Suspension bridges are an example of an original and unique Inka engineering technology. Q'eswachaka is an important example of sustainability from environmental and engineering perspectives.
The bridge is built of strong, locally harvested, and fully biodegradable materials. The Inka understood the characteristics of a variety of fibrous materials such as grass, cotton, and llama and other camelid wool. So, it was natural for the Inka to find an engineering solution using a locally abundant grass fiber that could be woven to make rope. Individual grass fibers can break and tear easily, but twisting and braiding them yields a stronger and more flexible material. This is because strength increases with more elements to share the load, or the forces, acting on them.
In suspension bridges, including Q'eswachaka, cables work through tension, or the stress resulting from a pulling force. However, if you pull a cable too much, it will break. The Inka understood this and used the engineering concept of tensile strength. The tensile strength of the grass cables, or how much they can be pulled from opposite directions before they break, is critical. The bridge builders also knew how much the cables could be stretched by the weight of the expected foot traffic on the bridge. The tensile strength of a grass rope depends on the type of grass, how much grass is used to make it, and how it is twisted and braided together with other ropes. Can you guess how big a load the largest cable of the Q'eswachaka can hold before it breaks? Each main cable, as thick as a man's thigh, can hold 5,175 pounds, or 2,347 kilograms, more than the weight of an average automobile or the combined weight of 12 llamas!
"In Spain and the rest of Europe at the time of the Inka Empire, most bridges were built as short arch spans which were acting in compression. By using tension, the Inka engineers were able to span much longer distances than the bridges in Europe at the time."
- Massachusetts Institute of Technology (MIT) Professor of Engineering Dr. John Ochsendorf, 2015
In Spain and the rest of Europe at the time of the Inka Empire, most bridges were built in the shape of short arches that used a pushing force, or compression. By using a pulling force, or tension, the Inka engineers were able to build bridges that were much longer than the bridges in Europe at the time.
- Massachusetts Institute of Technology (MIT) Professor of Engineering Dr. John Ochsendorf, 2015
Read additional quotes and paraphrased quotes from Spanish historians, contemporary engineers, and cultural experts to get insights about the innovative engineering qualities of the Inka Road and the Q'eswachaka bridge.
Suspension bridges did not exist in Europe at the time of the Inka Empire; instead, Europeans built stone arch bridges. Suspension bridges can span longer distances, but European engineers did not build this type of bridge for another 300 years. In suspension bridges, tension forces are most important, while in arch bridges, compression forces are what matter. Tension forces pull and stretch material in opposite directions, allowing a rope bridge to support itself and the load it carries. Compression forces squeeze and push material inward, causing the rocks of an arch bridge to press against each other to carry the load. Both types of bridges rely on abutments, the components of the bridge that take on pressure and dissipate it onto the Earth. In the case of Q'eswachaka, the abutments are made from massive rocks where the bridge's main cables are tied.
Ask a partner to work with you to demonstrate Force 1 and Force 2 as shown in the illustrations below. As you work with your partner, can you feel which way the forces are directed? Based on the definitions given above, which force is tension and which is compression?
Look at the bridge images below. Can you match Force 1 and Force 2 with each bridge type? Which bridge acts primarily on compression forces? Which bridge acts primarily on tension forces? Explain why and justify your choices.
Now think about how it felt to experience these forces. When pulling apart from your partner, what shape did your bodies resemble? How about when pushing against your partner's hands? Notice that when pulling apart from your partner, your bodies form a suspended curve much like the Q'eswachaka bridge, and when pushing toward each other, your bodies form an arch much like the Taft Bridge.