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The U.S. 9th Army crosses the Rhine on a temporary steel treadway pontoon bridge, 1945.
|Carries||Pedestrian, automobile, truck|
|Span range||Short to long|
|Material||Various: steel, concrete, boats, barrels, plastic floats, appropriate decking material|
|Movable||Generally no, but may have movable sections for watercraft passage|
A pontoon bridge or floating bridge or bridge of boats is a bridge that floats on water and in which barge- or boat-like pontoons 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.
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 Garigliano, the Battle of Oudenarde, and many others.
Pontoon bridges have been in use since ancient times.
In ancient China, the Zhou Dynasty Chinese text of the Shi Jing (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 or 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 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.
On October 22, 1420, Ghiyasu'd-Din Naqqah, the official diarist 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.—
The 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. Other spectacular pontoon bridges were Xerxes' Pontoon 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.—
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.— 
The emperor Caligula is said to have ridden a horse across a pontoon bridge stretching two miles between Baiae and Puteoli while wearing the armour of Alexander the Great to mock a soothsayer who had claimed he had "no more chance of becoming emperor than of riding a horse across the Bay of Baiae". Caligula's construction of the bridge cost a massive sum of money and added to discontent with his rule.
Although pontoons declined in use during the European Middle Ages, they were still used alongside regular boats to span rivers during campaigns, or to link communities which lacked resources to build permanent bridges. Towards the end of the medieval period, pontoon bridges became once again a standard part of military engineering (e.g. Battle of Calliano (1487)).
Early modern period
In the 1670s, the French devised the copper pontoon; after this point, rivers and canals ceased to present significant obstacles. The early modern period in pontoon use was dominated by the wars of the 18th and 19th centuries during which the art and science of pontoon bridging barely changed.
Pontoons were extensively used by both the armies and civilians throughout the first half of the 20th century and both World Wars.
The longest military pontoon bridge ever constructed across a river was built in 1995 by the 502nd and 38th Engineer Companies of the US Army's 130th Engineer Brigade, 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 (620 m). It was dismantled in 1996.
Modern variants of the pontoon bridge are still essential and in use 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. "By dawn on 4 April 2003, the 299th Engineer Company had emplaced a 185-meter long Assault Float Bridge - the first time in history that a bridge of its type was built in combat."  This took place during the 2003 invasion of Iraq by American and British forces. The 185-meter Assault Float Bridge was built to support retrograde operations because of the heavy-armor traffic crossing a partially destroyed highway span. That same night, the 299th also constructed a 40-metre (130 ft) 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. Four of the five longest floating bridges in the world are in Washington State.
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 called chesses to form a road surface, and the chesses were held in place with side guard 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 because of 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.
- A two-kilometre pontoon bridge was used by Persian Emperor Darius to cross the Bosphorus.
- The original London Bridge was a pontoon bridge.
- A two-mile bridge was built for Emperor Caligula at Baiae in 37 AD.
- Pontoon bridges saw extensive usage during the American Civil War. An innovative light-weight design known as Cumberland Pontoons were widely used during the Atlanta Campaign to transport soldiers and artillery across rivers in the South.
- During the Yugoslav wars of the 1990s, the Maslenica Bridge was destroyed and a short pontoon bridge was built by Croatian civilian and military authorities in July 1993 over a narrow sea outlet in the town of Maslenica, after the territory was retaken from Serbian Krajina. Between 1993 and 1995 the pontoon served as one of the two operational land links toward Dalmatia and Croat- and Bosnian Muslim-held areas of Bosnia-Herzegovina that did not go through Serb-held territory.
- Governor Albert D. Rosellini Bridge — Evergreen Point
- Completed 1963. Span 7,578 feet (2,310 m).
- Spans Lake Washington in Washington State, carrying State Route 520 from Seattle to Medina. A toll bridge until 1979, its common name is the 520 bridge or Evergreen Point Floating Bridge. It is the longest floating bridge in the world.
- Currently, the Evergreen Point Floating Bridge is once again being tolled; because of the age of the bridge and its likelihood of withstanding wind speeds over 75 miles per hour (121 km/h), it is planned to be replaced by a new floating bridge in 2014. Toll revenues are expected to cover approximately one-third of the estimated 4.65 billion dollar replacement cost.
- Lacey V. Murrow Memorial Bridge
- Original bridge completed in 1940 but sank in 1990 because of weather and mishaps in maintenance.
- Second bridge completed 1993. Span 6,620 feet (2,018 m).
- Spans Lake Washington in Washington State, carrying Interstate 90 traffic eastbound from Seattle to Mercer Island. A toll bridge until 1946, its common name is the I-90 bridge or Lake Washington Floating Bridge. It was the first floating bridge longer than a mile, and at the time was the longest floating structure in the world. It is now the second longest floating bridge in the world.
- Hood Canal Bridge
- Demerara Harbour Bridge
- Homer M. Hadley Memorial Bridge
- Completed 1989. Span 5,811 feet (1,771 m).
- Spans Lake Washington in Washington state, carrying Interstate 90 traffic westbound from Mercer Island to Seattle. It runs parallel to the Lacey V. Murrow Memorial Bridge, which carries eastbound Interstate 90 Traffic, and is also commonly referred to as the I-90 bridge or Lake Washington Floating Bridge. It is the fifth longest floating bridge in the world.
- Berbice Bridge
- Nordhordland Bridge
- Completed 1994. Span 4,086 feet (1,245 m) (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
- Bergøysund Floating Bridge
- Galata Bridge
- Howrah Bridge
- Okanagan Lake Bridge
- Completed 1958. Span 2,100 feet (640 m).
- Spans Okanagan Lake in British Columbia, carrying Highway 97 to Kelowna. This was the first floating bridge built in Canada. The old 3 lane floating bridge has been replaced by a new, 5 lane floating bridge. The new bridge — William R. Bennett Bridge — was completed on May 25, 2008.
- Dongjin Bridge in Ganzhou, China
- Eastbank Esplanade
- Completed 2001. Span 1,200 feet (366 m).
- Located in Portland, Oregon, it is the longest floating pedestrian bridge in the United States.
- Hobart Bridge
- Queen Emma Bridge
- Sunset Lake Floating Bridge
- Located in Brookfield, Vermont
- Built on logs in 1820, then upon tarred barrels in 1884, rebuilt using plastic barrels filled with styrofoam in 1978, caries light automobile traffic.
- Sozh Floating Bridge
- Floating Bridge, Dubai, United Arab Emirates
- Guangji Bridge, China
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 (130 km/h), gusting up to 120. Waves of 10–15 feet (3.0–4.6 m) battered the sides of the bridge, and within a few hours the western 3⁄4 mile (1.2 km) 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 was temporarily stored in the hollow chambers in the pontoons of the bridge (to prevent it from contaminating the lake). 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 from 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 because of 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, 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.
|Wikimedia Commons has media related to Pontoon bridge.|
- Bailey bridge for another bridge type with mobile military application.
- Bridge of boats - a temporary floating bridge built upon general-purpose watercraft
- Floating dock
- Mabey Logistic Support Bridge bailey type bridge that can be made into a multi-span bridge on pontoons
- Medium Girder Bridge for another bridge type with mobile military application.
- Mulberry Harbour - as used at D-Day
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- Graff, 87.
- Brook, 38.
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- "Combat Engineers Take a River In Their Stride" , December 1943, Popular Mechanics detailed World War Two article with rare photos of setting up of a pontoon bridge