The materials used in a bridge can vary depending on the type of bridge and load, but the most common materials are wood, iron, steel and concrete. Sometimes a bridge can be a combination of materials. Concrete reinforced with steel rods or bars is a prime example of materials combined to make a stronger bridge.
The type of material used in each bridge is determined by multiple factors, including the size, shape and type of load that will pass over it. Another important factor is the availability of materials. Bridges in the past were most often made of wood or stone. Wood was used for shorter spans and stone for archway-style bridges. As advances in technology brought steel and iron into the mix, bridges could be made sturdier, to last longer and cover larger expanses. Using iron, steel and concrete allows for a wide variety of bridges to be made.
Beam-style bridges, which can often be seen going over interstates, are typically a steel and concrete style bridge. Suspension bridges, like the Golden Gate Bridge, have concrete footers with towers built out of steel on top of them. The towers have steel cables attached that hold up the driving surface and allow the bridge some movement without breaking.