Beam bridges are some of the simplest bridges constructed today and their history can be traced back thousands of years. The oldest beam bridge is the Caravan Bridge, which was built over the Meles River near Izmir, Turkey, and dates to 850 BC. Beam bridges are generally constructed with one horizontal beam that features two supporters on either end. The highest railroad bridge in North America is the Metlac Railway beam bridge in Mexico, which was built in the 1980s and is sited at 430 feet up in the air.
Truss bridges are the oldest of modern bridges and the straight elements of which they consist were once made out of wood, but are now usually built with steel. A truss bridge consists of a series of triangular shaped beams for distributing and balancing weight. This unique style of the truss bridge means that it can be incorporated into other types of bridges, such as beam, suspension and cable-stayed. Truss bridges have many advantages, including their ability to bear heavy loads of weight and span long distances. They are relatively easy to construct at low cost compared to other types of bridges. One of the most famous historic truss railway bridges is the Lockport Railroad Bridge, which spans the Erie Canal at Lockport, New York.
A suspension bridge has two towers with cables that hold the bridge deck and suspender cables that support the weight below. Suspension bridges are generally the longest, most expensive and most impressive bridges in the world. The Niagara Falls Suspension Bridge, which was completed in 1869, was the world's first working railway suspension bridge and connected Niagara Falls, Ontario, with Niagara Falls, New York. Many railway suspension bridges followed, including the Manhattan bridge, which carries New York City subway trains between Manhattan and Brooklyn.
A cable-stayed bridge has several columns, often referred to as "towers," with cables supporting the deck of the bridge. The two main types of cable-stayed bridges are harp design and fan design. Harp design features cables that are parallel while fan design features cables that are attached to the towers. While cable-stayed bridges and suspension bridges may look the same, they actually function in entirely different ways. This is due in part because the cables are attached to the tower differently. Cable-stayed bridges may also be built with more than two towers, unlike suspension bridges. More than 10 trains per day travel on a cable-stayed bridge in crossing the Panama Canal.
