Inca Leapt Canyons With Fiber Bridges

MIT Students Plan to Stretch 60-Foot-Long Fiber Bridge Between Campus Buildings

Conquistadors from Spain came, they saw, and they were astonished. They had never seen anything in Europe like the bridges of Peru. Chroniclers wrote that the Spanish soldiers stood in awe and fear before the spans of braided fiber cables suspended across deep gorges in the Andes, narrow walkways sagging and swaying and looking so frail.

Yet the suspension bridges were familiar and vital links in the vast empire of the Inca, as they had been to Andean cultures for hundreds of years before the arrival of the Spanish in 1532. The people had not developed the stone arch or wheeled vehicles, but they were accomplished in the use of natural fibers for textiles, boats, sling weapons — even keeping inventories by a prewriting system of knots.

So bridges made of fiber ropes, some as thick as a man's torso, were the technological solution to the problem of road building in rugged terrain. By some estimates, at least 200 such suspension bridges spanned river gorges in the 16th century. One of the last of these, over the Apurimac River, inspired Thornton Wilder's novel "The Bridge of San Luis Rey."

Although scholars have studied the Inca road system's importance in forging and controlling the pre-Columbian empire, John A. Ochsendorf of the Massachusetts Institute of Technology here said, "Historians and archaeologists have neglected the role of bridges."

Ochsendorf's research on Inca suspension bridges, begun while he was an undergraduate at Cornell University, illustrates an engineering university's approach to archaeology, combining materials science and experimentation with the traditional fieldwork of observing and dating artifacts. Other universities conduct research in archaeological materials, but it has long been a specialty at MIT.

Students here are introduced to the multidisciplinary investigation of ancient technologies as applied in transforming resources into cultural hallmarks from household pottery to grand pyramids. In a course called "materials in human experience," students are making a 60-foot-long fiber bridge in the Peruvian style. On Saturday, they plan to stretch the bridge across a dry basin between two campus buildings.

In recent years, MIT archaeologists and scientists have joined forces in studies of early Peruvian ceramics, balsa rafts, and metal alloys; Egyptian glass and Roman concrete; and also the casting of bronze bells in Mexico. They discovered that Ecuadoreans, traveling by sea, introduced metallurgy to western Mexico. They even found how Mexicans added bits of morning-glory plants, which contain sulfur, in processing natural rubber into bouncing balls.

"Mexicans discovered vulcanization 3,500 years before Goodyear," said Dorothy Hosler, an MIT professor of archaeology and ancient technology. "The Spanish had never seen anything that bounced like the rubber balls of Mexico."

Heather Lechtman, an archaeologist of ancient technology who helped develop the MIT program, said that in learning "how objects were made, what they were made of and how they were used, we see people making decisions at various stages, and the choices involve engineering as well as culture."

From this perspective, she said, the choices are not always based only on what works well, but also are guided by ideological and aesthetic criteria. In the casting of early Mexican bells, attention was given to their ringing tone and their color; an unusually large amount of arsenic was added to copper to make the bronze shine like silver.

"If people use materials in different ways in different societies, that tells you something about those people," Lechtman said.

In the case of the Peruvian bridges, the builders relied on a technology well suited to the problem and their resources. The Spanish themselves demonstrated how appropriate the Peruvian technique was.

Ochsendorf, a specialist in early architecture and engineering, said the colonial government tried many times to erect European arch bridges across the canyons, and each attempt ended in fiasco until iron and steel were applied to bridge building. The Peruvians, knowing nothing of the arch or iron metallurgy, instead relied on what they knew best, fibers from cotton, grasses, and saplings, and llama and alpaca wool.

The Inca suspension bridges achieved clear spans of at least 150 feet, probably much greater. This was a longer span than any European masonry bridges at the time. The longest Roman bridge in Spain had a maximum span between supports of 95 feet. And none of these European bridges had to stretch across deep canyons.

The Peruvians apparently invented their fiber bridges independently of outside influences, Ochsendorf said, but these bridges were neither the first of their kind in the world nor the inspiration for the modern suspension bridge like the George Washington and Verrazano-Narrows Bridges in New York and the Golden Gate in San Francisco.

In a recent research paper, Ochsendorf wrote: "The Inca were the only ancient American civilization to develop suspension bridges. Similar bridges existed in other mountainous regions of the world, most notably in the Himalayas and in ancient China, where iron chain suspension bridges existed in the 3rd century B.C."

The first of the modern versions was erected in Britain in the late 18th century, the beginning of the Industrial Revolution. The longest one today connects two islands in Japan, with a span of more than 6,000 feet from tower to supporting tower. These bridges are really "hanging roadways," Ochsendorf said, to provide a fairly level surface for wheeled traffic.

In his authoritative 1984 book, "The Inka Road System," John Hyslop, who was an official of the Institute of Andean Research and associated with the American Museum of Natural History, compiled descriptions of the Inca bridges recorded by early travelers.

Garcilasco de la Vega, in 1604, reported on the cable-making techniques. The fibers, he wrote, were braided into ropes of the length necessary for the bridge. Three of these ropes were woven together to make a larger rope, and three of them were again braided to make a still larger rope, and so on. The thick cables were pulled across the river with small ropes and attached to stone abutments on each side.