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Winter 2006
The skyline of Florence is immediately identifiable by its iconic dome that has towered over the city since 1436.
Photo courtesy of Pennie Sabel
By Pennie L. Sabel
Photos courtesy of Pennie Sabel
Filippo Brunelleschi isn't one of the better-known, early Italian Renaissance architects; however, his legacy — the iconic dome of the Cathedral of Santa Maria del Fiori — is symbolic of Florence still to this day.
Brunelleschi was born in 1377 in Florence, the son of a prosperous lawyer. His father wanted him to follow in his footsteps, but Brunelleschi had his own ideas. Having an ability to solve mechanical problems, he apprenticed with a goldsmith and clockmaker, where he studied mathematics, mechanical devices and the science of motion with its complex gears, wheels and weights. There he built a number of unusual clocks, including one of the first alarm clocks.
Filippo Brunelleschi sits on the Piazza del Duomo gazing up at the architectural wonder that he created.
Photo courtesy of Pennie Sabel
Later in life, Brunelleschi, with his sculptor companion Donatello, went to Rome to study the ancient Roman architecture. They studied the proportions of columns and pediments, and determined the measurements specific to the three architectural orders — Doric, Ionic and Corinthian — that are governed by precise mathematical ratios. They also studied the ancient Roman methods of vaulting, as well as the dome of the Pantheon that would prove to be very useful when Brunelleschi began constructing the dome for the new cathedral in Florence.
On Aug. 19, 1418, a competition was announced in Florence for the design and vaulting of the dome to be built over the city's cathedral, which had been under construction for more than 100 years. The original design called for a grand octagonal pointed dome, which would be the highest and widest ever raised without support from buttresses; however, no one knew how to build it. The planners had faith that, in time, someone would come forward with the solution to this architectural and engineering puzzle.
The interior of the dome is the spherical shell that spans 143 feet in diameter and features the fresco of the Last Judgement by Giorgio Vasari and Federico Zucchari. On the climb up to the top of the dome, between the shells, you can stop and see the work of art up close.
Photo courtesy of Pennie Sabel
For such competitions it was customary for participants to build models so the officials could visually inspect what was being proposed. Brunelleschi's elaborate model was composed of brick with wood carvings done by Donatello and built in a courtyard near the cathedral — it spanned six feet and stood 12 feet high. The finalists in the competition were Brunelleschi and Lorenzo Ghiberti (sculptor, 1378-1455). After much political haggling, because Brunelleschi refused to divulge the technical details of his design, he was put in charge of building the dome.
Brunelleschi's design was unorthodox in that it required no centering, such as wooden support structures, to hold the stone and brick in place while the mortar cured. He knew there was not enough timber in Tuscany to build the scaffolding needed to support such a framework for the proposed, 143-foot-diameter dome. Using the double-walled Pantheon's dome as inspiration, Brunelleschi's design consisted of two, self-supporting shells; an inner spherical dome spanning the diameter; and a parallel outer shell to protect it from the weather. The shells would sit on the cathedral's octagonal drum — the vertical wall that carries the dome — and the dome itself would be crowned with a lantern, or windowed superstructure, that provides the dome's balancing force.
The great dome peeks out from behind the front of the Cathedral of Santa Maria del Fiori. At the top of the lantern, tourists who made the climb give perspective to the lantern.
Photo courtesy of Pennie Sabel
With his design approved by the cathedral's officials, Brunelleschi had to go about proving that it could, in fact, be built.
The cathedral walls are 140 feet high and the drum an additional 30 feet. The vaulting begins at 170 feet above the ground. The width of the inner dome tapers from seven feet at the base to five feet at the top; the exterior shell is two feet at the base, tapering to one foot at the oculus. The shells are supported by 24 circular stone ribs that are seven feet thick at the base and taper to five feet at the top, meeting in an open stone compression ring. The shells are built of stone for the first 46 feet, with lighter weight brick laid in a rotating herringbone pattern the rest of the way up.
The brick used are not the typical rectangular building blocks, but of Brunelleschi's design, consisting of rectangular, triangular, dovetailed, and bricks with flanges and special shapes to fit the angles of the octagon. In all, about four million bricks were used. To resist the outward thrust, a series of interlocking sandstone rings were embedded horizontally in the masonry, holding the structure together like a barrel.
Detail of the carving on the Carrara marble buttresses.
Photo courtesy of Pennie Sabel
The dome was to be built of brick, sandstone, limestone, marble and timber. At the time, sandstone and limestone quarries were plentiful around Florence, and the marble was brought from Carrara, more than 80 miles away.
Brunelleschi's next challenge was to get the building materials 170 feet to the top of the drum. This was no small task, for the beams in the sandstone rings alone weigh approximately 1,700 pounds. He again put his remarkable mechanical skills to work and designed a hoist with complex meshing parts and the world's first reverse gear clutch that allowed oxen, rather than men on a tread wheel, to raise and lower a load of stone or brick. The hoist required a specially made rope that was 600 feet long and weighed a thousand pounds.
The lantern at the top of the dome has eight hollow buttresses that line up with the eight ribs. The lantern acts as the common keystone.
Photo courtesy of Pennie Sabel
To raise the huge blocks of stone, Brunelleschi contrived a special mortise and tenon hanger. It was known as one of the most remarkable machines during the Renaissance; other architects and engineers, including Leonardo da Vinci, studied it. By the time the dome was completed, the oxen plodded for thousands of revolutions and raised approximately 70 million pounds of materials.
Getting the building materials up to the work area was one thing; moving the heavy blocks into position was yet another. Brunelleschi once again put his mind to work and designed a crane to move the loads laterally once they reached the top. The crane was built of eight pine beams, two elm trees and screws carved from walnut. The wooden mast was surmounted by a horizontal crossbeam that pivoted and consisted of screws, slideways and a counterweight. One screw moved the counterweight, while the other manipulated the load that was raised or lowered by a turnbuckle, allowing precise placement of the stone.
One of the eight ribs that form the octagonal shape of the dome and structurally support the masonry work.
Photo courtesy of Pennie Sabel
Layer by horizontal layer, the great dome rose over the city of Florence. Altogether, it took 140 years from the time the foundation stone was laid to the consecration of the Cathedral of Santa Maria del Fiori. On March 25, 1436, Pope Eugenius IV consecrated the cathedral at the Feast of the Annunciation, and the dome, deserving its own ceremony, was consecrated on Aug. 30, 1436, after being under construction for 16 years. Yet it still was not finished — the lantern was yet to be constructed on the top of the dome.
Author's Note
If you are interested in a more detailed study of the construction of the dome, read Brunelleschi's Dome: How a Renaissance Genius Reinvented Architecture by Ross King. The book was used as a reference for much of the material in this piece.
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By this time, the machines used in the construction of the dome itself had been removed from the interior of the cathedral. Once again, Brunelleschi's engineering genius was called upon to design and build another hoist and crane to move the pieces of Carrara marble to the top of the dome to build the lantern. The octagonal lantern sits on a marble base supported by the top sandstone chain. Eight hollow buttresses line up with the eight ribs of the dome and support 30-foot pilasters that are topped with Corinthian capitals. The lantern acts as the common keystone for each of the arches of the vaulting. Topping the lantern's dome is an eight-foot bronze ball and cross that stand 350 feet above the city streets of Florence.
Unfortunately, Brunelleschi did not live long enough to see the lantern completed. He died on April 15, 1446, at the age of 69. He was laid to rest in the crypt of the cathedral under a simple marble slab.
Specially designed bricks set in a rotating herringbone pattern were used for the top part of the dome to reduce the weight.
Photo courtesy of Pennie Sabel
He was called a genius in his own time and it was his work that helped to bring architects from being viewed as mechanical laborers to esteemed artists. In some ways, his work on the cathedral has never been surpassed. At 143 feet in diameter, it remains the largest masonry dome ever constructed — the dome of St. Peter's Basilica in Rome is 138 feet in diameter, St. Paul's Cathedral in London is 112 feet in diameter, and the U.S. Capitol Building dome is 95 feet.
There is a statue of Brunelleschi across from the cathedral. Seated and holding the tools of his trade, he gazes up at the dome as if keeping a watchful eye on his work of art.
The iconic dome as seen through the windows of the Museo dell' Opera del Duomo, the museum where treasures from the Duomo are sheltered for safe keeping.
Photo courtesy of Pennie Sabel
There are 463 steps leading from ground level to the lantern atop the dome that are between the inner and outer shell. But the view from the top is well worth the climb.
Photo courtesy of Pennie Sabel
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