ADVENTURES IN ARCHITECTURE
Il Duomo: Brunelleschi and the Dome of Santa Maria del Fiore
Episode Five: Lord of the Rings

by James Atkins, FAIA, FKIA
Illustrations by Jim Anderson

Summary: In our last episode, we followed Filippo and his artist friend, Donatello, as they explored the ancient ruins of Rome carefully studying construction techniques, measuring, and sketching their findings. Leaving the locals to think they were vagrants in search of abandoned treasures, they remained in their quest to learn about the superior Roman building technology for almost 13 years. Of particular interest was the Pantheon, the domed temple built by the emperor Hadrian in 125 AD. Knowing that Santa Maria del Fiore would eventually include a dome, Filippo measured and studied the Pantheon to learn of its design and construction.

Filippo eagerly responded to the Opera’s competition for the new dome for Santa Maria, announced in 1418, and he was selected, along with his arch-rival Lorenzo Ghiberti, as a capomaestro, or architect-of-record. Santa Maria’s supporting walls had been built without buttresses or perpendicular bracing walls, with the assumption that tension rings, or chains, could be used to counter the outward forces on those walls. However, the design and materials for the rings had yet to be determined.

Join us as Filippo takes charge in the development of the dome design and the rings that would support it. Such a task had never before been attempted, and completion of the project would require extraordinary talent and resolve. Let’s return to the 15th century as Filippo becomes lord of the rings.

Perception
All of the masters competing for the dome design, except for Filippo, assumed that centering would be required to support the heavy double-walled vault. Filippo never considered this method. On the contrary, he maintained that that the dome could not be built using centering and insisted that the Opera was obligated to build without it.

Given his limited influence in Florence and his lack of experience on a structure of such magnitude, he was ridiculed by the Opera and the townspeople. Ultimately, the Opera began to assume that such a structure could not be built, and they believed the earlier masters were naïve in thinking it could be done in the first place.

Filippo protested this conclusion, and the wardens responded by demanding he explain how it could be built. He continued to assert his position of building without centering, and twice he was angrily thrown out of the meetings by the wardens, who called him a madman who uttered nonsense. Filippo was quite humiliated and ashamed to go about the city. Nonetheless, he persisted with his beliefs and continued patiently to assert his design concept, using great diplomacy while curbing his volatile temper. (Ah, the quest for a project can guide us in remarkable ways!)

After a time, observing his continuing resolve, the wardens began to accept Filippo’s position and asked him to demonstrate his proposed method by using it in the construction of a small chapel in Florence named San Jacopo di Borgo Oltrarno, where a dome was scheduled to be built. Filippo accepted the request and demonstrated in this much smaller scale his proposed construction approach. The project was a success, and it was the first project in Florence to be vaulted using such a method.

Afterwards, the wardens requested a more detailed explanation of his proposed plan, and he responded with great patience and precision describing the process in great detail. They asked for his specifications in writing, and he produced a written work that was bound in a book and used to satisfy the lenders and suppliers of the marble and timber. (Hmmm, documentation … it was important then, it is important now.)

The specification below is excerpted from the book, The Life of Brunelleschi, by Antonio Manetti, originally written in the 1480s and translated by Catherine Enggass and published by The Pennsylvania State University Press in 1970.

. . . the cupola has the . . . measure of the pointed fifth . . . on its inner side . . . Let another outer cupola . . . be made to preserve it from moisture . . .

In this space [between the cupolas] the stairs are to be placed . . . the said cupola are to be encircled by six rings of . . . sandstone beams well tied together with bars of leaded iron and over the . . . sandstone beam . . . iron chains . . . ”

The specification continued in great detail explaining how the two shells would be tied together, as well as how the rainwater would be managed. The unit of measurement was the braccia, a measurement derived from the distance of the elbow to the fingertip, or about 23 inches.

Filippo’s specification was given in 1420, and it instilled confidence in the Opera and the townspeople, giving him great credibility and confidence. The perception by the wardens of his skill and talent had to be recognized for him to get the job. He had no relevant past projects to use as examples, and he had to rely on his persuasive abilities to make the difference.

The sandstone chains
By early June 1423, the wardens finally agreed upon a design for the chains. This design was quite complex, and one of Filippo’s carpenters was paid to construct a wooden model to demonstrate how the chains would be built.

The chains were somewhat elaborate, with two horizontal rings of stone concentrically placed. These two rings were connected with transverse connecting beams, attached on three-foot centers with iron clamps and dovetailed into the rings. The completed assembly formed a sturdy, rigid lattice that had to follow the inward curve of the masonry.

Although the rings were considered to be “hoops” whose tensile forces counteracted the outward push of the sides of the dome, Giovanni and Michelle Fanelli, in their book, Brunelleschi’s Cupola, state:

“ . . . the idea being that the longitudinal chains were a sort of hoop of reinforcing bars helping to contain the outward thrust of the segments . . . their effectiveness in performing this kind of structural role is quite negligible . . . after construction the chains broke along the cracks, and the lowest one was even interrupted . . . by four passages left between the inner-shell cavity and the upper galley of the drum . . . ”

So it appears that the “tension” rings were likely not performing in that manner at all, but instead acting more as a system for bonding the masonry.

It has a nice ring to it
The strata of the hills around Florence were made up of sandstone with high quartz content. The material had so much integrity that its prior use had been for grindstones. The chosen quarry for the rings was Cava di Trassinaia, which was just a few miles north of the city.

After the stones were rough-cut, they were tested for monolithic integrity with a hammer. A good beam would have a ring, not unlike striking metal. A dull sound would indicate a fissure or crack in the material.

Each of the eight sides of the concentric rings consisted of 12 beams, each 7 feet, 6 inches long by 1 foot, 5 inches thick, for a total of 96 beams. The connecting transverse beams in the lowest ring protruded through the sides of the dome just above the round windows and are visible from below.

The beams that made up the rings were tied together at their connecting ends with iron clamps. The forged iron clamps were then coated with lead to prevent rust and subsequent degradation.

There would be four stone chains in total, unlike the specification that called for six. A fifth ring would be made of wood. The chains were spaced about 35 feet apart vertically, and the upper chains were tilted in with the curve of the dome, making them much more difficult to install.

Initially, four wooden chains were planned, but as in architecture these days, material unavailability and subsequent design modifications overrode the original concept. The only wooden chain, installed 25 feet above the bottom stone chain, would prove to be quite challenging for several reasons.

Originally intended to be of oak and 20 feet in length, supply limitations would bring about a substitution of chestnut, with three wooden beams required on each leg of the octagon, or 24 in all. However, they would be connected with oak plates.

Although finding the chestnut beams proved difficult, curing them would be as daunting. They first had to be soaked in water for a month, and then they were buried in ox dung for almost as long. They were then placed on a bed of ashes and protected from the elements for several years. (Perhaps our limited supply of oxen has inhibited our advancements in building technology.)

Project delivery scheduling by contemporary methods would place the wooden chain on the critical path, and it is not surprising that only one wooden chain was installed. Some assert that Filippo had an ulterior motive for using the wooden chain: a ploy to undo his dreaded rival, Lorenzo.

The final blow
Filippo had never really gotten over the baptistery door competition, when both he and Lorenzo had been selected and he had refused to accept the commission. To add insult to injury, Lorenzo had been given equal status to Filippo in the dome competition. Also insulting was that both were paid the same salary of three florins per month. No doubt, Lorenzo would likely pursue equal credit for the dome itself. (Oh, those owner-mandated design team members! Every owner wants to play architect, even as far back as the 15th century.)

The wooden chain apparently gave Filippo the idea for a scheme to attack Lorenzo’s credibility and do in his rival once and for all. Feigning an illness, Filippo took to his bed and did not return to the site except in bandages. With Filippo incapacitated, responsibility for the wooden chain fell to Lorenzo. Since Filippo had not revealed his design, Lorenzo did not know what to do. Filippo’s condition appeared to worsen, and Lorenzo finally gave instructions as best he could, but he was not aware of the complex connections that Filippo had designed. This underscores the importance of effective communication among team members, not to mention a structured submittal review process.

After only a few wooden beams had been installed in the ring, Filippo made a dramatic recovery and reappeared on the site, immediately condemning the work of Lorenzo. The work was removed, and Lorenzo was exposed and disgraced before the wardens and the townspeople. Filippo took charge and began completion of the wooden chain. Filippo’s salary was increased, and Lorenzo’s was initially suspended, later to be reinstated at a fraction of Filippo’s. Lorenzo would become less and less involved with the dome and ultimately spend most of his time on other commissions. (Such mischief would not be possible today, as there are far too many lawyers among us itching to pursue defamation.)

Only one wooden chain was installed in the dome, and it is visible today on the ascent up the stairs between the two shells. (And, although Filippo was certain that he had dealt the final blow to Lorenzo, he would later find out otherwise. Don’t miss the continuing antics of Lorenzo in a future episode.)

Continuous iron chains
The original design called for iron chains to be installed within the masonry. However, only the existence of the lowest chain has been confirmed, because it is partially visible in the cavity between the two shells. Investigations with magnetic sensing equipment in the 1970s failed to confirm the presence of the chains, but large expenditures for iron components during dome construction are well documented. There is no reason to doubt the presence of the iron chains, but given the many design changes during construction, they may have been left out. The testing in the 1970s certainly supports this.

A model role
Filippo Brunelleschi accomplished something that neither his peers nor the ancient Romans had been able to do. Through his persistence, innovation, and technical genius, he conceived, designed, and built a monument not only to masonry construction but to dome design that prevails today.

The question arises: If this man of practical education and limited technical resources could do the great things he did in that era of antiquity, what is the potential for architects today with our magnificent universities, profound technologies, and unlimited resources? Perhaps we have it too easy. We surf the Web looking for answers rather than answering; we rely on others to solve rather than solving.

We should quest for problems and challenges that yet have no answers or solutions. We should seek to become lord of our own rings of accomplishment.