A trestle bridge can be made from either wood or metal, though in the early days of bridge construction, wood was more commonly used than metal. The term "trestle" refers to the support members that are built close together and stretched from the ground up to the bridge platform. Trestle bridges developed along with the railroads. At one time many of these towering structures existed and were used as a direct route across huge chasms.
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Characteristically, trestle bridges have a large deck, located at the top of the structure, where vehicular or train traffic can pass across the platform. Since guardrails on the deck are limited in size and height, trestle bridges are not usually built to hold automobile traffic. When it is put into use for these purposes, a trestle bridge is likely to run straight across a short span. On the other hand, trestle bridges for rail transport are much more common, because the two iron rails provides a secure means of travel across a lengthy span.
A trestle bridge contains at least two trestles that hold up the bridge. Each trestle has anywhere from four to six vertical members that support a horizontal beam, which holds the deck in place. These legs are tied together with horizontal and vertical crosspieces to form one unit, or trestle. The height of each trestle can be anywhere from 10 to 100 feet high. On a long span, a bridge might need a hundred or more such trestles to give the upper deck even support. Then as a final step, the bridge builders install numerous planks that joined together many trestles at one time. By necessity, these wooden members run parallel to the direction of travel and function as vital bracing that ties the whole structure together.
More often than not, 19th century railroad trestle bridges spanned large gorges or chasms, sometimes at a very high altitude. Because of dangers brought on by intermittent rain or seasonal flooding, these kind of structures rarely spanned a waterway. The main reason for this was that the huge lattice of lumber that supported the upper platform could also act as a gigantic matrix that could easily collect debris, carried downhill by flowing water and bring down the whole bridge. One innovation that lessened this problem and added to the appearance of the overall structure was the use of towers that were constructed with just a few trestles and placed a small distance apart.
Straight or Curved
The upper surface of a trestle bridge did not have to be a straight path across the void. On some occasions the route of the platform took on a curved shape. This was a design development that allowed engineers to reposition the direction of travel once on the other side. This type of construction was invariably associated with train travel.
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