The Spaghetti Bridge

Experiments by Arie van Scheepen and Marc Luitjens*

Imagine.

Imagine you are the sole survivor from a cargo ship wrecked in the middle of the Atlantic Ocean. You swim ashore on an isolated island far away from the civilized world. The island houses an ancient tribe, but they seem to have completely missed the industrial revolution. Anyway, they did save your life and you would love do something in return.

After a while you get around the language problem and the chief of the tribe gives you a tour of the island. The island is divided in two by a wide and deep gorge. Foaming white breakers beat the walls of the gorge and make it impossible for the people to reach the other side of the island. Unfortunately, this is the fertile part of the island rich with trees bearing the most delicious fruit. The poor tribe is stuck on the barren part of the island with little chance to enrich their diet.

The chief tells you that they only have a few miserable trees on their side that they have tried to use to build a bridge across the gorge. Yet the trees weren't long and strong enough to carry them with the loads of fruit.

You offer to help them, and start thinking about this problem. In the meantime, a container loaded with steel bars has washed ashore from your ship. Each one of them is too short, however, to cross the gap. You cannot afford to waste the precious steel so you need to do some model experiments first.

At some point you find some packs of spaghetti from the ship's cargo and you get an idea to build a construction. You remember that dry spaghetti is the perfect material for model experiments of steel bridges ...

Arie and Marc in front of their test rig.

The main problem is that just tying a few bars together to make a longer bridge won't make it strong enough to carry people with their loads. You vaguely remember what railway bridges looked like at home. They are made of triangles of steel bars to make them stiffer. Without these structures bridges as we know them now would not exist. They divide the forces (people and other loads) that are working on the bridge. The load then is not carried by a single part of the bridge, but is evenly distributed over all parts. This way you make stronger bridges with less material.

We need model experiments to test different constructions before you build the real thing. This is how engineers work. You don't want to have your bridge collapse the first time someone walks on it. The main point is that the material that you use for testing in the small scale model behaves exactly like the stuff (in this case steel) in the real bridge.

And here the spaghetti comes in. Spaghetti in a model bridge is just as strong as the steel in the big one. If you want to understand more about testing the strength of scale models read here:

Scale, Size and Mass

Let's say your model is 50 times smaller than a real bridge. Now don't forget: your model is not only 50 times shorter, but also 50 times less wide and 50 times less high! (You have seen the same problem with the Weigh a Dinosaur experiments.)

Now suppose you want the real bridge to carry 20 trucks weighing 10,000 kg each. Your model must then be able to carry a load that weighs 50 x 50 x 50 = 125,000 times less!

Ten trucks together weigh 200,000 kg. Divide this by 125,000 and you get 0.16 kg = 160 grammes. That's how much your model must at least support before it breaks.

Use different numbers for your own model.

We had lots of fun doing the experiments and the results really surprised us. After many tries, we found a construction that was many times stronger than the bars just tied together.Here is a description of the experiments.

Why not try them yourself?

>>Spaghetti Bridge Experiment

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Last Updated ( Thursday, 21 February 2008 )

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