Analisi strutturale di antiche torri sotto azione sismica

Le antiche torri e i campanili sono particolarmente sensibili agli scuotimenti sismici tanto da aver caratterizzato nei tempi passati i danni sismici di un territorio. Siano essi di pietra o di mattoni, isolati o compenetrati in costruzioni adiacenti, esibiscono danni tipici o meccanismi incipienti di crollo ormai catalogati e studiati. Le normative sismiche di ogni paese prevedono procedure di verifica convenzionale di queste costruzioni per sisma atteso.

Convenzionalità della procedura di verifica con coefficienti di garanzia che partono da quello di confidenza e finiscono su quello cosiddetto di struttura (il quale abbatte le azioni da spettro del sisma atteso sulla base della riserva di duttilità) sono alla base del calcolo. Questo si poggia su dati di input legati alla conoscenza dei parametri meccanici della muratura il cui reale reperimento non risulta codificato.

Per torri isolate l’analisi va condotta con modelli che contengono la deformabilità del suolo. Una semplice procedura utilizza parametri “condensati” in molle elastiche desumibili dalle formule di Gazetas o di Viggiani che tengono anche in conto la profondità del piano fondale. Le prove cross-hole, fornendo le velocità delle onde di taglio nei pressi della fondazione, consentono una migliore stima dei parametri contenuti nelle predette formule.

Il primo e più semplice procedimento di verifica consiste nel “beam model” in cui la torre, se isolata, si considera come una mensola con vincolo elastico alla base. Questo semplice modello consente ovviamente sia analisi dinamiche sia statiche equivalenti con carichi verticali ed orizzontali provenienti dallo spettro del sisma atteso nel sito. Per quanto riguarda la dinamica questo modello fornisce risultati accettabili invece per quanto riguarda le verifiche in relazione alle azioni orizzontali possono considerarsi accettabili solo se la torre ha modeste aperture, altrimenti le singolarità introdotte da queste rendono illusorie le verifiche anche se si tiene in conto la sezione ridotta per la aperture. Anche aspetti dovuti alla presenza o meno di scale solidali in muratura all’interno rendono questa procedura illusoria.

Una procedura più affinata è quella del push-over considerando il materiale non lineare e la discretizzazione ad elementi finiti. In questo caso i parametri del materiale aumentano e il loro reperimento in situ risulta complesso ed è opinabile la relazione fra prove non distruttive in situ e parametri di input dei programmi FEM.

Nell’articolo accluso (in inglese) sono esposte alcune procedure commentate in relazione ad un caso specifico riguardante la torre di Reno Centese investita dal sisma Emiliano 2012.

The bell tower of Reno Centese

The isolated bell tower of Reno Centese is located in the square of the village belonging to municipality of Cento, province of Ferrara, Italy (FE). Starting from the bottom, the tower is made by 1) a main prismatic body (square shape) with small circular openings, 2) a belfry and then 3) a spire (Figure 1). The total height of the tower is 29.25 m with a constant thickness of the walls (in the trunk) equal to 0.5 mm.
The main materials used are: bot piles as foundation, structure made by bricks that come from a demolition of previous tower in same place, external thin plaster as “sagramatura” that is a traditional technique in this geographic area, internal plaster made by traditional lime, stone decorations in the upper part of the tower and timber structures for staircase and internal decks.

Figure 1. The bell tower of Reno Centese

Main damages after 2012 Emilia's earthquake

During the night of 20 May 2012, an earthquake with a magnitude of 5.9 occurred in the provinces of Ferrara, Modena and Bologna causing severe damages in particular on masonry towers and bell towers. In the following two months, other seismic events occurred and, in particular, other six events with a magnitude greater than 5.
The tower of Reno Centese, Cento, during earthquake in 2012, was under “cosmetic” restoration so a scaffolding was present at the moment of the shock. An inclined severe crack due to an out-of plane rotation of the upper part of the bell tower occurred with successive sliding, and also two dislocations appeared in two directions [1]. Other cracks occurred also on north and east sides and the tower was considered by the experts prone to collapse (Figure 2). For this reason a “red area” was identified around, inside which all activities were blocked.
It is important to observe that all the flexural and shear cracks occurred in two separate zones: one located 1,5 m above the basement and the other one on the spire.

Figure 2. Main damages after Emilia's 2012 earthquake: a) diagonal crack (west side), b) dislocation A (20 cm), c) dislocation B (6 cm), d) cracks (north side)

Emergency strengthening intervention avoiding collapse

When the tower was defined by the experts close to collapse, one of the author proposed a “hard” and “unconventional” emergency intervention, based on the use of both spritz FRC and FRP wrapping. This was an invasive technique, but the tower did not fall down also after other shocks.
The final aim of the strengthening interventions was to achieve strength and stability of the bell tower higher than the ones before the earthquake, in order to prevent the structure to a possible successive seismic event.
The starting point was the application of fibre-reinforced projected cement mortar in order to close cracks. A truck with an arm of 52 m was used to execute the work in safety conditions. The new mortar produced also a confinement effect on the underlying masonry, because was characterized by higher compressive strength, tensile strength and fracture energy.
Then a double layer of fibers were applied horizontally and vertically, in particular the first layer was made by glass and the second by carbon, to remove the “red area” and to start the restoration works in the internal part of the bell tower (Figure 3).

Figure 3. Strengthening interventions: a) application of spritz FRC, b) FRP wrapping

After the strengthening works, the tower was monitored by Acoustic Emission Technique (AE), and no warnings were perceived during the successive seismic events. Sensors were installed close to cracks. In fact, during crack propagation, the elastic energy is released and produce some waves that are captured by the sensors. In this way, it is possible to monitor the damage process.