Ancient societies are known to have the experience of bridge building. However, the Romans have left a lasting legacy of durable usable structures that have lasted for more than 2000 years. Some of the remarkable features are the technology and the utilitarian diversity. Some of the designs were not only for human traffic but great aqueducts that supplied towns and plantations with great quantities of water vital to the maintenance of the city. One of the most impressive aqueducts built by the Romans is the Pont du Gard, completed in18 B. C. It carried water to a ancient city, Nemausus, a distance of more than thirty miles. The brilliance in the technology and its durability is still a testimony to the genius of the Roman engineering skill. (Dupre, 15)
There are some cardinal principles that every functioning bridge must adhere to regardless of size or utility. Every bridge must be able to tolerate and overcome the stress or forces it encounters. For it to function, it should carry its own weight, a weight which is called dead load; it should be able to carry the weight of the traffic for which it was intended, a weight which is called live or dynamic load. It must have the capability of resisting natural forces like wind, earthquake or other natural environmental stress or load.
The magnitude of a bridge is generally characterized by the length of its span, which is the distance from one end of the main support to the other. A plank across a stream is a span - the length of the plank. Longer bridges might require support from below; these columns are called piers. The major supports at the two extreme ends of the bridge are called adutments.
There are four types of forces that act on a bridge singly or in combination: tension, compression, shear, and torsion. The force that pulls or stretches apart is called tension. This is the exact opposite of compression, a force that pushes together. Shear is a sliding force while torsion produces a twisting effect. (Super Bridges)
The designs of bridges take four basic forms derived from nature: the Beam, a log across a stream, an arch that patterns rock formation, the suspension and the cable from a twig or branch hanging from a vine. Changing technology and the availability of different kinds of materials have greatly influenced the types of designs and structures over time. (Dupre, 12)