Dome or Brunelleschi’s Dome. In 1418 the worried Florentine fathers announced a contest for the ideal dome design, with a handsome prize of 200 gold florins and a shot at eternal fame for the winner. By the beginning of the 15th century, after a hundred years of construction, 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, 15ft high, 30 ft long, was standing in a side aisle of the unfinished building, and had long ago become 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, but enclosed in a thinner outer shell, partly supported by the inner dome, to keep out the weather. It was to stand on an unbuttressed octagonal drum. Neri's dome would need an internal defense against spreading (hoop stress), but none had yet been designed. The building of such a masonry dome posed many technical problems. Brunelleschi looked to the great dome of the Pantheon in Rome for solutions. The dome of the Pantheon is a single shell of concrete, the formula for which had long since been forgotten. Soil filled with silver coins had held the Pantheon dome aloft while its concrete set. This could not be the solution in the case of a dome this size and would put the church out of use. For the height and breadth of the dome designed by Neri, starting 171ft above the floor and spanning144 ft, there was not enough timber in Tuscany to build the scaffolding and forms. Brunelleschi chose to follow such design and employed a double shell, made of sandstone and marble. Brunelleschi would have to build the dome out of brick, due to its light weight compared to stone and being easier to form, and with nothing under it during construction. To illustrate his proposed structural plan, he constructed a wooden and brick model with the help of Donatello and Nanni di Banco, a model which is still displayed in the Museo dell'Opera del Duomo. The model served as a guide for the craftsmen, but was intentionally incomplete, so as to ensure Brunelleschi's control over the construction.
Brunelleschi's solutions were ingenious. The spreading problem was solved by a set of four internal horizontal stone and iron chains, serving as barrel hoops, embedded within the inner dome: one at the top, one at the bottom, with the remaining two evenly spaced between them. A fifth chain, made of wood, was placed between the first and second of the stone chains. Since the dome was octagonal rather than round, a simple chain, squeezing the dome like a barrel hoop, would have put all its pressure on the eight corners of the dome. The chains needed to be rigid octagons, stiff enough to hold their shape, so as not to deform the dome as they held it together.Each of Brunelleschi's stone chains was built like an octagonal railroad track with parallel rails and cross ties, all made of sandstone beams 17in in diameter and no more than 7.5ft long. The rails were connected end-to-end with lead-glazed iron splices. The cross ties and rails were notched together and then covered with the bricks and mortar of the inner dome. The cross ties of the bottom chain can be seen protruding from the drum at the base of the dome. The others are hidden. Each stone chain was supposed to be reinforced with a standard iron chain made of interlocking links, but a magnetic survey conducted in the 1970s failed to detect any evidence of iron chains, which if they exist are deeply embedded in the thick masonry walls. Brunelleschi also included vertical "ribs" set on the corners of the octagon, curving towards the center point. The Ribs, 13 feet deep, are supported by 16 concealed ribs radiating from center. The ribs had slits to take beams that supported platforms, thus allowing the work to progress upward without the need for scaffolding. A circular masonry dome can be built without supports, called centering, because each course of bricks is a horizontal arch that resists compression. In Florence, the octagonal inner dome was thick enough for an imaginary circle to be embedded in it at each level, a feature that would hold the dome up eventually, but could not hold the bricks in place while the mortar was still wet. Brunelleschi used a herringbone brick pattern to transfer the weight of the freshly laid bricks to the nearest vertical ribs of the non-circular dome.