ENHANCING RESILIENCE OF HISTORIC DOMES TO EARTHQUAKES. THE HISTORIC CHAINS OF THE BAPTISTERY OF SAN GIOVANNI AND OF THE DOME OF SANTA MARIA DEL FIORE IN FLORENCE

Over the centuries the historic city centre of Florence has been subjected to several earthquakes, some of high intensity as the events of September 28, 1453 (VII MCS) and May 18, 1895 (VIII MCS). These seismic events have affected two of the largest domes in Florence: the Baptistery of San Giovanni and the Dome of Santa Maria del Fiore. Within the last year, a in depth experimental campaign promoted by the Opera of Santa Maria del Fiore allowed to make new evaluations on the conservation of these monuments and on the effectiveness of the tie-rods strengthening techniques. Indeed, these two masonry domes, with significant geometric similarity, can represent, in virtue of their long history of cracks and strengthening, an archetypal case study able to allowing some general consideration on the efficacy of these primeval hidden structural elements, whose crucial role in ancient monument preservation is still under discussion. To this aim, the paper analyses the evolution of the cracking pattern of the Baptistery, combined to the stress measurements (carried out with the cooperation of the Italian CNR-IVALSA laboratory) on the tie-rods and a comparison with the typology of wooden tie-rods of Santa Maria del Fiore is presented in order to better understand their fundamental role in the conservation process. The effectiveness of the historic chains as a “natural” and “resilient” system of seismic protection is discussed, presenting the results of an integrated study between structural engineers and experts in conservation and restoration.