michelangelo’s dome -- 6/25/24

Today's selection-- from Saving Michelangelo's Dome by Wayne Kalayjian. The great architectural achievement of the dome at Saint Peter's Basilica in Rome:

 “Michelangelo's dome was a simple design made from three core ingredients: a pedestal, the double shell, and a crowning lantern. The pedestal was formed, in turn, from two components—a drum and an attic—which were largely installed before Michelangelo's death in 1564 but had sat dormant and open to the elements for another twenty-five lonely years above Rome's ancient hills, waiting for the money and know-how to finish the job. It was an impressive start, nonetheless, and these cylindrical walls of mortared brick and block now stood ten feet thick and some eight stories tall. Then, and with intuition as his guide, Michelangelo had had them buttressed by an impressive ring of stone columns and wing walls around the circumference as a muscular and pragmatic way (or so he thought) to resist the future dome's thrusting force, however large it turned out to be. 

“When Pope Sixtus V commissioned Giacomo della Porta to vault the dome and its double shell some thirty years later, in 1588, he stipulated that Michelangelo's hemispheric design be preserved in toto. But the harsh realities of construction and of gravity compelled Della Porta–again by intuition—to reduce the dome's thrust, which meant a change to the outer shell's shape into a more elliptical and forgiving silhouette. After gaining the pope's explicit consent, Della Porta assembled an elaborate production of timber scaffolds and falsework at the top of the attic from where he launched his thirty-two tapered skeletal ribs toward the sky. This colossal frame climbed another nine stories, was 150 feet across, and grew to seventeen feet thick in spots. To secure its stability and prevent its collapse during construction, the open webs between each rib were mortared together with layered severies of stone and brick that were up to ten feet deep. The process would have been delicate and dangerous under any circumstance, let alone the aggressive and arguably reckless schedule imposed by Sixtus V, but it was further complicated by the cast iron chains that had to be wrapped around the outer shell's circumference to keep the dome's thrusting force at bay. With the shells and the chains in place, Della Porta then lined the dome's outer face with sheets of lead and bronze to shield it from the weather. Next, the inside shell was slathered with mosaics and frescos, which took some thirteen years to decorate, using a novel and ingenious kind of timber scaffold that hovered high above the church floor and burrowed into the dome's inner face in the same way that a mountaineer clings onto a sheer granite cliff.

Main façade and dome of St. Peter's Basilica, seen from St. Peter's Square

“Michelangelo's dome was finished in 1593, when it was crowned by a cylindrical lantern made from stone. Now fully assembled, it stood among the most daring structures in the world and was about as tall as the Great Pyramid of Giza, standing some 450 feet above the streets of Vatican Hill. The dome alone (without its pedestal) was made from sixty-one million pounds of masonry, while the lantern was made from another five. For perspective, this was like stacking eighteen thousand cars neatly within a footprint of two square city blocks, and then gingerly hoisting them into position some thirty stories above the streets. By any standard, Della Porta and his deputy, Domenico Fontana, deserved enormous credit for pulling off such a perilous achievement in just five years and without the benefit of modern machinery and equipment.

“However, their ingenuity and tenacity could not contain the dome's thrusting force, nor keep the cracks away. So when Luigi Vanvitelli presented his troubling report to the pope in September 1742, he had likely anticipated that a counsel of architects and builders would convene soon after to figure out what might be done. But Pope Benedict XIV confounded them all when he instead sought guidance and advice from three mathematicians—of all people—who had little personal experience with construction and knew nothing about how domes were designed.”

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Wayne Kalayjian


Saving Michelangelo's Dome: How Three Mathematicians and a Pope Sparked an Architectural Revolution


Pegasus Books


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