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Pontoon bridges are floating bridges supported by barge-or-boat-like pontoons to support the bridge deck and its dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time. 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 alternate 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.

Military bridges

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.

Pontoon bridges have been in use since ancient times.

The Greek writer Herodotus in his Histories, records several pontoon bridges, the most spectacular of which were a pair of bridges across the Helespont by Xerxes I in 480 BC to transport his huge army into Europe:

and meanwhile other chief-constructors proceeded to make the bridges; and thus they made them: They put together fifty-oared galleys and triremes, three hundred and sixty to be under the bridge towards the Euxine Sea, and three hundred and fourteen to be under the other, the vessels lying in the direction of the stream of the Hellespont (though crosswise in respect to the Pontus), to support the tension of the ropes. They placed them together thus, and let down very large anchors, those on the one side towards the Pontus because of the winds which blow from within outwards, and on the other side, towards the West and the Egean, because of the South-East and South Winds. They left also an opening for a passage through, so that any who wished might be able to sail into the Pontus with small vessels, and also from the Pontus outwards. Having thus done, they proceeded to stretch tight the ropes, straining them with wooden windlasses, not now appointing the two kinds of rope to be used apart from one another, but assigning to each bridge two ropes of white flax and four of the papyrus ropes. The thickness and beauty of make was the same for both, but the flaxen ropes were heavier in proportion, and of this rope a cubit weighed one talent. When the passage was bridged over, they sawed up logs of wood, and making them equal in length to the breadth of the bridge they laid them above the stretched ropes, and having set them thus in order they again fastened them above. When this was done, they carried on brushwood, and having set the brushwood also in place, they carried on to it earth; and when they had stamped down the earth firmly, they built a barrier along on each side, so that the baggage-animals and horses might not be frightened by looking out over the sea.*

The late Roman writer Vegetius, in his work De Re Militari, wrote:

But the most commodious invention is that of the small boats hollowed out of one piece of timber and very light both by their make and the quality of the wood. The army always has a number of these boats upon carriages, together with a sufficient quantity of planks and iron nails. Thus with the help of cables to lash the boats together, a bridge is instantly constructed, which for the time has the solidity of a bridge of stone.*

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 in 1995 by the U.S. Army's 1st Armored Division as part of IFOR. It was assembled under adverse weather conditions across the Sava near Županja (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 * 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.

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.

  • Nordhordland Bridge
    • Completed 1994. Span, 4,086 feet (1,246 meters) (the floating bridge part).
    • Located near Bergen, Norway, the Nordhordland Bridge consists of a free-floating bridge and a high level cable-stayed bridge. The free-floating bridge has the longest laterally-unsupported span in the world. It is sometimes referred to as the Salhus Bridge.
    • Architect's web site

  • 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.

  • Queen Emma Bridge
    • A pontoon bridge from Punda to Otrabanda across the harbor of Willemstad on the island of Curaçao.*

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.
  • A minor disaster occurs if anchors or connections between the pontoon bridge segments fail. This may happen due overloading, extreme weather or flood. The bridge disintegrates and parts of it start to float away. Many cases are known.

See also

Bridges | Pontoon bridges

Pontonbrug | Most pontonowy | Pontonski most | Cầu phao

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Pontoon bridge".

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