Bridge Design

Types of Bridge Structure

The arch bridge is one of the oldest types of bridge design. It is founded on the principle of the Roman Arch, in which the two sides of a curved arch compress on each other at the peak, forcing tension to be pushed outward at the base. There it is controlled by supports at either end that brace the structure and make it stable enough to support a doorway, ceiling, or bridge. The ancient Romans built many arch bridges from stone and brick. Many of these 2,000-year-old structures still exist today, including the Pont du Gard aqueduct in France, which spans 300 yards at a peak height of 164 feet. The use of modern materials like steel and pressurized concrete enables contemporary arch bridges to be built with greater spans. While most arch bridges today extend to about 800 feet or less, one arch bridge in West Virginia, the New River Gorge Bridge, spans some 1,700 feet.

The beam bridge is another type of basic bridge construction that uses end supports to bear the load across its span. Beam bridges, however, use vertical or slightly angled piers to support horizontal beams that make up the deck of the bridge. The piers support the entire weight of the bridge and its traffic. The simple structure of beam bridges makes them a popular option to span short distances of approximately 250 feet or less. However, Louisiana's Lake Ponchartrain Causeway uses the beam [Page 294]construction methodology to form a massive over-water bridge extending over 24 miles. To make the beam approach viable for such a great distance, engineers had to link thousands of individual beam-based spans to create the full bridge extension.

Longer bridges generally use suspension design or modern cable-stayed technologies to accommodate vast spans. Suspension bridges use two tall towers to suspend cables that support the bridge deck. The Golden Gate Bridge in San Francisco is a classic example. The towers of a suspension bridge support the bulk of the weight as compression forces push down on the deck, and then move back up the cables to the towers, dissipating the compression into the ground. The cables accommodate the tension forces on a suspension bridge by transferring the tension to the bridge anchorages that ground the bridge to the earth.

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