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A trestle is a rigid frame used as a support, especially referring to a bridge composed of a number of short spans supported by such frames. In the context of trestle bridges, each supporting frame is generally referred to as a bent. Timber trestles were extensively used in the 19th century, making up from 1 to 3% of the total length of the average railroad. In the 21st century, steel and sometimes concrete trestles are commonly used to bridge particularly deep valleys while timber trestles remain common in certain areas.

Many timber trestles were built in the 19th and early 20th centuries with the expectation that they would be temporary. Timber trestles were used to get the railroad to its destination. Once the railroad was running, it was used to transport the material to replace trestles with more permanent works, transporting and dumping fill around some trestles and transporting stone or steel to replace others with more permanent bridges.

In the later 20th century, tools such as the earthmover made it cheaper to construct a high fill directly instead of first constructing a trestle from which to dump the fill. Timber trestles remain common in some applications, most notably for bridge approaches crossing floodways, where earth fill would dangerously obstruct floodwater.

Timber Trestles

One of the longest trestle spans created was for railroad traffic crossing the Great Salt Lakemarker on the Lucin Cutoffmarker in Utahmarker. It was replaced by a fill causeway in the 1960s, and is now being salvaged for its timber.

Many wooden roller coasters are built using design details similar to trestle bridges because it is so easy to make the roller coaster very high. Since loads are well distributed through large portions of the structure it is also resilient to the stresses imposed. The structure also naturally leads to a certain redundancy (provided that economic considerations are not overly dominant). Such wooden coasters, while limited in their path (not supporting loops), possess a certain ride character (owing to structural response) that is appreciated by fans of the type.

The Camas Prairie Railroad in northern Idahomarker utilized many timber trestles across the rolling Camas Prairie. The major trestle across Lawyers Canyon was the exception, constructed of steel.

Image:Wooden trestle bridge approach.JPG|A classic wood trestle using logs and beamsImage:IowaRiverFloodwayTrestle.jpg|Wooden trestle crossing a floodway.

Steel Trestles

The steel trestle shown below is a modern structure with a long expected lifetime compared to a wooden trestle. Being fire resistant in this brushy location is also an advantage. The approaches to the Kate Shelley High Bridgemarker are steel trestles.

New Orleansmarker utilizes steel trestles to support parts of I-10, the Pontchartrain Expressway, and Tulane Avenue.

Image:AlhambraTrestle.jpg|A steel trestle with plate girder spans double-bent steel towers.

Concrete Trestles

The new Kate Shelley High Bridgemarker is a concrete trestle.

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