Visual Inspection Geometric Survey and 3D Laser Scanning

Biological attack was visible on the belfry and was made of vegetation growth, mould and insect colonization. This was the main reason of the restoration intervention on the bell tower. However, during cleaning and recovery operations deep and wide cracks were detected on the belfry of the bell tower. Many bricks of gray tuff of various sizes that cover the outside of the belfry had clearly undergone detachment (Fig. 5). Only the belfry showed widespread cracks while there was no significant damage in the rest of the structure.

In particular, no crack starts from the bottom. Thus, the possibility that the crack pattern is due to a failure of the foundations may be excluded. The bell-tower was reinforced with four iron tie-rods at the same level of the structure. The visual inspection has revealed that the tie-rod on west fa?ade has no anchor (Figs. 6-8). Moreover, the state of conservation of the anchors has suggested that also other tie-rods may have undergone reductions in the tension state.

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This loss of tension in the tie-rods may have caused the opening of the cracks.

The geometric survey of the tower was performed using traditional instruments while for the belfry, due to much degradation of the surfaces and the complexity of the structure, a laser scanner instrument was used. This allowed more detail that is particularly useful for the next phase of material survey and provided all the data necessary to develop a 3D model to evaluate the seismic vulnerability of the structure. The laser scanner "Faro Focus 3D" based on the phase-shift technology was used.

This instrumentation offers many advantages: rapid data acquisition, precision appropriate to the purpose of the project, possibility to place the instrument near the surface even in confined spaces and without any additional equipment or cables. The metallic scaffolds around the tower prevented the view of the facades in their entirety and this greatly affected the choice of the number of scans. In particular, 16 scans were deployed in the latest levels to minimize shadow areas and ensure overlapping scans of at least about 30%. Scans were performed by setting the instrument according to the following parameters: Resolution=1/10; Quality=4x; scan time=3.05 min; distance of the points in mm / distance of 10 m= 15.340; number Points (in millions) = 7. Figs. 9-10 shows the plan, front and 3D views obtained by laser scanning.