Pontoon bridge

A pontoon bridge (or ponton bridge), also known as a floating bridge, is a bridge that uses floats or shallow-draft boats to support a continuous deck for pedestrian and vehicle travel. The buoyancy of the supports limits the maximum load that they can carry. Most pontoon bridges are temporary and used in wartime and civil emergencies. There are permanent pontoon bridges in civilian use that can carry highway traffic; generally, the relatively high potential for collapse and sinking (e.g. due to waves and collisions) and high continuous maintenance costs makes pontoons unattractive for most civilian construction. Permanent floating bridges are useful for sheltered water crossings if it is not considered economically feasible to suspend a bridge from anchored piers (such as in deep water). Such bridges can require a section that is elevated or can be raised or removed to allow waterborne traffic to pass. Notable permanent pontoon bridges include the Evergreen Point Floating Bridge, Hood Canal Bridge, and the Nordhordland Bridge. Pontoon bridges have been in use since ancient times and have been used to great advantage in many battles throughout history, such as the Battle of Garigliano, the Battle of Oudenarde, the crossing of the Rhine during World War II, the Yom Kippur War, Operation Badr, the Iran–Iraq War's Operation Dawn 8, and most recently in the Russian invasion of Ukraine.

Definition A pontoon bridge is a collection of specialized, shallow draft boats or floats, connected together to cross a river or canal, with a track or deck attached on top. The water buoyancy supports the boats, limiting the maximum load to the total and point buoyancy of the pontoons or boats. The supporting boats or floats can be open or closed, temporary or permanent in installation, and made of rubber, metal, wood, or concrete. The decking may be temporary or permanent, and constructed out of wood, modular metal, or asphalt or concrete over a metal frame.

Etymology The spelling "ponton" in English dates from at least 1870. The use continued in references found in U.S. patents during the 1890s. It continued to be spelled in that fashion through World War II, when temporary floating bridges were used extensively throughout the European theatre. U.S. combat engineers commonly pronounced the word "ponton" rather than "pontoon" and U.S. military manuals spelled it using a single 'o'. The U.S. military differentiated between the bridge itself ("ponton") and the floats used to provide buoyancy ("pontoon"). The original word was derived from Old French ponton, from Latin ponto ("ferryboat"), from pons ("bridge").

Design

When designing a pontoon bridge, the civil engineer must take into consideration Archimedes' principle: Each pontoon can support a load equal to the mass of the water that it displaces. This load includes the mass of the bridge and the pontoon itself. If the maximum load of a bridge section is exceeded, one or more pontoons become submerged. Flexible connections have to allow for one section of the bridge to be weighted down more heavily than the other parts. The roadway across the pontoons should be relatively light, so as not to limit the carrying capacity of the pontoons. The connection of the bridge to shore requires the design of approaches that are not too steep, protect the bank from erosion and provide for movements of the bridge during (tidal) changes of the water level. Floating bridges were historically constructed using wood. Pontoons were formed by simply lashing several barrels together, by rafts of timbers, or by using boats. Each bridge section consisted of one or more pontoons, which were maneuvered into position and then anchored underwater or on land. The pontoons were linked together using wooden stringers called balks. The balks were covered by a series of cross planks called chesses to form the road surface, and the chesses were secured with side guard rails. A floating bridge can be built in a series of sections, starting from an anchored point on the shore. Modern pontoon bridges usually use pre-fabricated floating structures. Most pontoon bridges are designed for temporary use, but bridges across water bodies with a constant water level can remain in place much longer. Hobart Bridge, a long pontoon bridge built 1943 in Hobart, Tasmania was only replaced after 21 years. The fourth Galata Bridge that spans the Golden Horn in Istanbul, Turkey was built in 1912 and operated for 80 years. Provisional and lightweight pontoon bridges are easily damaged. The bridge can be dislodged or inundated when the load limit of the bridge is exceeded. The bridge can be induced to sway or oscillate in a hazardous manner from the swell, from a storm, a flood or a fast moving load. Ice or floating objects (flotsam) can accumulate on the pontoons, increasing the drag from river current and potentially damaging the bridge. See below for floating pontoon failures and disasters.

Historic uses

Ancient China 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 (221–207 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):

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