Be a Bridge Maker

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What kind of bridge would you build with the following materials?

  • 1 rectangular piece of cardboard about 4 feet (1.2 meters) long by 1 foot (0.3 meters) wide
  • 2 chairs
Figure 1. Illustration by Mary Ellen Hawkins, 2016.

For example, you can place the two chairs facing each other and span the cardboard between the seats. This is called a beam, or plank, bridge. This type of bridge is not very strong. Test it out by placing a heavy load (a stack of books) on it, and see how many it can hold before it collapses (Fig. 1).

What forces are primarily at play in this type of bridge, compression or tension?

To make the plank bridge stronger, modify it so that it becomes a suspension bridge. Include the following materials:

  • 2 pieces of rope, each about 9 feet (2.75 meters) long
  • 12 pieces of string, each about 2 feet (0.6 meters) long

Stretch the ropes above the cardboard, over the backs of the chairs, and have two friends pull them tight by sitting on the floor behind each of the chairs (Fig. 2).

Figure 2. Illustration by Mary Ellen Hawkins, 2016.

Tie the tops of 6 pieces of string to one of the ropes, at equally spaced intervals between the two chairs. Do the same with the other lengths of string, but on the other rope (Fig. 3).

Figure 3. Illustration by Mary Ellen Hawkins, 2016.

Secure the loose ends of the strings with duct tape onto the cardboard below.

Now you have a suspension bridge. Ask your friends to pull on the ropes behind each of the chairs as you start placing books on top of the bridge. Experiment with the pulling forces and with various loads to test the strength of your bridge.

What forces are primarily at play in this type of bridge, compression or tension? Where are the forces felt as you add more books? How is this bridge similar and how is it different from Q'eswachaka?

"Of course we teach the youth what the bridge has done and its meaning. They learn by watching and participating. I always hear them say that this bridge is the work of our fathers and Inka ancestors, and that's why we need to take care of it and keep it."

- Mr. Victoriano Arizapana Huallhua, Chakacamayuc (Quechua Bridge Master, Huinchiri, Cusco, Peru)-March, 2015

Learn More

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.

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