One of the first engineering feats of yesteryear that overcame simple wooden beam construction was the arch. So successful was this achievement that most all civilizations of antiquity used it. In the beginning, they were used for bridges, but later, due to their strength of supporting heavy loads, they were also used in the construction of buildings, such as the Gothic structures that were built in the Middle Ages.
The process to build an is as follows:
Abutments are constructed on both sides of the bridge. These will be used to keep the arch in place, because without abutments, the thrusts on both sides of the arch will push outward with no structure to hold it in its place, resulting in the arch falling down.
Once the abutments are created, scaffolding is constructed for the workers to start building the arch, which is created on both ends in a curved fashion using bricks, until they all meet at the center. On the scaffolding are wooden (or other material) columns that will support the arch until the arch is completed. The scaffolding and columns are removed and the arch is solely supporting itself via its weight being thrust (or pushed) down to each of the bricks on both sides. This thrust continues to the abutments, which take on the load.
Viaducts are a series of arches that stretch across a body of water or land.
To see how actual construction of an arch bridge works, click here.