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 end in localized focus of excessive stresses. However, the most vulnerable a part of the tower is the belfry as a result of massive openings leading to slender pillars with excessive top lots. In the case examine, the belfry plays an much more important function because of the lesions within the arches and the ineffectiveness of the tie-rods.
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Moreover, the biological attack related to the crack sample indicated that pressing interventions are wanted for the structural preservation. Thus, the seismic safety of the bell tower within the current state was strongly conditioned by the steadiness of the belfry. The injury at the degree of the belfry is as a result of roof unfold with the ensuing outward thrust that is not restrained by an efficient system of chaining and/or hooping. Thus, a horizontal thrust is applied to the structure of the belfry that is very susceptible as a outcome of several elements:
- high slenderness of the masonry walls that are excessive compared to the cross-section;
- poor mechanical properties of masonry;
- voids and detachments of the coating composed of squared grey tuff blocks from the interior core of the belfry;
- absence of bands of considerable measurement above the cell openings and modest coupling guaranteed by the arches.
In order to improve the structural behaviour of the belfry, each restore and strengthening interventions had been executed at a local stage. A collection of chrome steel reinforcing rings was applied in 4 completely different horizontal sections of the tower to restrict the masonry and improve the connection between the contiguous partitions.
The single chain system consists of a couple of 30 mm diameter stainless-steel bars threaded on the edges to allow tensioning via bolting on anchoring nook plates.
Thus, the chaining system applies lateral forces to the nook plates and, due to this fact, to the masonry at the edges of the octagonal plan. In this manner, the hoops connect horizontally the person masonry partitions of the belfry and provides passive confinement in the event of their lateral actions. In the identical time, the pre-tensioning supplies an energetic confinement subtle on various parts of the walls, counteracts the creep damage when conducting the native interventions of injection, rebuilding, pointing of the mortar joints and restoring materials deterioration. In order for the metal reinforcing rings to be efficient and avoid unwanted displacements, the energetic confinement motion utilized on the belfry should respect the radial symmetry. For this function, it is necessary that the tensioning of the bars takes place concurrently on all sides to guarantee that the lateral forces are applied in radial path on the corners of the octagonal plan. This behaviour was obtained by monitoring the stress state of the chrome steel bars. To this goal, load cells had been installed on some of the chains to carry out two main actions:
- determine the tightening torque to succeed in in all chains the pre-tightening force of about 500 daN;
- control over time the clamping pressure that may change as a result of seasonal excursions but additionally to the pure “discharge” over time.
The monitoring system included eight load cells put in in pairs on all 4 levels and mounted on the upper bar on reverse sides, respectively on the church side and the City Hall aspect. The load cells have been connected to a data acquisition unit mounted at the first access level of the tower. It was decided to perform three tightening phases similar to the next values of the axial load: one hundred, 300 and 500 daN. In this way, it was potential to check the start-up of the chains without overloading the construction. During the three phases, it was essential to repeat the tightening on the varied levels several instances, since, as expected, the tightening of an higher stage corresponds to variations at the decrease stage. The repair and strengthening intervention satisfies each compatibility, sturdiness, and reversibility that are elementary issues for the restoration of cultural heritage constructions.
Moreover, it not only enhances resistance in opposition to vertical hundreds, but also enhance the behaviour beneath seismic actions, avoiding local collapse mechanisms that involve the person piers of the belfry or the walls supporting the roof. In fact, the steel reinforcing rings and the compensation of lesions permits guaranteeing a more practical connection of the piers to the horizontal wall bands. This results in a world behaviour of the belfry underneath horizontal actions and avoids the onset of native collapse mechanisms that negatively affect the seismic security 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’s only essential to carry out the LV2 degree evaluation for the belfry and the LV1 stage evaluation for the complete constructing.