Cracks in a Great Dome
by Roberto Suro
(from The New York Times, 1987)
FLORENCE - As the lofty cupola atop this city’s Duomo, or cathedral, was being completed in 1434, a visitor from Rome described it as “a structure so great, rising above the skies, large enough to shelter all the people of Tuscany in its shadow, built without the help of any centering or of much woodworking, of a craftsmanship perhaps not even the ancients knew or understood.”
In fact nothing like it had ever been built: two vaults, one inside the other, rising 299 feet, topped by 66-foot marble lantern—all held up without external buttresses.
The design was so bold and novel that a little insurance policy was built in. A bronze plate was a small hole in it was affixed at the top so that in the summer solstice a ray of sunlight shot down to a mark on the floor. Aside from its astronomical value, the device was a way of checking whether the dome was moving off its center of gravity.
The sun still strikes the same spot each year, but those caring for the dome worry that it has recently embarked on a course of self-destruction. When or if a disaster could take place is anybody’s guess, experts say. What is certain is that for two years four fissures that run from the top of the dome to the church foundations and from the inside walls to the outer skin have been steadily getting wider.
Now everything from lasers to plumb lines is being used to gather information on the dome. By the end of this summer it will be hooked up to no fewer than 300 measuring devices, giving it a solid claim as the world’s most carefully monitored structure.
Changes in temperature will be recorded in dozens of spots, as will the rise and fall of the water table beneath the foundation. But most of all the effort aims at tracking movement, up, down and sideways. Data from earlier measurements show that the giant cupola is not a lifeless stack of stone and brick. Instead, some architects see it is a collection of many active masses that generate their own thermodynamic energy. They sometimes move apart and sometimes collide, a bit like the tectonic plates of the earth’s surface.
The dome, designed by Filippo
Brunelleschi, is one of the great symbols of the Florentine Renaissance. Its construction helped mark
“The challenge we face now,” says Gastone Petrini, an architect who oversees the monitoring program, “is to use all the best science and technology available to try and understand the spirit and life of this dome. And here we are dealing with mysteries 500 years old.
Construction of Santa Maria del Fiore, best known as Il Duomo, had
been under way for a century when Brunelleschi began considering how to build a
dome across a space 148 feet in diameter.
The only models available to him were the Pantheon in
Over the course of 28 years Brunelleschi carefully designed every part of the structure, from the eight arched ribs that form its skeleton to the intricate patterns of brickwork that absorb and transfer its weight. The intensely geometric logic of the structure is as complex and unseen as that of a Bach cantata. No other dome has ever been built exactly the same way.
Even before construction was finished people began noticing cracks. At least once a century since then studies have been organized to figure out what was happening. In the 1600’s, after a long debate, it was decided Il Duomo did not need iron chains like those placed around St. Peter’s dome to hold it together, and by 1757 a fairly detailed survey of the cracks was completed.
In 1934, Pier Luigi Nervi, the architect and engineer who helped develop the uses of reinforced concrete, made one of the first breakthroughs. Leading a study commission, he noted that the cracks opened and closed with the seasons. In the winter, when the cupola’s stone and bricks contract, the cracks widened. In the summer, when the materials expand, the cracks closed together.
Most modern constructions include expansion joints, small voids that materials can fill when they expand. Brunelleschi’s dome apparently developed its own expansion joints in the form of these cracks without doing catastrophic damage to the building, according to Paolo Alberto Rossi, a Florentine architect who has studied the dome for decades.
A survey completed in 1984 counted a total of 493 cracks of various sizes, sorted into categories identified by the letter A through D. All have formed in a remarkable symmetrical pattern. The most important of the cracks, says Mr. Rossi, who serves on the commission now monitoring the dome, are four dubbed the “A” fissures, which cut through the length and breadth of the dome and divide the structure into quarters.
Mr. Nervi’s study noted that the major cracks, such as the A fissures, which opened to a width of about three millimeters in winter, never closed entirely when the dome swelled in summer. One supposition is that plaster used to patch the cracks over the years and crumbling building materials have jammed the fissures. Concern over this phenomenon prompted the creation of a modest monitoring program that has been gathering data since 1955.
Left to its own devices, the dome probably could have gone on for a few more centuries without needing a large-scale intervention, says Mr. Rossi. But then man intervened.
One of the mysteries of Brunelleschi’s design involves 48 holes, each exactly 60 centimeters square, that were left in the base of the dome and that are open on the inside and covered by the outer skin of the dome. It had long been assumed that the holes simply served as mounts for the scaffold used when frescos were painted on the inside of the dome.
In 1978 a government culture agency decided to restore the frescoes, and a private company contracted to build a scaffold for the work decided to fill the holes with concrete so that steel beams could be anchored in them.
In 1985, Lando Bartoli, another local architect, noticed that additional cracks were quickly forming around the holes. He loudly raised an alarm, which earned him a place on the current study commission.
“This structure could have lived naturally for a thousand years maybe, but we have gravely reduced its life expectancy whatever it may have been,” he said. In the summer, Mr. Bartoli explained, the four major masses separated by the A fissures generate energy as they expand into the fissures, but now, at the base of the dome, the masses run into concrete where there was once empty space. The energy that was once dissipated with the closing of the fissures is now exerted against the concrete blocks and is being transferred elsewhere into the structure.
None of the experts on the commission is sure exactly what is happening inside the dome right now, under the hot sun of the Tuscan summer. They believe the concrete acts as a fulcrum so that when the masses push together below the holes, they are forced apart above. Last summer measurements showed that the A fissures in the dome widened instead of closed during the hot season for the first time. The same phenomenon is being repeated this year.
Mr. Rossi believes that with the disruption caused by the blocked holes the dome’s outward thrust at the base is being increased to potentially disastrous levels, and he proposes binding the base in some way.
Mr. Petrini, who supervises work on the dome, said: “our first priority now must be diagnostic. We have to fully understand what’s happening before devising an intervention.”