Spawning from old Etruscan bridge building techniques, the Romans expanded on the classical column and lintel method to a much more sturdy voussoir arch with keystone. The column and lintel was a moderately secure technique composed basically of having one large flat stone or many wooded beams span a distance. The problem was that when a large load,such as a Roman legion, would cross it the structure would fail and the bridge would collapse. The Roman voussier arch was a breakthrough in structural stability. Combined with the pazzola concrete technology it was one of the sturdiest methods of construction the world had ever seen. The way the arch functioned was really quite simple actually.    Imagine a ring of precisely cut stone are fitted together as the image above shows. Then a rope is wrapped around the stones as tightly as possible. The force of the rope causes the reaction of the stones to shift inward and closer together making a tighter more secure bond. The Romans just applied this method to their arches.

   This bridge locatd in Alcanara spain is a remarkable example of the Romans highly advanced method of manipulating concrete and arches. Towerining an amazing 200 feet in the air this bridge is still in use today.