Truss bridges are old and durable, made up of pillars, ropes, and diagonal webs forming triangles. They support themselves using tension and compression. Different types include the king post, queen post, and Howe truss, with longer spans possible in modern variations. Examples include the New Fork River Bridge, Bridge over the River Kwai, Navajo Bridge, Firth of Forth Bridge, and Chinese Garden Bridge.
The truss bridge is one of the oldest and most resistant bridge types. The basic design of the truss consists of three basic components: the pillars, the ropes and the diagonal “cobwebs” that give the truss its characteristic triangular textured appearance. More elaborate truss bridges may also sport struts, swing bracing, and windbracing.
At first glance, a truss bridge appears to be a conglomerate of triangles linked together in a streamlined mass. These triangles shape the support structure of the truss design, but the foundation of the truss bridge consists of two or more straight horizontal beams, called “chords.” Smaller beams called “webs” connect larger beams at diagonal angles, giving the appearance of triangles fitted together in a huge jigsaw puzzle. The pillars form the “legs” of the bridge to keep it raised off the ground or water.
Truss engineering is a marvel of bridge technology, with some very old bridges still in excellent working condition. Triangular nets between the long horizontal ropes prevent the ropes from flexing or bending. Unlike the arch bridge, the truss bridge does not support the bridge from underneath the roadway. Unlike the suspension bridge, the truss does not support the bridge from above the roadway. The truss bridge supports the structure itself, using tension and compression to bring the individual members together into a tight, solid and rigid structure.
The first and simplest type of truss bridge is the king truss. Two rays lean towards each other to form a triangle. A long parallel beam, or chord, forms the base of the triangle. Hanging from the apex of the triangle to the string is a strong tension rod. This type of truss is still used today for short bridges and modern home construction and is best suited for spans of 35 feet (10.67 meters) or less.
The queen trellis is a more elaborate adaptation of the king post, featuring a small square within the large triangle. The additional nets form a tighter, more rigid structure, capable of withstanding heavier loads and longer spans up to 70 feet (21.3 meters). In the 1840s, engineer William Howe developed his Howe truss design, which connected multiple diagonal nets to two parallel strings. The Howe truss load can handle spans of 150 feet (45.72 meters). Modern engineers have built numerous variations of the truss bridge that can handle even longer spans.
Examples of truss bridges abound around the world. The New Fork River Bridge in Sublette County, Wyoming is a fine example of an actual trestle. In Thailand, the Bridge over the River Kwai, built by WWII prisoners and popularized by the film of the same name, is a steel truss bridge. A stunning truss arch bridge called the Navajo Bridge is located in Lees Ferry, Arizona. The Scottish Firth of Forth Bridge, completed in 1890 and 350 feet (106.68 meters) long is an example of a cantilever truss, and the Chinese Garden Bridge in Shanghai sports a unique camelback truss.
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