Pontoon bridge

Pontoon bridge, bridge held up by pontoons (flat-bottomed boats), sealed metal tubes, or other floating objects. Because it can be built relatively quickly and is made up of materials that are transportable, it is especially convenient for military purposes. An army can set it up when invading or defending a territory to transport troops and equipment across a river when no bridge exists, or when the enemy has destroyed a once-existing bridge. A pontoon bridge was used as early as 480 B.C. by the Persian army. More recently, pontoon bridges were important to U.S. troops in Europe during World War II, where many bridges had been annihilated. Because a pontoon bridge blocks navigation, it is not a viable permanent structure. However, several long-span permanent concrete pontoon bridges exist. Washington has three, and Tasmania and Istanbul have one each.