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.
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