What are the bridges made of?

Usually bridges are made of steel and/or concrete. Concrete bridges have steel reinforcement embedded within the concrete, because concrete is weak in tension. Steel is strong in tension and compression, which makes suspension bridges practical. Every nut and bolt, steel member size and thickness, and every other component on a bridge has been located and sized by using mathematics to calculate all the forces applied to each component of the bridge, which must be resisted by the materials used.