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The Renaissance
Brunelleschi's Plan
His ideas were indeed revolutionary, and so numerous that I'll only mention a few here. For one thing, the dome isn't a dome. Brunelleschi realized correctly that a dome that size would but unstable (given the building materials of the day), so he designed a cupola that is eight pointed arches. These are so smoothly integrated, however, that the initial visual impact is of a dome.
The cupola is not one dome, but two, one inside the other. This was one of the points that caused others to think Brunelleschi was loony, but again only Filippo understood the matter correctly. The cupola is a double-shell construction, with the two shells being solid at the base and then, higher up, with open space between and ribbing connected the two. The double-shell gives the cupola strength and lightness.
The lantern, which is the tower-like construction on top of the dome, Brunelleschi designed to be huge. When the Florentines learned of the great size of the lantern, they thought Brunelleschi had gone too far in his presumption. Once again, though, Filippo had it right--the great weight of the lantern is needed to lock the eight arches of the dome into place.
Filippo saw to matters both great and small. He understood that wind would imperil a structure that large, and designed into the construction holes and fluting that would ease the stress. He understood even that rain would add much weight and designed the gutters and spouts so as to move the water quickly from the surface. He designed in stairwells, so that it is possible to climb all the way up into the lantern.
The completed cathedral rises nearly 400 feet, including a prodigious 70 feet for the lantern alone. The dome itself is 90 feet high and spans 140 feet at its base. It's an astounding achievement.