Science

The World's Longest Floating Bridge Is No Longer Dying

At more than 7,580 feet long, Seattle’s Governor Albert D. Rosellini Bridge is already the world’s longest floating bridge. But after 50 years of service the four-lane concrete span has aged worse than Gerard Depardieu. Its pontoon supports can no longer endure the region’s strong storms and the bridge itself is likely to collapse under a moderate earthquake. But Washington can rebuild it, they have the power, they have the technology.

More commonly known as State Road 502 (SR 502), the Rosellini bridge is a key thoroughfare across Lake Washington and a vital transportation artery for Microsoft employees commuting between Redmond on the eastern shore and Seattle on the Western. On average, it carries 115,000 cars and 190,000 people on the daily. A series of 33 77-ton pontoons support the floating span and anchor it to the lake bottom 214 feet underwater. The bridge had been in service since 1963 but, by 1997, had fallen into a state of such disrepair that it regularly had to be closed during high winds. It was also sitting a full foot lower in the water than it did in the 60s, and was at risk of full collapse should a moderate to severe tremor strike the area. Myint Lwin, WSDOT’s chief bridge engineer, estimated its complete failure — even with seismic retrofitting — by 2017. So the state of Washington went ahead and built a newer, stronger, better designed bridge atop the skeleton of the old.

The new $US4.65 billion SR 502, which is due to open in the middle of next year, is significantly larger than its predecessor. The new span’s floating section measures 7,710 feet long — 130 feet longer than the previous one — and will carry six lanes of traffic as well as pedestrians and cyclists in a 14-foot wide foot path. It even incorporates a series of five viewpoints along the span for tourists to snap pictures of Seattle from the water.

The Washington DoT has more than doubled the number of concrete pontoons supporting the new bridge, relying on 77 such floaters compared to the original’s 33. Each pontoon measures 360 feet long, 75 feet wide and 28 feet tall (though only the top six feet break the water’s surface) and anchors itself to the lake bottom using no less than 58 concrete anchors tethered with 3-inch diameter steel cables. What’s really cool is that each tether terminates in a remotely-controlled hydraulic jack that can increase or decrease line tension in response to the bridge’s movements during storms. With these new weight-spreading systems in place, SR 502 is expected to weather 90 MPH gusts and a “1,000-year earthquake event.”

But, like many large public sector projects, the rebuilding of SR 502 has not been without scandal. Aberdeen-based Kiewit Construction, the company responsible for building the bridge’s pontoons, has repeatedly come under fire for shoddy workmanship. An anonymous inspector for Quality Assurance subcontractor team, O’Neill Environmental, told Komo News that workers installed the wrong size rebar in the wrong locations, if at all. “To me it’s just a disaster; it’s a disaster waiting to happen,” said the inspector, “I won’t drive across that bridge when they have it built.” Still it can’t be any worse than what Tacoma has to put up with:

[Wired - Komo News - About - Wikipedia - Washington DoT 1, 2, 3 - Images: WS DoT]

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