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Pontoon bridge
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A pontoon bridge or floating bridge is a bridge that floats on water, 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.
Military bridges Pontoon bridges are especially useful in wartime as river crossings.
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A pontoon bridge or floating bridge is a bridge that floats on water, 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.
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.
Ancient ChinaThe ancient China, the Zhou Dynasty Chinese text of the Shi Qing (Book of Odes) records that King Wen of Zhou was the first to create a pontoon bridge in the 11th century BC. However, the historian Joseph Needham has pointed out that in all likely scenarios, the temporary pontoon bridge was invented during the 9th century BC - 8th century BC in China, as this part was perhaps a later addition to the book (considering how the book had been edited up until the Han Dynasty, 202 BC - 220 AD). Although earlier temporary pontoon bridges had been made in China, the first secure and permanent ones (and linked with iron chains) in China came first during the Qin Dynasty (221 BC - 207 BC). The later Song Dynasty (960 - 1279 AD) Chinese statesman Cao Cheng once wrote of early pontoon bridges in China (spelling of Chinese in Wade-Giles format):
The Chhun Chhiu Hou Chuan says that in the 58th year of the Zhou King Nan (257 BC), there was invented in the Qin State the floating bridge (fou chhiao) with which to cross rivers. But the Ta Ming ode in the Shih Ching (Book of Odes) says (of King Wen) that he ‘joined boats and made of them a bridge’ over the River Wei. Sun Yen comments that this shows that the boats were arranged in a row, like the beams (of a house) with boards laid (transversely) across them, which is just the same as the pontoon bridge of today. Tu Yu also thought this...Cheng Khang Chheng says that the Zhou people invented it and used it whenever they had occasion to do so, but the Qin people, to whom they handed it down, were the first to fasten it securely together (for permanent use).
During the Eastern Han Dynasty (25 - 220 AD), the Chinese created a very large pontoon bridge that spanned across the width of the Yellow River. There was also the rebellion of Gongsun Shu in 33 AD, where a large pontoon bridge with fortified posts was constructed across the Yangtze River, eventually broken through with ramming ships by official Han troops under Commander Cen Peng. During the late Eastern Han into the Three Kingdoms period, during the Battle of Chibi in 208 AD, the Prime Minister Cao Cao once linked the majority of his fleet together with iron chains, which proved to be a fatal mistake once he was thwarted with a fire attack by Sun Quan's fleet.
The armies of Emperor Taizu of Song had a large pontoon bridge built across the Yangtze River in 974 in order to secure supply lines during the Song Dynasty's conquest of the Southern Tang.
On October 22, 1420, Ghiyasu'd-Din Naqqah, an envoy of the embassy sent by the Timurid ruler of Persia, Mirza Shahrukh (r. 1404–1447), to the Ming Dynasty of China during the reign of the Yongle Emperor (r. 1402–1424), recorded his sight and travel over a large floating pontoon bridge at Lanzhou (constructed earlier in 1372) as he crossed the Yellow River on this day. He wrote that it was:
...composed of twenty three boats, of great excellence and strength attached together by a long chain of iron as thick as a man's thigh, and this was moored on each side to an iron post as thick as a man's waist extending a distance of ten cubits on the land and planted firmly in the ground, the boats being fastened to this chain by means of big hooks. There were placed big wooden planks over the boats so firmly and evenly that all the animals were made to pass over it without difficulty.
Greco-Roman eraThe Greek writer Herodotus in his Histories, records several pontoon bridges. For Emperor Darius I The Great of Persia (522 BC - 485 BC), the Greek Mandrocles of Samos once engineered a pontoon bridge that stretched across the Bosporus, linking Asia to Europe, so that Darius could pursue the fleeing Scythians as well as move his army into position in the Balkans to overwhelm Macedon. Another spectacular pontoon bridge was a pair of floating bridges across the Hellespont 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.
Modern usage
The longest military pontoon bridge ever constructed across a river was built in 1995 by the 586th Engineer Company, 36th Engineer Group out of Fort Benning, Georgia 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 . It was disassembled in 1996.
Modern variants of the pontoon bridge are still essential and in use (as of 2007) by modern armies. As an example, the American Army has developed a version dubbed the "Assault Float Ribbon Bridge". It was constructed during combat by the 299th Multi-role Bridge Company, USAR on the Euphrates River at Objective Peach near Al Musayib on the night of 3 April 2003. This took place during the 2003 invasion of Iraq also called "Operation Iraqi Freedom" by American and British forces. 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. Examples of the construction and use of pontoon bridges during combat operations date back through World War II and earlier .
DesignWhen 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 called chesses to form a road surface, and the chesses 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.
Submerged floating-tube bridges have been considered for use across ocean straits and even across entire oceans. 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.
Notable uses of pontoon bridges
Historic
Disasters Floating bridges can be vulnerable to inclement weather, especially strong winds.
- In 1979, the longest floating bridge crossing salt water, the Hood Canal 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. Specifically, the sidewalks were being removed to widen the traffic lanes to the standards mandated by the Interstate Highway System. Engineers realized that jackhammers could not be employed to remove the sidewalks without risking compromising the structural integrity of the entire bridge. As such, a unique process called hydrodemolition was employed, in which powerful jets of water are used to blast away concrete, bit by bit. The water used in this process, however, trickled into the hollow chambers in the pontoons of the bridge, which were supposed to be filled with air, and provide the bridge its necessary flotation. Furthermore, watertight doors on the pontoons were removed to facilitate the work. During a week of rain and strong winds, the watertight doors were not closed and the pontoons filled with water from the storm, in addition to the water taken on as a result of the hydrodemolition. 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. When the Lacey V. Murrow Memorial Bridge sank (above), it severed the anchor cables of the bridge parallel to it. A powerful tugboat pulled on that bridge against the wind during a subsequent storm, and prevented further damage.
See also
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