[Aztlan] INCA SUSPENSION BRIDGES

[michael ruggeri]

How the Inca Leapt Canyons



By JOHN NOBLE WILFORD
Published: May 8, 2007
CAMBRIDGE, Mass. — 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.

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, 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.

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.

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.




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