How Floating Bridges Work

Surely ancient peoples must have looked longingly at unreachable shores on the other side of rivers and wished for bridges to carry them there. Wishes alone couldn't build those bridges, but war could.

Most early floating bridges were built for the purposes of war. The Chinese, Romans, Greeks, Mongols and other peoples all used versions of pontoon bridges to move soldiers and equipment, usually across rivers too deep to ford.

The most primitive floating bridges were wooden boats placed in rows with planks laid across them to support foot traffic, horses and wheeled carts. At each shore these bridges were secured, often with ropes, to keep them from drifting with the current or wind.

One of the most storied ancient floating bridges was created at the behest of the Persian king Xerxes. Xerxes set out to conquer Greece in 481 B.C., but first, his huge army had to cross the Hellespont, a strait about 4,409 feet (1.3 km) wide.

Xerxes commanded his men to build a floating bridge, but a violent storm subsequently destroyed their work. To show his anger toward the sea, Xerxes told his men to symbolically whip the water 300 times, and he also had leg shackles thrown into the sea. He punished the engineers of the failed bridge by beheading them.

A second bridge was built, and nearly 400 ships were used to keep its surface afloat. The boats were all tied together with heavy flaxen and papyrus ropes and weighted with heavy anchors to hold them in place, and there was an opening left so that small vessels navigating the strait could still pass the bridge.

Logs were used for the bridge's surface, and these were topped with brush and soil. And there were barriers on each side so that horses (and perhaps men) wouldn't be spooked by walking so close to their potential watery ends. This one survived, letting hundreds of thousands of soldiers pass over the strait. Ultimately, this army attacked and captured Athens.

Many wars since then have seen floating bridges in action, including the American Civil War and both World Wars. They continue to be vital for campaigns that must cross bodies of water to engage (or run away from) enemies.

Modern pontoon bridges are extremely portable and can often ride on a semi-truck trailer. They usually combine an inflatable underbelly that's topped with a metal structure that supports large vehicles and men.

Not all pontoon bridges are built for warfare. Keep reading, and you'll see how more permanent ones are constructed for more peaceful ends.

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.

Every geographical area is different, so engineers pick a bridge style best suited to each location. When they decide to go with a floating bridge, the pontoons are usually built on shore near the construction site and then towed into place by barges and anchored. Then the support elements and deck of the bridge are added to the top.

In many cases, engineers must also account for water traffic, such as barges or military ships. For these situations, the bridge may have an elevated section (called a high rise) under which boats may pass. Or there may be a drawbridge that can be raised, although in this configuration, bridge traffic must halt.

Engineers use various techniques to keep floating bridges stationary and stable. Most use a combination of anchors and mooring lines. The anchors weigh many tons and along with the mooring lines let the bridge flex under stress without breaking.

Those are the easy parts. When it comes to making usable, durable floating bridges, it's often the shorelines that cause the most headaches. That's because shores are rarely near the surface of the water level, and often, the shore is jagged and at a much higher elevation.

That means engineers must design approaches that aren't too steep and let vehicles descend safely to the bridge's surface. Often, they build up the shoreline slowly by adding soil or rock to create a gentler ascent. Or they drive supports into the soil beneath the water to support a deck that's angled downwards toward floating portions. Still other bridges use tunnels to approach the watery part of their journey.

As engineers have honed their knowledge base, their feats have become more and more impressive. And nowhere is there a more awe-inspiring floating bridge than in Seattle. On the next page, you'll read all about the biggest pontoon bridge around.

The Governor Albert D. Rossellini Bridge—Evergreen Point bridge (often called just the SR 520 bridge) in Seattle is the longest floating bridge on the planet. It is 7,497 feet (2,285 meters), or 1.4 miles (metric), long and spans Lake Washington, letting vehicles pass to and from Seattle and Bellevue, Washington.

After three years of construction, the four-lane toll bridge opened in 1963. It wasn't the first floating bridge on Lake Washington; that was actually the Lake Washington Floating Bridge, which was built in 1940 and wound up as part of Interstate 90. But the SR 520 bridge is significantly bigger than its forbearer.

It's four lanes and 60 feet wide and is supported by 33 pontoons. Those individual pontoons are enormous, at around two and a half stories tall, and 360 feet (110 meters) long -- that's longer than four gas tanker semi-trucks placed end to end. The pontoons are secured by 62 anchors that weigh around 77 tons each.

If those numbers don't float your brain's boat, consider this -- the SR 520 is getting a major upgrade. A new version of the bridge will have six lanes and a bicycle path. It will be 115 feet wide and have 77 pontoons, each of which will be 360 feet long, 75 feet high and 28 feet tall. Perhaps most incredibly, each pontoon will weigh around 11,000 tons, which is the equivalent of 23 Boeing 747 jets [source: Washington DOT].

The bridge is also upgradeable. Currently, the two center lanes will be for HOV (high-occupancy vehicles) only, but in the future, a light-rail system could be placed here instead (although this would require even more pontoons to support the additional weight).

You might wonder why in the world Washington State would build such a mammoth structure. The Seattle metropolitan area is home to large companies with equally large workforces. All of those people need efficient transportation options, and the direct route across Lake Washington is the fastest. But that route is curved, which makes a suspension bridge impossible, and the lake is too deep for a conventional bridge.

But the bridge won't come cheap. The budget for the new crossing is currently $4.65 billion [source: Washington DOT].

For that kind of price tag, let's hope this bridge lasts for a very long time. But because Murphy's Law always seems to assert itself in the end, something will break -- floating bridges are no exception. Keep reading, and you'll find out more about famous bridges and their infamous moments.

Natural disasters can ruin a lot of human creations, and floating bridges are no exception. Wind, waves and corrosion are all major enemies of these mostly concrete and steel bridges. And no matter how much planning goes into bridge construction or how much engineers "overdesign" bridges to account for unexpectedly extreme circumstances, floating bridges can and do fail.

As engineers plan floating bridges, they take local conditions into account, by trying to anticipate characteristics of wind and wave movement. They must also consider the possibility of unusual events, such as ship strikes or wildly powerful storms. Usually, designers design bridges to withstand a storm so strong that it occurs only once every 100 years, and the bridges themselves are purposely built to allow for some flexing as temperature, wind and waves change.

In the event of a strong storm with high winds and waves, traffic is often suspended, and if the bridge has a navigational opening, such as a draw bridge, it's opened to relieve stress. That's what owners of the Hood Canal bridge did in 1979 during a windstorm in February 1979, to no avail.

During the storm, the doors to the tops of the bridge's western pontoons either blew open or were left open on accident. Water from rain and waves 10 to 15 feet high began to slowly fill the pontoons, while waves slammed against the bridge for hours until it began to break apart and was finally destroyed.

And in November 1990, renovation crews were working on the Lake Washington bridge, and as the crews departed for Thanksgiving weekend, a major storm approached. With the permission of engineers, work crews had been storing wastewater from their project in some of the bridge's huge concrete pontoons. To do so, they'd temporarily removed the pontoons' watertight doors. As the storm moved through, seawater and rain filled the pontoons, and slowly, as live TV captured the disaster, the bridge ripped apart and sank.

Both bridges were eventually replaced using updated engineering knowledge and construction techniques. Yet fears remain that massive floating bridges are not durable enough to justify their great initial expense. Floating bridges are also notorious for being extremely expensive to maintain. They are subjected to so many natural forces that they must be constantly monitored for cracks and other deficiencies, which require time and equipment to repair.

These types of problems don't stop engineers from proposing and erecting floating bridges. Keep reading, and you'll find out more about famous bridges across the globe.

Although Washington State is ground zero for the most numerous and biggest floating bridges, it's by no means the only place you'll find them. Countries around the world maintain their own permanent, pontoon-based constructions.

As with the Seattle area, in Georgetown, Guyana, water (in the form of the Demerara River) was making it hard for the city to grow economically. So in 1978, Guyana built the Demerara Harbor bridge to connect communities and allow for more commercial expansion.

At 1.2 miles (2 kilometers) long, it's the fourth-longest pontoon bridge in the world. It supports a two-lane road and incorporates 122 steel pontoons. Like its Washington cousins, this bridge has suffered its share of problems: deterioration due to lack of maintenance, ship collisions and the sinking of sections due to heavy tides.

As in Washington, Norway has its share of water near population centers. That fact helped spawn a need for the Nordhordland Bridge, which has a floating section that's 4,088 feet (1,246 meters) long. There are 10 pontoons under the bridge of varying sizes, which help the deck cross a fjord and connect to a more traditional, cable-stayed bridge.

Not all currently existing bridges are so large or so new. The Dongjin Bridge in China, for example, has been in use since the Song Dynasty, which took place between 960 and 1279. This (400 meter) bridge still uses wooden boats -- about 100 of them -- to provide buoyancy for the wooden plank deck. The boats are secured to each other with chains.

Regardless of their construction or age, floating bridges have been a part of human transportation for centuries. Not only have they conquered many winds and waves, but they've also stood the test of time. And they'll be around for a long time to come, as long as people still need to find their way across deep waters.