An engineer's masterful grasp of physics, the properties and availability of various building materials, and construction techniques have made it possible to build modern bridges that carry heavier loads, span greater distances, and use less material than ever before. In determining which of the basic bridge designs to use when building a bridge, an engineer must consider, among other things, the length of the span, the surrounding terrain, the bridge's intended use, and the cost of materials and labor.

Beam bridges, the simplest and most cost-effective kind of span, consist of beams made of wood, iron, or steel -- the stiffer, the better -- laid horizontally atop evenly spaced supports called piers. Although single-beam bridges rarely span longer than 200 feet, bridges made by joining several beams can run almost endlessly. Stiffening trusses may be added to help the bridge support heavier loads.

Arch bridges, typically steel or stone structures, are easily identified by their graceful, curved shape. Capable of spanning longer distances -- 200 to 800 feet -- without the use of intermediate support piers, these compressive structures can be built high over deep rivers or gorges. Fixed supports, called abutments, at each end of an arch prevent it from spreading apart at its base.

A suspension bridge's signature features -- its long steel cables and rising towers -- support the weight of a suspended deck and the traffic it's designed to carry. The cables are anchored at each end of the bridge into solid concrete blocks, which pull on the suspension cables and keep them taut. Capable of spanning more than a mile, suspension bridges are often used over large bodies of water, like harbor entrances. Because suspension bridges typically have only two foundational piers, on top of which the towers stand, obstacles to shipping activity passing below the deck are minimized.

Cable-stayed bridges are similar in some ways to suspension bridges but differ in how they support a roadway. In both designs, the towers support the entire weight of the roadway. But close inspection reveals that in a cable-stayed bridge, the roadway is directly linked to the towers by large, high-tension cables. In a suspension bridge, however, the roadway is connected first to smaller, vertical suspension cables and those are then connected to the long steel cables that drape over the towers and anchor at both ends of the bridge.