Pontoon bridges are floating bridges supported by floating pontoons 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 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.
Submerged floating-tube bridges have been considered for use across ocean straits and even across entire oceans. The construction of such a tunnel was featured in the alternative history novel A Transatlantic Tunnel, Hurrah! by Harry Harrison. 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. No submerged floating tunnel exists in the world at present.
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 the Garigliano, the Battle of Oudenarde, and many others.
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 to form a road surface, and the planks were held in place with side 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 due to 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.
The longest military pontoon bridge ever constructed across a river was built by the U.S. Army's 1st Armored Division in 1995. It was assembled under adverse weather conditions across the Sava River between Croatia and Bosnia, and had a total length of 2,034 feet. It was disassembled in 1996.