By around 1413 or 1414, construction was completed to the top of the drum, and the question of how to construct the dome could no longer be ignored.
Several circumstances of the existing construction increased the difficulty of building a dome and influenced the design of its main features.
The width of the dome's base measures 138 1/2 feet, a distance exceeded by only the ancient Roman Pantheon, whose width is 143 feet. (The second largest pre-Renaissance dome is that of the sixth-century Byzantine church Hagia Sophia in Istanbul, which is 108 feet wide and 180 feet high). The immense size of the dome of Florence cathedral can be appreciated by a size comparison with other world monuments.
An octagonal drum rose from an octagonal arrangement of supports at the crossing. The drum, whose individual sections of wall were pierced by oculi, provided a support that was uniform in width and relatively narrow in relation to the span. Consequently, it was not possible to use thick-walled, solid-core construction like that used for the concrete dome of the Pantheon, whose supporting walls are twenty feet thick.
Because the span was so wide, centering was impractical. No single beam could begin to span such a wide opening (centering for large vaults was generally supported at the level of the springing line), and centering built up from the ground level, a distance of 180 feet, would have required a forest of timber.
An administrative board overseeing the cathedral's construction consulted Brunelleschi and others in making its decisions about the dome's design. The dome’s three main features were an octagonal plan, a vertical profile, and double-shell construction.
Because the drum had already been built in an octagonal shape, the dome also had to be octagonal in plan.
Domes having straight-sided plans are called domical vaults.
The design of a double-shell dome was a unique solution to the problem of lightening its weight. Earlier efforts to lighten the weight of vaulting included the Roman use of coffering and the Italian-Gothic use of rib vaulting.
The dome's steeper-than-hemispherical shape provides a reduction of stress at the haunch, the point of an arch about halfway between the crown and the springing line, by concentrating weight over its base. (This principle explains the use of pointed arches in the construction of Gothic cathedrals.)
Had a steep dome not been structurally advantageous, Brunelleschi would probably have used a hemispherically shaped dome like that of the Pantheon and all his subsequent domes.
After the completion in 1412 or 1413 of the drum, whose span and thickness had been determined in the previous century, the overseers of the Cathedral works still sought a way to engineer such a large structure.
A competition was held in 1418, and the commission was awarded to Brunelleschi and Ghiberti jointly. Their collaboration was not congenial, and in 1425 Ghiberti withdrew, leaving the job to Brunelleschi, whose ideas had been dominant.
Using ancient models that he studied in Rome, Brunelleschi devised a technique of masonry construction using a herringbone pattern that interlocked the successive courses so that each course was supported by the previous one during construction.
The lower courses were built of stone and the upper ones were of brick.
The construction progressed as a series of rings, whose lateral plane curved downward toward the center. Once a ring was completed, it became self-supporting because its inward-leaning stones or bricks pushed against each other.
In order to reinforce the haunch, the point where the outward thrust is greatest, Brunelleschi formed a collar of oak timbers fastened together by iron rings.
Brunelleschi devised mechanical aids such as a crane-like machine that could lift building materials to wherever needed. This saved workers from having to carry heavy building materials up ladders to the construction level. These mechanical devices exemplified a new Renaissance problem-solving attitude and bolstered Brunelleschi's reputation as an architect.
Construction of the dome was completed in 1436, and it was the genius of Brunelleschi that made it possible despite so many adverse conditions.
Although other large domes were built in the Renaissance, such as the dome of St. Peter's, which spans 137.5 feet, their construction was facilitated by stronger supporting systems: pendentives rising from massive piers were incorporated into the initial plans and built from the start.
Furthermore, later architects had the benefit of Brunelleschi's example. Michelangelo, upon receiving the commission for St. Peter's, studied Brunelleschi's dome and obtained measurements of its dimensions. Although Michelangelo's dome differed in shape and design, it was similar to Brunelleschi's in having double-shell construction.
In the sixteenth century, the dome's interior was painted with a scene of the Last Judgment using a complex program devised by Vincenzo Borghini. The painting was begun by Giorgio Vasari and finished by Federico Zuccaro.
The exedrae contributed to the dome end's similarity to a centralized design because they added mass to the center and echoed the overall circularity. Semicircular projections were an important feature in many centralized churches.
The alternation of pairs of Corinthian half-columns with apsidal niches that are carved in relief with fan-shaped shell motifs gives the outer facings of the exedrae a highly sculptural quality.
In 1436, Brunelleschi's design for the lantern was selected in another competition. Construction was not begun for ten years, and Brunelleschi died within months of its being begun. The lantern was largely built according to Brunelleschi's designs by his follower Michelozzo, who worked on it until 1461.
The lantern's design employs novel variations of Classical forms.
Below the conical roof, which has flute-like contours, are eight blind arches. Horizontal moldings connect the arches into a continuous line.
Eight round-headed windows in the lower section help light the rotunda below.
Between the windows of the lantern, eight console-topped arches function as flying buttresses to transfer the thrust of the lantern's sides to the dome's eight major ribs. These buttressing devices are ornamented with carved flutes, shells, and scrolls forming consoles.
In the next century, another competition was held to determine the design for the trim that would cover the unfinished band of wall between the drum and the dome.
Stonework along one of the eight facings was carried out in 1508 according to the winning entry by Baccio d'Agnola. Michelangelo reportedly compared it to a cage for crickets, and construction was halted.
Today the single section stands out, and the bold simplicity of the undecorated drum is generally considered to be more attuned to the dome's boldness of conception.
In the sixteenth century, the windows piercing the dome were filled in on the inner shell, which reduced the interior illumination.
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Dome of Florence Cathedral, 1420-36