Washington State’s Impressive 7,700-Foot Floating Bridge

Washington state is the proud home to the longest floating bridge in the world and one of just four in existence. The SR 520 Bridge in Seattle is both colorful and iconic. Built for $4.5 million, this bridge replaced an older bridge in the same area. Drivers going westbound on State Route 520 can enjoy the recently opened bridge. Soon, the other half of the bridge will open for eastbound drivers.

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The SR 520 Bridge stretches 7,700 feet. While impressive, the vast length has raised a few questions. For one thing, some people are curious as to why a floating bridge would need to span a lake and whether it can accommodate traffic from both vehicles and trains.

Benefits of a Floating Bridge

Around the Seattle area, floating bridges are nothing new. For the SR 520, it will span Lake Washington, which is more than 200 feet deep. Unfortunately, the bottom consists of soft silt that would make it extremely difficult, if not impossible, to build a more conventional suspension bridge.

With a suspension bridge, towers must be rooted. In addition, each tower would need to be roughly 630 feet tall, which is 50 percent higher than the towers supporting the Brooklyn Bridge. Simply put, the soft silt would create a major issue and a suspension bridge would be extremely expensive.

There is also a political challenge, as explained by Dave Becher, state engineer with the DOT and project construction manager for the SR 520 Bridge project. For residents living in lofty towers with panoramic views of the lake, a suspension bridge would be a major obstruction.

When it comes to floating bridges, Washington has become an authority. For these projects, the state relies on the expertise of engineers from various countries. A prime example with the SR 520 Bridge is that a Japanese delegation attended the grand opening.

Long History of Floating Bridges

Floating bridges are actually older than the state of Washington. In fact, historians claim that these innovative bridges date back to 480 BC and perhaps even earlier. At that time, 300 ships were used by King Xerxes to move his army across the Dardanelles. The concept of using ships as a bridge grew, which is evident in the sophisticated SR 520 Bridge.

The SR 520 Bridge is supported by 77 massive boxes of air. The largest pontoon measures 360 feet long, 75 feet wide, and 28 feet tall. With the design, the upward force exerted by the water equals the downward force on the boxes, allowing roughly 20 feet of each pontoon to settle beneath the water’s surface while 7 feet remain above water.

The concrete used for the SR 520 Bridge project consisted of a special mix. Even the pouring process was unique, which ultimately prevents the pontoons from cracking. On the inside of the
pontoons are smaller compartments that are completely watertight. That way, if a leak were to develop, it would involve just one compartment, thereby dramatically reducing the risk.

The pontoons are also designed with an H2O invasion sensor. If something is detected, an alert is sent to the bridge maintenance facility operated by the DOT, as well as the communications center and appropriate transportation officials.

Safety First

To ensure safety in the instance of high winds, the SR 520 Bridge can withstand 89 mph winds. To keep the bridge securely in place, 58 large anchors are secured to the bottom of the lake. Each of these anchors is then affixed to cables of 3-inch steel.

The anchors used for the predecessor to the SR 520 Bridge were designed to sink into soft silt. For the earlier bridge, different anchors were used, one type weighing 107 tons and another type weighing 450 tons. The current anchors offer stability while preventing the bridge from wobbling caused by waves.

Overall, this new Seattle bridge is safe and secure. However, there are records of things going awry with floating bridges. As an example, a portion of the Hood Canal floating bridge fell during a storm in 1979. Then, in 1990, a crew performing maintenance on a closed 1940s bridge accidentally left the hatches to the pontoons open, allowing them to flood. The bridge’s compartments also experienced small cracks, intensifying the problem.

Henry Petroski, an engineer, explained that in the 1990 incident, the failure led to a domino effect. As expected, engineers now have a much better understanding of the technology and mechanisms that go into the building of floating bridges. Because of this, the state transportation agency in Washington feels completely confident that, one day, light rail will be also installed on the SR 520 Bridge.

Original article: http://www.wired.com/2016/04/takes-keep-7700-foot-floating-bridge-doom