A floating bridge supported by pontoons. The structure may be temporary, as for military usage, or permanent, where deep water and adverse ground conditions make piers expensive. Typically the pontoons consist of flat bottomed boats, hollow metal cylinders, or concrete rafts. Three permanent concrete pontoon bridges cross L Washington in Seattle, USA.
| Pontoon bridge | |
|---|---|
| Temporary military river crossing ca. 1940 | |
| Ancestor | None - this is a foundational type |
| Related | None |
| Descendant | None |
| Carries | Pedestrian, automobile, truck |
| Span range | Short to long |
| Material | Barious: steel, concrete, boats, barrels, plastic floats, appropriate decking material |
| Movable | Generally no, but may have movable sections for watercraft passage |
| Design effort | low |
| Falsework required | No |
Pontoon bridges are floating bridges supported by barge-or-boat-like pontoons to support the bridge deck and its dynamic loads. Permanent floating bridges are useful for sheltered water-crossings where it is not considered economically feasible to suspend a bridge from anchored piers. 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.
Military bridges
Pontoon bridges are especially useful in wartime as river crossings.
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:
The late Roman writer Vegetius, in his work De Re Militari, wrote:
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.
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.
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. 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.
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. A toll bridge until 1979, its common name is the Evergreen Point Floating Bridge. It is the longest floating bridge in the world. Murrow Memorial Bridge Original bridge completed in 1940 but sank in 1990 due to weather and mishaps in maintenance Second bridge completed 1993. A toll bridge until 1946, its common name is the 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 the second longest floating bridge in the world. It is the third longest floating bridge in the world. Located immediately south of Georgetown, Guyana, it is constructed with steel pontoon units and is the fourth longest floating bridge in the world. Murrow Memorial Bridge, which carries eastbound Interstate 90 Traffic, and is also commonly referred to as the Lake Washington Floating Bridge. It is the fifth longest floating bridge in the world. 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 old 3 lane floating bridge is currently being replaced by a new, 5 lane floating bridge. One of the bridges, the Dongjin Bridge, can still be seen. Located in Portland, Oregon, it is the longest floating pedestrian bridge in the United States. Hobart Bridge Completed 1943 Spanned the Derwent River at Hobart, Tasmania Constructed of hollow concrete pontoons, it was replaced by a new bridge in 1964 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 only floating bridge crossing salt water, the Hood Canal Bridge, was subjected to winds of 80 miles per hour, gusting up to 120.
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