Why Bridges Don't Fall Down

A bridge stays up by turning the downward pull of gravity into forces its materials can quietly absorb. The two main ones are tension (being stretched) and compression (being squeezed). Take an arch bridge. When you stand on it, your weight pushes down, but the curved shape redirects that push sideways and down into the supports at each end, squeezing the stones together. Stone is fantastic at resisting compression, so it barely notices. A suspension bridge does the opposite: the deck hangs from cables that are in tension, and steel cable is superb at being pulled. Good design means matching the material to the force: use stone and concrete where things get squeezed, steel where things get stretched. A bridge fails when a part meets a force its material is bad at, or when a load exceeds what was planned for.