The arch bridge works by distributing compression through its semicircular form into a horizontal thrust restrained by its abutments, or legs. Longer bridges often consist of a series of arches, as the greater the curvature, the more tension on the bridge's underside. If the arch is too big, tension overcomes the natural strength of the semicircular design.
Though there are architectural variations to arch bridges, the basic structure remains the same. In the past, arch bridges were often made of stones, with a keystone in the middle that bore the bulk of the weight. The more weight the bridge bore, the stronger its structure became. Due to their inherent weight, these bridges required extremely sturdy foundations. Most modern arch bridges are made of reinforced concrete.
Arch bridges are durable. The oldest standing arch bridge in the world, the Arkadiko Bridge on the Peloponnese in Greece, was built around 1300 B.C. but is still used today. Many Roman bridges and aqueducts, built in segmented arches, still stand. Medieval builders improved the span and height of arch bridges by modifying the thickness of the arch barrels. Modern arch bridges include the Lupu Bridge in Shanghai, which is more than 1,800 feet long, and the New River Gorge Bridge in West Virginia, which is 1,700 feet long.