History Has Much to Teach

A Simple Roman Arch

From the very beginnings of time, bridges have been essential in spanning not only rivers, but great distances that humans cannot simply leap across. Some of the first bridges were hewn out of natural stone formations, but as humanity's exploits continued to expand, so did their ability to build bridges. Some of the first "artificial" bridges were built out of stone in the form of arches by the Roman Empire, as a way to transport not only goods but troops throughout their vast domains.

As time progressed, bridges came to span even more vast distances and become ever more complex. Instead of building an arch, which simply utilized compression for it's support (a force that we'll talk about later) people began to take advantage of tension, a force that is opposite to compression. As a result, suddenly bridges were not only built downwards, but also upwards. Perhaps one of the most stunning examples of the use of tension in bridge design is the Rainbow Bridge in Tokyo. 

The resulting forces can be seen on the diagram to the left. The blue lines represent the forces of tension acting on certain parts of the bridge, while the red arrows signal where the forces of compression are acting. 

Though there is far more history to bridges than what was outlined here, this serves as a concise look at how bridge building has changed from the past to the present.