The effectiveness of ancient “hidden” tie rods in masonry dome preservation: San Giovanni in Florence

The last surveys and inspections carried out on the wooden tie-rods encircling the two domes of the Baptistery of San Giovanni and of the Cathedral of Santa Maria del Fiore in Florence, promoted by the Opera del Duomo of Florence, allow to make new considerations on these monuments and on the effectiveness of these strengthening techniques. In this paper, the evolution of the cracking pattern of San Giovanni Baptistery, combined to the stress measurements carried out (CNR-IVALSA laboratory) on the tie-rods systems, is analyzed. In addition, a brief comparison with the typology of wooden (and also “hidden”) tie-rods inserted in Santa Maria del Fiore dome is presented in order to better understand their fundamental role in the conservation process of these great structures. Numerical analyses, carried out considering both simplified approaches and refined finite element models, are employed to deepening the effectiveness of the “hidden” tie-rods.