Squeeze and Stretch: The Forces That Hold Up Bridges

The Golden Gate Bridge in San Francisco is longer than 15 football fields laid end to end, and thousands of cars roll across it every hour without it sagging into the water. It stays up because of two invisible forces fighting a quiet tug-of-war inside every beam and cable. One force squeezes. The other stretches. Learn to spot them and you can understand how almost any bridge stands.

In this lesson you will: • Learn the two main forces in a structure: compression and tension. • Find where each force is hiding in a simple bridge. • Explain why a rope works in one job but not another. • Test which shape carries the most weight.

Compression is a force that pushes a material together, or squeezes it. When you stand on a soda can and it stays firm, the can is under compression. When you press both ends of a sponge toward each other, you are squeezing it — that is compression too. In a bridge, the tall towers and the thick columns underneath are under compression. The whole weight of the bridge and the cars pushes straight down on them, squeezing them against the ground.

Tension is the opposite. It is a force that pulls a material apart, or stretches it. When you play tug-of-war, the rope is under tension — both teams pull it in opposite directions. The long steel cables that swoop between the towers of a suspension bridge are under tension. They are being stretched tight as they hold up the road hanging below them. A cable is great at tension but useless at compression — you cannot push on a rope.

Look closely at a metal bridge and you will see triangles everywhere. A square frame can lean over and collapse into a flat diamond when you push its corner. A triangle cannot squash flat unless one of its sides actually breaks. Because triangles hold their shape, engineers build bridge frames, called trusses, out of rows of triangles. Each bar in the truss is either being squeezed (compression) or stretched (tension), and together they share the load.