ADVENTURES IN ARCHITECTURE
Il Duomo: Brunelleschi and Innovations of the Dome of Santa Maria del Fiore
Episode 6: Innovations of the Dome

by Jim Atkins, FAIA, FKIA
Illustrations by Jim Anderson

Summary: In our last episode, Filippo became lord of the rings as he solved structural design challenges that allowed the dome to stand without buttresses. We followed his exploits as he argued his design concept with the wardens of the Opera del Duomo, first being ridiculed and cast away, and ultimately keeping his cool and convincing them that his design was the best way to go.

As architects, we dream at night of such a successful and rewarding experience.

We followed his path, creating the stone rings and the troublesome wooden ring, experiencing his accomplishments in achieving the unique design that is celebrated today. We examined the enigmatic iron rings, and their supposed, but questionable existence within the dome structure.

But the infamous rings alone did not allow this masonry dome of domes to stand as the great architectural icon that we know today. The design required for this structure to consist of other, less celebrated elements that, combined with the rings, lend to its overall structural integrity. These other components were integral to Filippo’s amazing design and no doubt worthy participants in its success.

Return with us now to the 15th century where we will examine these amazing elements of the dome; important contributors to its integrity and performance. These innovative components, some added by design and some possibly by necessity, reinforce the unchanging dynamic nature of architecture.

A dome within a dome
The existing technology at the time Brunelleschi appeared on the scene did not support the construction of a span the size of Santa Maria del Fiore. Although the Romans had made great accomplishments in dome construction, specifically with the Pantheon, their technology had been long disregarded and forgotten.

The solution to the dome of Santa Maria del Fiore lay with the two shells, and existing built examples were the baptisteries of Florence and Pisa. Brunelleschi was no doubt guided by these two successful designs, although they were much smaller in diameter than Santa Maria.

The technology of the double shell was later used by Michelangelo on the dome for St. Peter’s Basilica in Rome. He was a great admirer of Brunelleschi and was quite humble when referring to Filippo’s work: “It would be difficult to equal…impossible to surpass. I shall build its sister, bigger perhaps but not more beautiful.”

The sandstone rings
The sandstone rings were built in a “lattice” design, and when used as they are in the dome, they may well be behaving like a truss, giving diaphragm strength to the two shells as well as acting in tension, holding the sides together. Given that the tension aspects of the rings have been questioned by Giovanni and Michelle Fanelli in their book, Brunelleschi’s Cupola, as we observed in the last episode, the truss effect may be playing a more vital role in the dome’s structural behavior than is typically acknowledged.

Perhaps the most practical aspect of the two domes is its accommodation of the stairway up to the lantern, a design element that has allowed Il Duomo over the centuries to become a much more interactive tourist attraction than it otherwise would have been. Visitors can ascend up one side of the dome to the lower gallery before continuing on to the lantern, and descend from the upper balcony back down to the cathedral floor below. (We will explore this adventurous route in greater detail in a later episode.)

Filippo also used the stairs within the two shells to allow access for the workers during construction, much like we do in buildings today when we install the stairway components as soon as the building frame and decking have been completed. In the 15th century, this allowed workers to access the building floors without having to manage more hazardous temporary wooden ladders. They did not fear worker’s comp claims as we would today, they were more fearful of losing the workers.

The horizontal arches
Another significant design element of the dome is the 144 horizontal arches attached to the outer shell at the vertical corner ribs intersecting the eight octagonal sides. These bracing arches occur in the upper two-thirds of the dome between the two shells. They actually perform as braces around the vertical ribs, adding depth to the structural performance of the rib itself. They almost touch the inner dome at their greatest point of protrusion. Interestingly, they are not mentioned in Filippo’s original specification.

On the contrary, this design feature was first mentioned in the 1426 amendments, and they were possibly intended as a temporary measure to brace the assembly during the construction process. They are about 3 feet in length and 2 feet in depth, and they are visible from the cavity between the two shells.

We cannot help but wonder why this design feature was incorporated. Was it a secret component that Filippo was withholding in order to conceal his complete dome design? Or was it an added detail designed during construction that was required to make his design perform; perhaps something that today would be issued as an architect’s supplemental instruction? We may never know for sure, but it is possible that our man came up with this detail on the fly; just like we do on occasion when a Request for Information brings with it the reality that our original design must be tweaked in order to be constructed and function properly.

The herringbone brick pattern
There are four million bricks in the dome of Santa Maria del Fiore, and they are quite different from those that you and I specify for buildings today. Some masonry projects these days may have a few special brick shapes, but a significant number of bricks in Il Duomo were unique in form. In addition to rectangular shapes, there are triangular bricks and other unusually shaped ones that were required to fit between the inward-curving triangular segments between the “ribs” of the dome.

Instead of laying the bricks in conventional horizontal courses, they were placed on end to transfer the forces downward along the curvature of the dome. Had they been laid conventionally, they would tend to slip inward since they would be inclined at the same angle as the dome itself.

The shapes and placement of the bricks resulted in a “herringbone” pattern. It is not known how Brunelleschi developed this unique design, but it had been previously used in Persian and Byzantine domes. Some speculate that he may have visited these areas during his earlier self-studies.

The purpose of using brick instead of sandstone or heavier materials in the upper levels of the dome was to decrease weight and thus reduce the inward forces. Each brick was molded for its position, with faces angled to conform to the inclination of the dome. The forms for the bricks were wood, and, according to the biography of Brunelleschi written by Manetti in the 1480s, Filippo personally inspected every brick.

… not a small stone or brick was placed which he did not wish to examine to see whether it was correct and if it was well-fired and cleaned … He personally went to the brickyards … He seemed to be the master of everything.

Obviously, Filippo’s construction phase services were more comprehensive than those we provide today. Can you imagine personally inspecting every brick in a building today?

The herringbone design was a structural success because the intermediate vertical bricks locked the horizontal bricks into position, creating a self-supporting arch even before the mortar had completely cured. This ingenious approach allowed construction to go forward without the elaborate wooden centering and without having to wait for the mortar to reach its full compressive strength.

The time required for the mortar to set was critical to job progress, and it is speculated that Filippo used sodium carbonate, or soda ash, to accelerate the process. This was a remarkable Roman innovation that was lost for centuries but somehow was “rediscovered” in the time of Brunelleschi.

The mixing of the mortar was done high up on the dome so it could be used while it was still in a plastic state. The lime, sand, and water were hoisted to the top where the mixing took place. Replicas of the tools used for the masonry work are on display as you descend from the upper gallery down to the floor of the cathedral.

Assuming that Brunelleschi had visited buildings constructed in this manner and had some prior understanding and confidence in the design, it is remarkable that he attempted a dome of such scale, especially when this construction technique had not been previously used by the Florentines. It is more remarkable that no dome larger than Santa Maria del Fiore has been constructed in the six centuries hence.

Hazards of the trade
Since centering was not used to construct the dome, the masons had to work on the raw masonry precipice with nothing between them and the floor far below. This precarious staging was obviously alarming to the masons as they worked, so Filippo built a platform inside the vault that served as both a safety net and a visual screen. It served no purpose as a structural support.

Safety harnesses were also used for protection, and the mason’s wine was diluted with one-third part of water. Apparently they imbibed during working hours, which would no doubt these days put OSHA into a tailspin. Those who did not comply with the rules were fined the equivalent of 11 days’ work. Workers were also forbidden to ride in or transport their tools in the wood boxes used on the hoist to lift materials to the top.

Although OSHA was not around to keep things in order, these safety methods were very effective in that only one death occurred during construction of the dome. This is quite an accomplishment considering the number of workers, the length of construction time, and the wine.

Unemployment was also a threat to the masons. As the dome grew toward completion, fewer masons were needed, and many were laid off. There was also some discontent with the wages, and “having selfishly unionized themselves,” according to Manetti in his biography of Brunelleschi; they went on strike for higher pay. Such strikes were not only unknown to the Florentines, they were against the law. The wealthy guilds ruled the workers harshly, allowing few accommodations.

When the strike occurred in 1426, Filippo moved swiftly and fired the lot, hiring Lombards to replace them. The masons, realizing they were faced with permanent unemployment, humbly asked for their jobs back. Filippo immediately rehired them at a lower rate.

Again, we can see that many of the issues and challenges that occur in architecture and construction today were alive and well in the 1400s.

The many innovations that Brunelleschi designed and developed to construct the dome were extraordinary, original inventions that awe engineers, even today. But equally remarkable were the devices used to lift materials to the lofty regions where the dome was being constructed. The technology for lifting materials of this size and weight did not exist at the time Filippo created the magnificent dome of Santa Maria del Fiore.

He was without question one of the most talented and prolific architects of his time. His involvement and craftsmanship in other trades reinforced his architectural skills and allowed him to be the great builder he was. We should all strive to be as thirsty as Filippo to learn of the artistry outside of architecture as well as within.

Until next time, good luck out there.