The Romans gained much of their engineering skill from the Etruscans and drew on Etruscan and Asian models for the semicircular arch. From them, the Romans learned the use of the keystone arch, which enabled them to build extremely strong and durable structures. Many of these engineering and architectural projects are still standing. Some are still in use after two thousand years like this bridge in Spain.
             Early Roman architects were influenced by Greek post-and-lintel construction. But the Greek design was limited in its capabilities to span large distances and being able to bear heavy loads while not falling down of its own weight. Post and lintel construction comprises a flat piece of stone bridging a space between two upright supports. Post and lintel supports have a flaw. When a heavy weight is placed on the middle of the span too much stress may be put on the stone and it can break in the middle.
             The Romans solved this problem by using a type of construction called voussoir arch with keystone. The engineering principle of the arch is quite simple. The circle is the strongest structural shape. The arch is just half of this perfect form.
             To create a voussoir arch, tapered stone blocks were cut then arranged like the diagram at the right. It was then stood up on its ends. The ends rested on piers made of stone blocks or bricks mortared together with pozzolana cement in the typical Roman arch bridge. The cement was named after a local mountain that the ingredients of the cement came from.
             The weight of the stone and concrete of the bridge itself compressed the tapered stones together, making the arch an extremely strong structure. During construction, the voussoir's were supported by a temporary wooden frame until the keystone was inserted. The Roman invention of the arch allowed architects to build larger structures than ever before. The extension of the arch idea lead t
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