Bridge design has changed and improved over time.
The “clapper bridge” is one of the earliest types of bridges. This is a low bridge that is made of flat stones or logs. The “clapper bridge” is a natural bridge.
Ancient Romans refined bridge building. They used the arch design and incorporated natural cement.
Asians used a cantilever design which allowed them to build over long spans.
The Rennaissance had an effect on bridge building in that Leonardo da Vinci and Galileo developed theories about the strength of building materials. This information helped architects make strong structures from light materials such as metal.
Later bridges were built out of iron, instead of stone, brick, clay or timber. The use of metal improved bridge design. The Brooklyn Bridge is one such bridge.
The most recent achievement in bridge building is called prestressing which helps prevent concrete from cracking. Today, most bridges are made of prestressed concrete and steel.
For more detailed information about bridge design and an overview of the forces that affect bridges visit www.engineeringtufts.edu.