Architecture and Repair of The Belfry Tower

The tower is next to the church creating horizontal constraints that increase the seismic vulnerability of the tower by limiting its slenderness and creating stiffening zones that result in localized concentration of high stresses. However, the most vulnerable part of the tower is the belfry due to the large openings leading to slender pillars with high top masses. In the case study, the belfry plays an even more critical role due to the lesions in the arches and the ineffectiveness of the tie-rods.

Moreover, the biological attack associated with the crack pattern indicated that urgent interventions are needed for the structural preservation. Thus, the seismic safety of the bell tower in the current state was strongly conditioned by the stability of the belfry. The damage at the level of the belfry is due to the roof spread with the consequent outward thrust that is not restrained by an effective system of chaining and/or hooping. Thus, a horizontal thrust is applied to the structure of the belfry that is very vulnerable due to several factors:

1. high slenderness of the masonry walls that are high compared to the cross-section;
2. poor mechanical properties of masonry;
3. voids and detachments of the coating composed of squared gray tuff blocks from the internal core of the belfry;
4. absence of bands of considerable size above the cell openings and modest coupling guaranteed by the arches.
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In order to improve the structural behaviour of the belfry, both repair and strengthening interventions were executed at a local level. A series of stainless steel reinforcing rings was applied in four different horizontal sections of the tower to confine the masonry and improve the connection between the contiguous walls.

The single chain system consists of a couple of 30 mm diameter stainless steel bars threaded at the edges to allow tensioning through bolting on anchoring corner plates.

Thus, the chaining system applies lateral forces to the corner plates and, therefore, to the masonry at the edges of the octagonal plan. In this way, the hoops connect horizontally the individual masonry walls of the belfry and give passive confinement in the event of their lateral movements. In the same time, the pre-tensioning provides an active confinement diffused on various portions of the walls, counteracts the creep damage when conducting the local interventions of injection, rebuilding, pointing of the mortar joints and restoring material deterioration. In order for the steel reinforcing rings to be effective and avoid unwanted displacements, the active confinement action applied on the belfry should respect the radial symmetry. For this purpose, it is necessary that the tensioning of the bars takes place simultaneously on all sides to ensure that the lateral forces are applied in radial direction on the corners of the octagonal plan. This behaviour was obtained by monitoring the stress state of the stainless steel bars. To this aim, load cells were installed on some of the chains to carry out two main actions:

• determine the tightening torque to reach in all chains the pre-tightening force of about 500 daN;
• control over time the clamping force that may change due to seasonal excursions but also to the natural "discharge" over time.

The monitoring system included eight load cells installed in pairs on all four levels and mounted on the upper bar on opposite sides, respectively on the church side and the City Hall side. The load cells were connected to a data acquisition unit mounted at the first access level of the tower. It was decided to perform three tightening phases corresponding to the following values of the axial load: 100, 300 and 500 daN. In this way, it was possible to check the start-up of the chains without overloading the structure. During the three phases, it was necessary to repeat the tightening at the various levels several times, since, as expected, the tightening of an upper level corresponds to variations at the lower level. The repair and strengthening intervention satisfies both compatibility, durability, and reversibility that are fundamental issues for the restoration of cultural heritage constructions.

Moreover, it not only enhances resistance against vertical loads, but also improve the behaviour under seismic actions, avoiding local collapse mechanisms that involve the individual piers of the belfry or the walls supporting the roof. In fact, the steel reinforcing rings and the compensation of lesions allows guaranteeing a more effective connection of the piers to the horizontal wall bands. This leads to a global behaviour of the belfry under horizontal actions and avoids the onset of local collapse mechanisms that negatively affect the seismic safety of the structure. Finally, it should be underlined that these interventions are aimed at strengthening and repairing the deteriorated and damaged masonry of the belfry. Thus, the intervention project is configured as "repair or local intervention". Therefore, according with the Italian Guidelines (DPCM 2011) it is only necessary to perform the LV2 level assessment for the belfry and the LV1 level assessment for the entire building.