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The Original Materials Science

Students build a suspension bridge like the Inca’s
August 15, 2007

When 16th-century Spanish invaders reached the Andes Mountains, the Inca’s innovative use of a humble material filled them with fear and awe. To cross deep gorges punctuating their main ­highway, the Inca twisted grasses into sturdy ropes for suspension bridges that could, without supports, span 150 feet–­making them about half again as long as any other bridge the Spaniards had ever seen.

Alice Chang ’10 and her classmates assembled Chaka Stata in 12 hours on a May Saturday. The bridge remained in place until the end of June.

This past spring, 14 MIT students traversed the chasm between ancient civilization and modern engineering, working about 70 hours to create their own fiber suspension bridge across the moat behind the Stata Center. The Chaka Stata project (chaka means bridge in Quechua, the language of the Inca) was part of Materials in Human Experience, a class that explores materials’ physical characteristics–and asks why different peoples selected certain materials and how they’ve used them.

The Inca’s many uses for fiber included an accounting device called a quipu, slings to hurl stones in combat, and bridges. To create the Chaka Stata, students twisted and braided 50 miles of sisal twine into six primary cables, four for the floor and two for handrails; each cable was able to sustain 5,000 pounds of force. Made from the fibers of an agave called maguey, the twine is twice as strong as the local grasses used in the Keshwa-chaka, one of the last bridges of Inca origin still functioning in Peru. (Locals have created new cables for the bridge at annual three-day festivals for 600 years.) But the Inca built bridges with other natural fibers, like woven vines and perhaps sisal.

On a Saturday in May, the students spent 12 hours assembling the bridge (which hung about six feet above the moat at its lowest point), using concrete blocks and boulders to anchor its 70-foot span. “The bridge was overdesigned, the way all engineering constructions are, so it could take many more people than could ever stand on it,” says professor of archaeology and ancient technology Heather Lechtman, who was one of three class instructors.

Chemical-engineering major Alice Chang ‘10 quickly discovered the hazards of handling 850 pounds of twine: splinters and chafing. She also developed respect for the Inca’s engineering savvy. “[The class] takes a look at the past–culturally, and through an engineering lens,” says Chang. “You wouldn’t really expect it at a place like MIT, where it’s all about future technology.”

No shortcuts were allowed, even though the three-inch-thick main cables were about 170 feet long. “When they realized how much time it was going to take to twist all those cords … some of them wanted to use machines that would do the twisting for them,” says Lechtman. But she nixed that idea.

Using the Inca’s preferred walkway material would have entailed collecting thousands of twigs, so Lechtman proposed flexible wood-slat sand-dune fencing, to follow the bridge’s curve. This time, the students objected.

“One of them said, ‘Professor ­Lecht­man, we’ve gone to so much trouble to make this as close as we can to the way the Andean peoples made it–we’re not going to use slatted fence,’” Lechtman recalls. “And I said, ‘Good for you.’” They opted for fencing made of hollow reeds instead. “These students really got it,” says Lechtman. “They understood what the people of the Inca Empire were capable of.”

John Ochsendorf, associate professor of architecture, has studied Inca suspension bridges for more than a decade and consulted on the Chaka Stata project. “I was completely blown away, because it looked so much like the Keshwa-chaka,” he says.

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