INCA SUSPENSION BRIDGES



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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