Floating bridges are uncommon in large part because the kinds of situations and environmental criteria for them are unusual. These bridges are mostly needed to connect heavily populated areas where there is a very wide and very deep body of water, along with extremely soft lake- or ocean-bottom soil, factors that prohibit conventional bridge piers (the structure on which the bridge surface rests).
They are cost-effective in comparison to traditional bridges in scenarios where the water is more than 100 feet (30 meters) deep, and a half mile (900 meters) wide. Areas with very strong winds or waves generally make floating bridges unfeasible.
Contemporary floating bridges are generally made with steel, concrete, wood or a fusion of these components. Because seawater is present around some floating bridges, concrete comes in especially handy, as it is resistant to saltwater corrosion and adequately dampens vibrations from traffic, wind and waves.
Engineers must plan not only for longitudinal forces from water and wind, but also for the weight from traffic. That's where buoyancy comes in. Pontoons are simply hollow, watertight vessels. When you have pontoons in great numbers (or great size), they are very buoyant and can support a weight equivalent to the amount of water they displace, even when the pontoons are made of heavy material such as concrete.
There are two basic categories of pontoon bridges: continuous and separate pontoon. As the name implies, a continuous pontoon stretches the entire length of the bridge. Sometimes the top of the pontoon is overlaid with the deck that supports traffic. Separate pontoon bridges use multiple pontoons spaced along the bridge's length.
Getting bridges to float is the easy part. On the next page, you'll read more about bridge design and how land -- not water -- presents challenges to floating bridge plans.