The exact date and location of the initial historic transition from simpler bridges to arch-supported types is now lost to us. The development and use of the arched bridge has been attributed variously to the Indus Valley Civilization of around 2500 B.C.E. the Mesopotamians, Egyptians, Sumerians, and Chinese; and the Etruscans and Romans, who built most of the surviving early arch-based architecture in Europe.
Early arches were corbeled, not really an arch as we understand the term today. A corbel is a projecting, stone supporting piece. It is a simple example of a cantilever. Such an arch is constructed by progressively corbeling from the two sides with horizontal joints until they meet at a midpoint. At the top, where the two sides meet, a capstone is placed.
The Romans, aided by their invention of a cement material to bind stone together, refined the technigues of arch construction. Arch-based Roman bridges and aqueducts may be seen today throughout many citiesin Europe and the Middle East.
The basic arch design proper can be likened to a beam curved to form a semicircle and prevented from straightening and spreading by strong abutments at either end. Traditionally, the shape of a stone arch is made from wedge-shaped blocks, carefully cut to fit perfectly together.
Known as "voussoirs," these blocks gradually take the curve of the arch from a central, vertical keystone down to the outermost, horizontal footers. The weight of the bridge users pushes downward onto the keystone, and its wedge shape transfers that energy outward onto the voussoirs, thereby spreading the forces sideways and around the arch instead of straight downward. The development of the arch enabled longer and stronger bridges to be made.
