What if bridges could float?

A bridge helps you cross water. Most bridges stand on strong legs.

But what if a bridge could float on top of the water, like a boat?

Picture a bridge. You probably imagine a road held up on tall legs, called piers, that stand on the ground under the water. That works well in shallow rivers. But what if the water is very deep, and the bottom is soft mud?

Engineers have a surprising answer: let the bridge float. In this lab we explore floating bridges: how they stay up, what happens when the wind blows, and why some lakes are just too deep for piers.

Conventional bridges carry loads down through piers into the ground. Where water is deep and the bed is soft, reaching firm ground can be impractical. A floating bridge instead carries its weight, and its traffic, on the water itself.

This thought experiment asks what changes when a bridge floats. It follows three threads: how pontoons provide support, how wind and waves load a structure that sits on the surface, and why deep, soft-bottomed lakes favor floating designs. Engineering facts come from the Washington State Department of Transportation (WSDOT), King County, HistoryLink and the engineer-run Practical Engineering; comparisons with a world of floating bridges are ideas.

Some real bridges float! The longest one is on a big lake in Washington State.

It sits on giant hollow concrete boxes. They are called pontoons.

Floating bridges are real. The SR 520 bridge across Lake Washington, near Seattle, opened in April 2016, and Guinness World Records named it the world's longest floating bridge.

It rests on 77 pontoons: large, watertight concrete boxes. The biggest ones are 360 feet long, 75 feet wide, nearly 30 feet tall and weigh 11,000 tons each. WSDOT built them on land, floated them out and towed them with tugboats to the lake, where they were joined end to end.

WSDOT's SR 520 floating bridge opened in April 2016 and was named the world's longest floating bridge by Guinness World Records. It is supported and stabilized by 77 concrete pontoons. The 21 longitudinal pontoons, built in a casting basin in Aberdeen, Washington, are each 360 feet long, 75 feet wide, nearly 30 feet tall and 11,000 tons; 44 smaller supplemental stability pontoons were built in Tacoma.

The pontoons were floated out, towed by tugs to Lake Washington and joined there. At the peak of construction more than 650 workers, including steelworkers, carpenters, welders and crane operators, built them.

To float, a bridge pushes water out of the way. The water pushes back up.

If the push up is as big as the bridge's weight, the bridge floats!

How can a concrete bridge float? WSDOT explains it simply: the pontoons push aside water, and the weight of that water equals the weight of the bridge, including the cars and trucks on it. Hollow, watertight boxes push aside a lot of water without weighing too much themselves.

But floating changes everything else too. A floating bridge sits low on the water, so waves can reach it, and it would block boats unless engineers leave a way through. Floating bridges solve one problem and bring new ones.

A floating body is in equilibrium when the weight of displaced water equals its own weight. WSDOT describes the pontoons in exactly these terms: though made of heavy concrete, they displace a weight of water equal to the bridge's weight, traffic included. Their hollow, watertight construction is what makes this possible.

Floating brings new design problems. The deck sits near the surface, exposed to wind and waves; the structure must be held in place; and it forms a barrier to navigation. On the SR 520 bridge, elevated approach spans at each end let vessels pass beneath before the road drops to the water, as the Practical Engineering channel describes.

Lake Washington is very deep. Under the water is thick, soft mud.

Building tall legs all the way to hard ground would be very hard. So engineers made the bridge float!

Lake Washington was carved by glaciers, and King County measures it at up to 214 feet (65 meters) deep. Below the water lies a layer of soft clay and mud about a hundred feet thick. Building piers all the way down to solid ground would have needed enormous supports.

So in 1921, an engineer named Homer Hadley suggested a bridge that did not touch the bottom at all. His idea led to the Lacey V. Murrow Bridge, which opened in 1940. To keep floating bridges from drifting, engineers tie them to anchors on the lake bed with long cables.

Practical Engineering's account of Lake Washington's crossings explains the choice: the glacially carved lake exceeds 200 feet in places (King County lists a maximum depth of 214 feet, 65.2 m), over a roughly 100-foot layer of soft clay and mud, so piers founded on firm ground would have been enormous and too costly to consider. Homer Hadley proposed a floating concrete bridge in 1921; the Lacey V. Murrow Bridge opened in 1940.

Mooring is essential. Long cables run to anchors: concrete slabs on the bed, piles, or, in deep water or soft soil, anchors jetted down into the mud. WSDOT notes the SR 520 bridge uses heavier anchors than its predecessor, some over 400 tons, with stronger anchor cables.

A floating bridge sits low on the water. On windy days, waves can splash onto the road.

Sometimes the bridge closes until the wind calms down.

Because a floating bridge sits so close to the water, strong winds can push waves right onto the road. In Washington State, each floating bridge has a wind limit, and if the wind gets stronger than that, the bridge is closed to traffic until it calms down.

The old SR 520 bridge, opened in 1963, had pontoons that were at risk in windstorms. Its replacement has more than twice as many pontoons (77 instead of 33) and is designed to handle sustained winds of 89 miles per hour, what engineers call a 100-year storm: one so strong it has only about a 1-in-100 chance of happening in any year.

Floating bridges sit close to the water, so in high winds waves can break onto the deck and the structure can flex beneath traffic. Washington's transportation department sets a wind threshold for each floating bridge and closes it when that threshold is exceeded (Practical Engineering); HistoryLink's 2003 account, for example, describes the Hood Canal Bridge closing to vehicles whenever winds held at 40 mph for 15 minutes.

WSDOT replaced the 1963 SR 520 bridge partly because its pontoons were vulnerable to windstorms and strong waves. The new bridge has 77 pontoons instead of 33 and is designed for sustained winds of 89 mph (about 143 km/h), equivalent to a 100-year storm. Keeping pontoons watertight matters too: in a storm on February 13, 1979, water got into the Hood Canal Bridge's pontoons, through maintenance hatches a witness saw standing open, and its western half sank. The bridge was closed to traffic at the time, and it reopened, rebuilt, in 1982 (HistoryLink).