recording. Cracking corresponds both to diffuse cracking in the continuum elements of the sandstone blocks and to opening or sliding of the discrete interfaces between blocks.  
Another case was the Hospital San Giovanni in Turin (Italy) [25], a masonry building complex initiated in 1680, under the design of the Italian architect [PERSON] (1610-1683) (Figure 7). The ground floor of the complex was recently chosen to host an important fossil collection from the Regional Museum of Natural Science.  
Figure 8: (a) Vault subjected to the dead load: principal compression stress contour; (b) 3D vault deformed mesh; (c) time-displacement diagrams at the springing and (d) close to the keystone.  
Figure 7: Hospital San Giovanni: photograph inside the vault.  
Due to this change of use, an assessment of the structural load capacity of the masonry vault beneath the first floor was necessary because the fossil collection involves a significant increase in the vault load. During the _in situ_ load test, we recorded the acoustic emissions from the vault, as well as displacements of the vault and strains in the steel rods. We compared the experimental data with the numerical results obtained from finite element modeling of cracking and crushing. After validation, the model allowed us to assess the ultimate load-bearing capacity of the vault. The 3D model provided a slightly better estimate of the displacements close to the abutment (Figure 8). From the direct comparison of the cumulative AE counting distribution with the deformation of the structure, we determined that the two diagrams (Figure 9) match each other in shape quite well. In the present case, if the time scale is plotted without rescaling, the two curves do not match identically. Nevertheless, AE are confirmed as a useful tool to measure damage, specifically in the case of very stiff vaulted structures.