Resource: Grass Bridge

Suspension bridges such as San Francisco's Golden Gate Bridge and New York's Brooklyn Bridge are among the engineering wonders of the world. They owe their existence to a technological breakthrough: the development of high-tension steel from iron. But what do people do when the only material they have in abundance is grass? They use it. That's what the Inca people of what is now South America have done for hundreds of years.

Until recently, little more was known about traditional grass suspension bridges than that several hundred or more of these natural-fiber structures once spanned formidable obstacles — the canyons and rivers of the Andes Mountains. These bridges enabled the Inca Empire, the largest empire of the pre-Industrial world, to flourish and to expand into new territory. When the Spanish conquistadors arrived in the 16^th century, they marveled at this technological achievement. The bridges, built from nothing more than braided grass and wood fibers, spanned distances longer than those of any bridges in Europe at the time.

Although these structures were made from natural fibers, modern researchers know that they were more than just ropes thrown across canyons. Rather, they were engineered — deliberately designed and built — in much the same way that modern suspension bridges are. For the foundations, the Inca created pairs of stone abutments on either side of a crossing. These served as the anchors between which a roadway was suspended. As depicted in the video segment, which features the only Inca bridge crossing still in use, each of the four or more thick main cables that made up the roadway was actually a braid of three thinner cords. In turn, these thinner cords were each made up of two-dozen two-ply strands of grass that nearby villagers had twisted together by hand. The roadway was covered with a mat made of branches. Two additional cables served as protective handrails, which were secured to the roadway by vertical ties.

Grass bridges tend to sag in the middle under their own weight and to sway in high winds, but despite their flimsy appearance, recent tests show that their design is structurally sound. In fact, they meet the benchmark for today's safety standard, which is to build a bridge three to four times stronger than the loads it will have to support.

Inca bridges, like Roman arch bridges, are examples of very successful bridge designs that have survived for centuries and continue to carry traffic today. Each of these bridge types was made using materials readily available to the people building them.

To learn more about how suspension bridges are designed to balance the forces that act on them, check out Clifton Suspension Bridge.

To learn more about arch bridges, which date back to the Roman Empire, check out Arch Bridge.

• Did you ever think that grass could be strong enough for building a bridge? How did the Inca work with grass to make it strong enough?
• Why do you think the Inca chose grass as the material for building their bridge?
• What constraints did the Inca have in solving their design problem? (limited materials, 150-foot width to cross)
• Compare suspension bridges that you have crossed to the one made by the Inca. How are they similar? How are they different? What material is the bridge that you crossed made of and why do you think the engineers chose this material? How are engineers influenced in their bridge designs by what will have to cross the bridge?