"As hangs the flexible line, so but inverted will stand the rigid arch." This dictum on structural forces, formulated by Robert Hooke in 1675, remains the basis for understanding both suspension bridges and masonry buildings. But as John Ochsendorf, assistant professor of architecture at MIT, explained in his Program in Science, Technology and Society colloquium, "Medieval Architectural Technology: New Lessons from Master Builders," on April 30, we can learn a lot from studying exactly why and under what loads the flexible line will keep hanging and the rigid arch will keep standing.
Ochsendorf studies historical design procedures in traditional structures, creates tools to analyze and assess the safety of masonry buildings and looks at what traditional building methods can teach modern architects and engineers about sustainability. He studied civil and environmental engineering under David Billington at Princeton and was able to study the work of medieval masons firsthand while completing his doctorate at King's College, Cambridge (United Kingdom).
His colloquium presentation opened with an image of the magnificent fan-vaulting on the ceiling of King's College Chapel (completed in 1515), which spans 42 feet and hovers 84 feet above the pavement, yet its constituent blocks are only four inches thick. "You'd be hard-pressed to find someone in the world (today) who could sign off on this and say this is a safe structure," he said. "If some architect, maybe an MIT graduate, proposed this geometry…almost no building code in the world would allow it to be built. And yet it's been standing for 500 years."
But such buildings do sometimes come down, as did the 13th-century Upper Church of Saint Francis in Assisi, Italy. It had stood for 700 years before September 1997, when its vault, weakened by an earthquake, was brought down by an aftershock. The collapse killed four people and destroyed priceless frescoes by Giotto and Cimabue. With the example of Assisi in mind, Ochsendorf said, "One of the questions of our research group is, 'Which one is next?'"
Along with his graduate students, Ochsendorf has been developing interactive tools to explore the geometries of different arches--work that he plans to continue next year as a fellow at the American Academy in Rome. Using these tools, you can move building elements around virtually, imitating the ravages of gravity, and show at what point a particular configuration becomes unstable. As part of his work in this area, he has collaborated in a Columbia University survey of Romanesque churches of the 10th and 11th centuries in the Bourbonnais region of central France, led by Professor Stephen Murray (see www.learn.columbia.edu/bourbonnais/). He looks for patterns in the churches' original layouts, noting which of these have failed over their millennium of existence.
History has a lot to teach us about technology transfer and the lack thereof, Ochsendorf noted. In 1532, Spanish conquistadors in Peru first encountered the long-span suspension bridges, built entirely out of vegetable fiber, that the Incas used to tie together their vast empire--"Hooke's chain," as Ochsendorf put it. The Spanish were so terrified of these bridges that, according to contemporary accounts, they crawled over them on hands and knees even as their horses and cannons passed across in safety.
Ochsendorf believes that their fear sprang from incomprehension: This technology, based on tension rather than compression, was utterly at odds with their understanding of how the world worked. Spanish colonial attempts to use their familiar compression technology to span these deep Peruvian river valleys were failures, costly in materials and lives, while the Incan feat of suspension engineering would not be matched in the European sphere until the 19th century. (At MIT this semester, an undergraduate class in materials science is reproducing a small Incan suspension bridge near the Stata Center. See story.)
Ochsendorf believes that the masters responsible for these buildings have a lot to teach modern architects and engineers about sustainability in building design. It is no accident that older masonry buildings, built with traditional techniques and materials, survive earthquakes far better than modern ones, or that often the only buildings left standing in European cities bombed during World War II were the Gothic cathedrals.
Stability is one consideration; -resources are another. Architecture, he notes, is "waste in transit," and in our heavily built world, flimsy construction projects are burning through natural resources as never before. But the trend may be moving in the other direction.
Ochsendorf recently consulted on a new conference center in southern England. The client wanted a building that would last for 500 years, one that could be made with nontoxic local materials and have zero energy consumption. Ochsendorf's team brought in Spanish masons to make lightweight timbrel vaulting, made of tile and light mortar, that would support a "green" roof. The building was made with blocks of rammed local chalk. "We were concerned about our vaults, that if they put a modern riding mower on top it would be a nasty point load on our thin tile shell--they're only six inches thick," said Ochsendorf, "and the client said, 'Oh, that's not a problem. I'll use sheep.'"
A version of this article appeared in MIT Tech Talk on May 16, 2007 (download PDF).
