Florence Cathedral began in the late thirteenth century, but its immense crossing remained without a finished dome for generations. Filippo Brunelleschi developed an ingenious masonry solution in the fifteenth century. The completed dome became a landmark of Florence and a major achievement of Renaissance construction.
History
Santa Maria del Fiore was built on the site of Florence's second cathedral dedicated to Saint Reparata; the first bishop's seat was the Basilica of San Lorenzo, the initial building of which was consecrated as a church in 393 by St. Ambrose of Milan. The ancient structure, founded in the early 5th century and having undergone many repairs, was crumbling with age, according to the 14th-century Nuova Cronica of Giovanni Villani, and was no longer large enough to serve the growing population of the city. Other major Tuscan cities had undertaken ambitious reconstructions of their cathedrals during the Late Medieval period, such as Pisa and particularly Siena where the enormous proposed extensions were never completed.
The city council of Florence approved Arnolfo di Cambio's design for the new church in 1294. Di Cambio was also architect of the church of Santa Croce and the Palazzo Vecchio. He designed three wide naves ending under an octagonal dome, with the middle nave covering the area of Santa Reparata. The first stone was laid on 9 September 1296, by Cardinal Valeriana, the first papal legate ever sent to Florence. The building of this vast project was to last 140 years; Arnolfo's plan for the eastern end, although maintained in concept, was greatly expanded in size.
Dome
After a hundred years of construction and by the beginning of the 15th century, the structure was still missing its dome. The basic features of the dome had been designed by Arnolfo di Cambio in 1296. His brick model, 4.6 m (15.1 ft) high, 9.2 m (30.2 ft) long, was standing in a side aisle of the unfinished building, and had long been sacrosanct. It called for an octagonal dome higher and wider than any that had ever been built, with no external buttresses to keep it from spreading and falling under its own weight.
The commitment to reject traditional Gothic buttresses had been made when Neri di Fioravanti's model was chosen over a competing one by Giovanni di Lapo Ghini. That architectural choice, in 1367, was one of the first events of the Italian Renaissance, marking a break with the Medieval Gothic style and a return to the classic Mediterranean dome. Italian architects regarded Gothic flying buttresses as ugly makeshifts. Furthermore, the use of buttresses was forbidden in Florence, as the style was favored by central Italy's traditional enemies to the north. Neri's model depicted a massive inner dome, open at the top to admit light, like Rome's Pantheon, partly supported by the inner dome, but enclosed in a thinner outer shell, to keep out the weather. It was to stand on an unbuttressed octagonal drum.
Cracking of the dome
The unreinforced masonry that Brunelleschi used to construct the dome is weak in tension which leads to cracking when tensile stresses exceed the limited masonry tensile strength. The material is especially susceptible to damage from seismic loading due to its heterogeneity and many surfaces between different materials (stones to mortar connection). The first written evidence about the presence of cracks appears in a report by Gherardo Silvani report dated 18 September 1639 which refers to "peli" ("hairs"). In 1694 Giambattista Nelli and Vincenzo Viviani surveyed the cracks with Nelli recording that there were two major cracks with a maximum width of 29 mm (1.1 in). They believed that the cracks were caused by the weight of the dome, and the resulting horizontal thrusts on the pillars. A commission, headed by Vincenzo Viviani, carried out investigations in 1695 and came to the conclusion that the cracking was due to the dead weight of the buildings.