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Pontoon bridge

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(Redirected from Floating bridge)
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EastbankEsplanade.jpg
A pontoon bridge

Pontoon bridges are floating bridges supported by floating pontoons with sufficient buoyancy to support the bridge and dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time[1] (http://www.structurae.de/en/structures/stype/stype51.php). Permanent floating bridges are useful for sheltered water-crossings where it is not considered economically feasible to suspend a bridge from anchored piers. Such bridges can require a section that is elevated, or can be raised or removed, to allow ships to pass.

Submerged floating-tube bridges have been considered for use across ocean straits and even across entire oceans. The construction of such a tunnel was featured in the alternative history novel A Transatlantic Tunnel, Hurrah! by Harry Harrison. It is estimated that a submerged floating tunnel would be two to three times more costly to build than a floating bridge, and the technology remains unproven. No submerged floating tunnel exists in the world at present.

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Military bridges

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Temporary military river crossing ca. 1940

Pontoon bridges are especially useful in wartime as river crossings. Such bridges are usually temporary, and are sometimes destroyed after crossing (to keep the enemy from using them), or collapsed and carried (if on a long march). They were used to great advantage in many battles throughout time, including the Battle of the Garigliano, the Battle of Oudenarde, and many others.

When designing a pontoon bridge, the engineer must take into consideration the maximum amount of load that it is intended to support. Each pontoon can support a load equal to the mass of the water that it displaces, but this load also includes the mass of the bridge itself. If the maximum load of a bridge section is exceeded, one or more pontoons become submerged and will proceed to sink. The roadway across the pontoons must also be able to support the load, yet be light enough not to limit their carrying capacity.

Prior to the advent of modern military pontoon bridge-building equipment, floating bridges were typically constructed using wood. Such a wooden floating bridge could be built in a series of sections, starting from an anchored point on the shore. Pontoons were formed using boats; several barrels lashed together; rafts of timbers, or some combination of these. Each bridge section consisted of one or more pontoons, which were maneuvered into position and then anchored. These pontoons were then linked together using wooden stringers called balks. The balks were then covered by a series of cross planks to form a road surface, and the planks were held in place with side rails. The bridge was repeatedly extended in this manner until the opposite bank was reached.

Precautions are needed to protect a pontoon bridge from becoming damaged. The bridge can be dislodged or inundated whenever the load limit of the bridge is exceeded. A pontoon bridge can also become overloaded when one section of the bridge is weighted down much more heavily than the other parts. The bridge can be induced to sway or oscillate in a hazardous manner due to the regular stride of a group of soldiers, or from other types of repeated loads. Drift and heavy floating objects can also accumulate on the pontoons, increasing the drag from river current and potentially damaging the bridge.

The longest military pontoon bridge ever constructed across a river was built by the U.S. Army's 1st Armored Division in 1995. It was assembled under adverse weather conditions across the Sava River between Croatia and Bosnia, and had a total length of 2,034 feet. It was disassembled in 1996.

The first military pontoon bridge ever to be constructed during combat was built by the 299th Multi-role Bridge Company, USAR [2] (http://299th.luddite.net/) on the Euphrates River at Objective Peach near Al Musayib on the night of 03 April 2003 during Operation: Iraqi Freedom. The 185-meter Assault Float Bridge was built to support retrograde operations due to the heavy armor traffic crossing a partially destroyed highway span. That same night, the 299th also constructed a 40-meter single-story Medium Girder Bridge to patch the damage done to the highway span. The 299th was part of the U.S. Army's 3rd Infantry Division as they crossed the border into Iraq on 20 March 2003.

Permanent floating bridges

  • Demerara Harbor Bridge
    • Completed 1978. Span, 6,074 feet.
    • Located in Georgetown, Guyana, it is constructed with steel pontoon units and is the fourth longest floating bridge in the world.
  • Dongjin Bridge in Ganzhou, China
    • Pontoon bridges have been constructed over the Zhang and Gong rivers since the Song Dynasty (960-1279).
    • One of the bridges, the Dongjin Bridge, can still be seen.
    • It is 400 metres long, made up of wooden planks placed on around 100 wooden boats linked together with iron chains.
  • Nordhordland Floating Bridge
    • Completed 1999. Span, 4,086 feet.
    • Located near Bergen, Norway, this free-floating bridge has the longest laterally-unsupported span in the world. It is also called the Salhusbrua pontoon bridge.
  • There is a pontoon bridge from Punda to Otrabanda across the harbor of Willemstad on Curaçao.[3] (http://www.boldts.net/album/CaribCur9.shtml)

Disasters

Floating bridges can be vulnerable to inclement weather, especially strong winds.

  • In 1979, the Hood Canal floating bridge was subjected to winds of 80 miles per hour, gusting up to 120. Waves of 10 to 15 feet battered the sides of the bridge, and within a few hours the western 3/4 mile of the structure had sunk. It has since been rebuilt.
  • In 1990, the 1940 Lacey V. Murrow Memorial Bridge was closed for renovations. Multiple 6-foot holes were cut into the sides of the pontoons to facilitate the work. After a week of rain and strong winds, the inundated bridge broke apart and sank. The bridge was rebuilt in 1993.

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