Pontoon bridges are floating structures supported by floating docks, useful for temporary or permanent crossings. Submerged floating pipe bridges are expensive and untested. They are useful in war and require careful engineering to support loads. Wooden bridges were common before modern equipment. Precautions are needed to protect pontoon bridges from damage. The longest military pontoon bridge was built by the US Army in 1995 across the Sava River, Croatia and Bosnia, and was dismantled in 1996.
Pontoon bridges are floating bridges supported by floating docks with sufficient buoyancy to support the bridge and dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time. Permanent floating bridges are useful for protected water crossings where it is not considered economically feasible to suspend a bridge with anchored piers. Such bridges may require a raised section, or one that can be raised or removed, to allow vessels to pass.
Submerged floating pipe bridges were considered for use across oceanic straits and even across entire oceans. The construction of such a tunnel was featured in the alternate history novel A Transatlantic Tunnel, Hurrah! by Harry Harrison. It is estimated that a submerged floating tunnel would be two to three times more expensive to build than a floating bridge, and the technology has not yet been demonstrated. There is currently no submerged floating tunnel in the world.
Pontoon bridges are especially useful in times of war as river crossings. Such bridges are usually temporary and are sometimes destroyed after crossing (to prevent the enemy from using them) or collapsed and carried away (if on a long march). They were used to great advantage in many battles over time, including the battle of Garigliano, the battle of Oudenarde and many others.
When designing a pontoon bridge, the engineer must consider the maximum amount of load it is intended to support. Each pontoon can support a load equal to the mass of the water 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 piers are submerged and proceed to sink. The roadway crossing the pontoons must also be capable of carrying the load, but be light enough not to limit their carrying capacity.
Before the advent of modern military pontoon bridge construction equipment, floating bridges were generally constructed using wood. Such a wooden floating bridge could be built in a series of sections, starting at an anchor point on the shore. Pontoons were formed using boats; several barrels tied together; wooden rafts, or a combination thereof. 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 crossbars called balks. The beams were then covered with a series of cross planks to form a road surface, and the planks were held in place with side rails. The bridge was extended several times in this way until it reached the opposite bank.
Precautions are needed to protect a pontoon bridge from damage. The bridge can be moved or flooded whenever the bridge load limit is exceeded. A pontoon bridge can also be overloaded when one section of the bridge is weighted much more heavily than the other parts. The bridge can be caused to sway or sway dangerously by the smooth step of a group of soldiers, or by other types of repetitive loads. Driftwood and heavy floating objects can also build up on the pontoons, increasing the river’s current resistance and potentially damaging the bridge.
The longest military pontoon bridge ever built across a river was built by the US Army 1st Armored Division in 1995. It was assembled under severe weather conditions across the Sava River between Croatia and Bosnia and had a total length of 2.034. It was dismantled in 1996.
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