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Last edited 27 Apr 2021
|The Evergreen Point Floating Bridge in Washington State, carrying State Route 520 over Lake Washington, seen from the north side bicycle and pedestrian trail from the west high rise. It is the world's longest pontoon bridge.|
Bridges are a common feature of the built environment and one of the key elements of civil engineering. Movable bridges are designed in a way that allows them to accommodate different situations and the flow of different types of traffic. One kind of movable bridge is a pontoon (or ponton) bridge. These are also referred to as floating bridges.
Pontoon bridges are constructed on large, water-tight concrete floats, boats, barges or other buoyant structures that are rigidly connected end-to-end, upon which the road deck is constructed. Individual bridge pontoons are built on the land before being floated and towed to the bridge site. Once there, they are connected at each end to grounded approach structures, starting at the edge of the floating structure and then pieced together towards the bridge’s centre. Cables hold the pontoons in place, connected to buried anchors.
The use of floating bridges is typically limited by the wind and wave forces that are likely to be experienced. The ‘fetch’ is the unobstructed clear distance over the water that wind can travel to the bridge. The longer the fetch, the higher the wind and wave forces will generally be. Pontoons bend, heave and twist as a result of wind and wave (and sometimes tidal) forces, creating large stresses in the pontoons and anchor system. However, even in the event of a ‘100-year storm’ (a storm that statistically has 1% chance of occurring in any given year), the pontoons are designed to survive.
 Strategic pontoon bridges
The earliest recorded use of a pontoon bridge goes back to Ancient China. While texts indicate use by the Zhou Dynasty in the 11th century BC, it is believed that temporary pontoon bridges were used several centuries earlier and permanent pontoon bridges came about in the 2nd century BC during the Qin Dynasty.
Temporary pontoon bridges were frequently used in Ancient Greek and Roman times. This continued in the middle ages, when they were frequently used in military applications.
As tactical structures, they were used during emergency situations, such as natural disasters or wartime conflicts. They were put in place either when conventional bridge construction was thought to be cost prohibitive or when additional flexibility (such as elevation or removal for the passing of waterway traffic) was necessary.
 Permanent pontoon bridges
There are some permanent pontoon bridges in use around the world in places such as Australia, India, Canada and so on. As permanent structures, pontoon bridges generally require deep water and are practical in places where the softness of the ground beneath the bridge makes it unsuitable for safe pier construction.
In March 2016, Washington State in northwest America opened the world’s longest floating bridge. The ‘State Route 520 Bridge’ (SR520), as it is known, is a six-lane pontoon structure 2.4 km in length. There are three additional pontoon bridges in Washington State.
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