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.
How the Inca Leapt Canyons
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.” Dr. 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 M.I.T.
Ancient technologies influenced cultures
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.
Rubber invented by Mexicans 3,500 years before the Goodyear patent
In recent years, M.I.T. 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 M.I.T. 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 M.I.T. 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,” Professor 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.
The colonial government tried many times to erect European arch bridges, and failed where the Incans succeeded
Dr. 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, Dr. 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.
The Incas were the only ancient civilization to build suspension bridges
In a recent research paper, Dr. 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 third 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,” Dr. 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. Three of the big cables served as the floor of the bridge, which often was at least four to five feet wide, and two others served as handrails. Pieces of wood were tied to the cable floor.
Finally, the floor was strewn with branches to give firm footing for beasts of burden. More branches and pieces of wood were strung to make walls along the entire length of the bridge. The side covering, one chronicler said, was such that “if a horse fell on all fours, it could not fall off the bridge.” Still, it took a while for the Spanish to adjust to the bridges and to coax their horses to cross them. The bridges trembled underfoot and swayed dangerously in stiff winds.
Ephraim G. Squier, a visitor to Peru from the United States in the 1870s, said of the Apurimac River bridge: “It is usual for the traveler to time his day’s journey so as to reach the bridge in the morning, before the strong wind sets in; for, during the greater part of the day, it sweeps up the Canyon of the Apurimac with great force, and then the bridge sways like a gigantic hammock, and crossing is next to impossible.”
Other travelers noted that in many cases, two suspension bridges stood side by side. Some said that one was for the lords and gentry, the other for commoners; or one for men, the other for women. Recent scholars have suggested that it was more likely that one bridge served as a backup for the other, considering the need for frequent repairs of frayed and worn ropes. The last existing Inca suspension bridge, at Huinchiri, near Cuzco, is virtually rebuilt each year.
People from the villages on either side hold a three-day festival and gather stiff grasses for producing more than 50,000 feet of cord. Finally, the cord is braided into 150-foot replacement cables. In the M.I.T. class project, 14 students met two evenings a week and occasional afternoons to braid the ropes for a Peruvian bridge replica 60 feet long and 2 feet wide. They were allowed one important shortcut: some 50 miles of twine already prepared from sisal, a stronger fiber than the materials used by the Inca.
Some of the time thus gained was invested in steps the Inca had never thought of. The twine and the completed ropes were submitted to stress tests, load-bearing measurements and X-rays. “We have proof-tested the stuff at every step as we go along,” said Linn W. Hobbs, a materials science professor and one of the principal teachers of the course. The students incorporated 12 strands of twine for each primary rope.
Then three of these 12-ply ropes were braided into the major cables, each 120 feet long — 60 feet for the span and 30 feet at each end for tying the bridge to concrete anchors. One afternoon last week, several of the students stretched ropes down a long corridor, braiding one of the main cables. While one student knelt to make the braid and three students down the line did some nimble footwork to keep the separate ropes from entangling, Zack Jackowski, a sophomore, put a foot firmly down on the just-completed braid. “It’s important to get the braids as tight as possible,” Mr. Jackowski said.
“A little twist, pull it back hard, hold the twist you just put in.” No doubt the students will escape the fate of Brother Juniper, the Franciscan missionary in Wilder’s novel who investigated the five people who perished in the collapse of the bridge of San Luis Rey. Brother Juniper hoped to discern scientific evidence of divine intervention in human affairs, examples of “the wicked visited by destruction and the good called early to Heaven.”
Alas, he could not; there is some of both good and evil in people. So his written account was judged heretical. He and his manuscript were burned at the stake. If the students’ bridge holds, they will have learned one lesson: engineering, in antiquity as now, is the process of finding a way through and over the challenges of environment and culture.