Brunelleschi's Dome

about the stability of their design. His doubts illustrate a fear that haunted architects in the Middle Ages. Today a patron who hires an engineer takes it for granted that the end product will stand, even through earthquakes and hurricanes. But in the Middle Ages and the Renaissance, before the science of statics was developed, a patron enjoyed no such assurance, and it was not uncommon for buildings to fall down soon after completion, or even during the building process itself. The bell towers in both Pisa and Bologna began to lean while still under construction because of subsidence in the underlying soil, while the vaults in the cathedrals at both Beauvais and Troyes collapsed a relatively short time after being raised. The superstitious attributed these failures to supernatural causes, but to the more knowledgeable the real culprits were the architects and builders who had made fundamental (though imperfectly understood) errors in design.
In the end Giovanni’s concerns led the wardens to stipulate that, although Neri’s model would be adopted, the pillars that supported the dome should be enlarged. But enlarging the pillars would create perhaps even greater problems. Their dimensions were directly related to those of the octagonal tribune, whose perimeter they would form. The foundations for an octagon of 62 braccia had already been begun: would this groundwork have to be undone? Even more serious, the diameter of the tribune could not be enlarged without a corresponding increase in the span of the cupola. Was it possible to build a dome with a span even larger than 62 braccia, still without the use of any visible supports?
These questions were addressed at the meeting in August 1367, in which the wardens opted for a dome that would be 10 braccia wider than the one previously planned. Three months later, in keeping with Florentine democracy—and also, perhaps, with a desire on the part of the wardens to spread the responsibility as widely as possible—the plan was endorsed by a referendum of Florence’s citizens.
The decision to adopt Neri di Fioravanti’s design represents a remarkable leap of faith. No dome approaching this span had been built since antiquity, and with a mean diameter of 143 feet and 6 inches it would exceed that of even the Roman Pantheon, which for over a thousand years had been the world’s largest dome by far. And the cupola of Santa Maria del Fiore would not only be the widest vault ever built: it would also be the highest. The walls of the cathedral were already 140 feet high, above which a tambour (or drum) on which the dome was to rest would rise another 30 feet. The purpose of this tambour was to elevate the dome—to serve, in effect, as a pedestal, raising the dome even higher above the city.4 Vaulting for the cupola would therefore begin at an incredible height of 170 feet, much highe