Resource: Designing a Paper Bridge

You can think of a bridge as any supportive structure that spans a gap. Beam bridges are the simplest kind of bridge. A beam, typically made of wood, iron, or steel, is laid horizontally across the tops of two or more supports, called piers. A beam's strength depends primarily on the material it's made from and the distance it has to span unsupported.

When you put weight on a beam and it sags, the top compresses and the bottom stretches. The stronger the material, the more force is needed to compress and stretch it, and the more weight a bridge made out of that material will support before it sags. Steel is very strong under pressing force (compression) as well as under stretching force (tension), so a steel beam will bend less in the middle than a wooden plank when weight is applied to it, and is better suited to span longer distances.

You can model a beam bridge with paper and books. A flat piece of paper furls and falls through under the load of even a single penny. You can enhance its ability to support pennies somewhat by piling on additional pieces of paper. Beam thickness is another factor that influences how much weight a bridge can support before it fails.

Bridges come in other designs, too, some of which you can model with paper and books. You may discover that a curved arch design will support more pennies than a flat beam design. An arch is naturally strong in compression, making it well suited to support the weight that presses down on a bridge. Because it is especially difficult to bend along creases, paper folded like a fan or an accordion will also support considerable weight before failing. With no obviously weak points in the design, weight can be distributed more evenly along this "corrugated" paper bridge.

Why do you think folding the paper works to hold up the pennies? Explain, using a sketch.

Can you think of other bridges that use triangles to provide support? Using the paper bridge as an example, explain how the triangles help.

Describe the pushes and pulls that are acting on the flat paper bridge and the accordion-folded bridge.

Can you think of another way to fold the sheet of paper to make a strong bridge? Before you test it, analyze the bridge in term of pushes and pulls, compression and tension.

What other shapes of paper are stronger than flat paper?