Domes: Easy When You Know How

In the second century A.D., Roman emperor Hadrian commissioned the building of a temple to commemorate the gods of all the planets. Mind you, Rome was a city filled with temples, forums, and stadiums. What was one more building?

A great deal, as it turned out.

The temple was the Pantheon. While the architect is unknown, the Pantheon, a domed building that was the first Roman building to break with Greek architecture, and was destined to become the most influential building of all antiquity. It was a beguilingly simple design incorporating the circle and a straight line—a rotunda coupled with a rectangular entry or porch. To top it off (literally), the architect placed the rotunda beneath a dome, at the apex of which was a circular opening known as the oculus, that let in sunlight by day, and starlight by night.

Flash forward 1300 years, to the 15th Century, and two as-yet unknown Florentine artists—Donatello and Filippo Brunelleschi (pronounced "broon-ell-es-kee")—have sojourned south to study the glory that was Rome, which by this time was mostly ruins. One building, however, was remarkably intact—the Pantheon.

The people of 15th-Century Rome, while Christians, were mostly illiterate and superstitious. They wondered what drew these travelers to Rome. It couldn’t be to see the pagan buildings, chief of which was the Pantheon, could it? Because it was still standing, the Pantheon most certainly was the work of the devil; that was the belief of the people of 15th- century Rome. Had they admitted the real reason, Donatello and Brunelleschi very likely would have been burned at the stake as heretics. Instead, they said they were digging through the rubble for Roman coins, which was accepted as perfectly rational behavior. Who didn’t dig for coins? While Donatello secretly studied Roman statuary, Brunelleschi made a secret study of the Pantheon.

Back in Florence, the citizens of that fair city were looking for someone with the know-how to design and build a dome to top the magnificent and unfinished Santa Maria del Fiore. Brunelleschi was in Rome gathering data to be that someone.

As Brunelleschi studied the Pantheon, the idea gradually came to him. As architect of the Florence Dome, he faced the same problem Hadrian’s architect faced—how to build a free-standing dome without the walls pushing outward and causing the building to collapse. Both architects worked with the same materials—brick and mortar. Hadrian’s architect solved the problem with immense weight. He built the masonry walls incredibly thick and set the dome partway down within the rotunda, so that the weight of the walls pressing downward exceeded the weight of the dome pressing outward. It was as simple as that, and when it occurred to Brunelleschi he surely smiled. Still, it was no help to him. The walls of Sainta Maria del Fiore were finished and not nearly thick enough to offset the great weight of a masonry dome. It seemed the only solution was to build flying buttresses as wall supports, but the church planners already had ruled out that possibility. Brunelleschi was stumped. There had to be another way.

One day inside the Pantheon, while peering up at the five rows of the geometrically shaped coffers that circle the inside of the dome, the solution came to him. The coffers themselves, square indentions in the cement vaulting, had no structural purpose. They were merely decorative, an arresting pattern of squares that diminished in size and depth as they rose upward and inward. To Brunelleschi, the pattern suggested a framework. A light inside him went on. This was the very solution he was seeking—a framework, yes, of oak beams tethered by steel chains that would encircle the dome and prevent it from spreading outward. The great weight of the dome therefore would be forced directly down upon the walls.

It’s easy when you know how, he would say later.

ROME VIS-A-VIS FLORENCE

When Brunelleschi returned to Florence, he presented his idea as the only viable solution, and was awarded the commission.

Brunelleschi didn’t know it then, but his problems were just beginning. Emperor Hadrian had commissioned his temple during a period of tremendous construction in Rome. Unlike Brunelleschi, the Emperor was served by a thriving industry that produced bricks and mortar of unlimited supply, with abundant manpower and the latest in technology.

On the other hand, Brunelleschi was pretty much left to his own devices. Hadrian was shipped tons of bricks and mortar on a moment’s notice; he could call up a forest of lumber necessary to rise to a height of 143 feet above the rotunda floor, in order to support the forms onto which cement was poured to create the massive dome, which was 142 feet in circumference. Indeed, as emperor, Hadrian had the wealth of the Roman Empire at his disposal: unheard of riches plundered from far-off lands and an inexhaustible supply of slave labor. His mere wish commanded wealth and forces from throughout the known world, from as far north as Britain, as far East as present-day Iraq, and as far south as deep within the northern rim of the African continent. Such was the wealth and power of Rome, when it was finished, the Pantheon’s cement dome was covered in a sheet gold, and its walls adorned with silver and gold ornamentation.

To build his dome, Brunelleschi had to create an industry and develop a new technology. As it was, there weren’t bricks enough, or trees enough, or tools sophisticated enough, to fulfill his vision.

About the only thing the construction of the two domes had in common was abundant funding. But the funding of both was achieved by vastly different economies. Both drew upon wealth from far off places. The wealth that built the Pantheon came as the spoils of distant wars. While the wealth that built Santa Maria del Fiore came as the result of trade, investment and banking.

Indeed, the Medici, the Florentine family which put up much of the money, were traders of cloth, and bankers, with branches as far away as London and Hamburg, Bruges and Geneva, and throughout all Italy. While Imperial Rome was centered around the forum, and built by slave labor and the treasuries of conquered lands, Florence was centered around the marketplace and built on trade and capitalism. There were other crucial differences, too.

The people of Florence, residing in a republic, were building a Christian church; the Emperor of Rome, whose word was law, was building a pagan temple. Both domes were designed by men of vision and genius, but built in vastly different worlds under vastly different circumstances. Indeed, while Hadrian’s dome was clad in purest gold, Brunelleschi’s dome was clad in common terra cotta tiles.

Among Brunelleschi’s biggest headaches was continued shortages. He didn’t have the necessary lumber to erect scaffolding, bracing and forms required to reach a height of 250 feet, so he was forced to build his dome with bricks and mortar rather than with poured concrete. To do so, he developed a sophisticated means of laying bricks in such a way that friction held in them in place until the cement dried. He also had to learn how to make cement itself, as the knowledge had been lost with the collapse of the Roman Empire. To make bricks fast enough, he built a number of kilns on the outskirts of Florence. To get the bricks to the church, he had to build special boats. Finally, he developed special pulleys with reverse gears, and swing cranes, to lift bricks to a height of a 30-story building. Lastly, he designed special leather harnesses to prevent his workers from falling to their death.

Hadrian’s architect did not have such problems, which is not to slight the ingenuity of his accomplishment. Building a dome with so great a circumference as the Pantheon dome,had never been done before, and wouldn’t be done again until the 15th Century—by Brunelleschi. Hadrian’s dome was made entirely of cement. Steel reinforcement was not an option. Either the cement cured and the dome held up,or it didn’t. The solution was to decrease the thickness of the dome as it rose, from a thickness of 21 feet at the base, to approximately four feet at the top. Equally necessary was to decrease the weight of the cement as it rose, with a mixture of increasingly lighter aggregate, shifting to an aggregate of pumice and small empty bottles at the top.

Five thousand tons of cement was required, poured in one continuous fill, which required an army of workers, and careful monitoring by a staff of trained supervisors. When the cement dried, the forms were removed and the dome held up.

It’s easy when you know how, Hadrian's architect may have said.

Two thousand years later, the Pantheon dome still holds up. In our time, engineers have concluded that the aggregate of pumice and bottles decreased the dome’s weight by approximately 80 percent, which helps explain the dome’s longevity. However, all these years later, Brunelleschi’s dome is still clad in terra cotta tiles, while the gold and silver that adorned the Pantheon disappeared long ago. Today, Hadrian’s dome is clad in something quite mundane—lead.

The influence of the Pantheon can be seen in many of the buildings of Andrea Palladio, the 16th Century architect.

In America, the influence of the Pantheon dome can be seen in Thomas Jefferson’s Monticello, in the University of Virginia domed library, and in the Jefferson Memorial in Washington D.C.

The influence of Brunelleschi’s dome can be seen in the dome of St. Peter’s Basilica in Rome, the dome of St. Paul’s Cathedral in London, and in the dome of the United States Capitol Building.

For its day, the construction of the Pantheon was every bit as inventive as that of Santa Maria del Fiore. The difference was in how and why they were built, reflective of their respective worlds they inhabited. Entire books have been written about both domes and their influence on architecture down to our day. Centuries pass, but the hold these domes have on our imagination is timeless.

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