Permanent floating bridges are essentially boutique structures that only make sense for certain rare kinds of sites: unusually deep bodies of water and bodies of water with very soft bottoms, where piers are impractical.

1. Lacey V. Murrow Memorial Bridge, Seattle

The Lacey V. Murrow Memorial Bridge is a floating bridge in the Seattle metropolitan area of the U.S. state of Washington. It is one of the Interstate 90 floating bridges that carries the eastbound lanes of Interstate 90 across Lake Washington from Seattle to Mercer Island. The world’s first floating bridge to be built using concrete pontoons, it was designed by engineer Homer Hadley, and constructed by the Puget Sound Bridge and Dredging Co., at a price tag of $9 million. While undergoing reconstruction in 1990, an 850-m-long section of the bridge sank when a storm filled one pontoon. The rebuilt bridge reopened in 1993, at a cost of $93 million.

2. Hood Canal Bridge, U.S.A

The Hood Canal Bridge is a floating bridge in the northwest United States, located in western Washington. It carries State Route 104 across Hood Canal of Puget Sound and connects the Olympic and Kitsap Peninsulas. The Hood Canal (actually a fjord, not a canal) of Puget Sound is one such place. More than 7,000 feet wide near its mouth and 55 miles long, Hood Canal separates the Olympic and Kitsap peninsulas. People who wanted to travel from one side to the other either had to take ferries across the choppy stretch or drive around. The people of Hood Canal had their design, but there were still many questions about how to actually build it. Not only did the bridge have to be both flexible and strong (combined wind and water currents can move the bridge roughly 16.5 feet in either direction), but it was unclear how the salt water would effect the buoyancy of the pontoons. Salt water tends to make things float higher, which in this case could make the bridge more unstable and subject to high winds, currents and waves. Many people thought the bridge wouldn’t be possible.

3. Homer M. Hadley Memorial Bridge, U.S.A

The Third Lake Washington Bridge, officially the Homer M. Hadley Memorial Bridge, is a floating bridge in the Seattle metropolitan area of the U.S. state of Washington. It is one of the Interstate 90 floating bridges, carrying the westbound lanes of Interstate 90 across Lake Washington between Mercer Island and Seattle. The floating bridge is the fifth-longest of its kind in the world, at 5,811 feet. When the bridge was built, parallel to the Lacey V. Murrow Memorial Bridge, two reversible high-occupancy vehicle (HOV) lanes were set up to accommodate the traffic flow between Seattle and the suburban Eastside (westbound in the morning, eastbound in the evenings). Prior to 2017, it also carried two reversible lanes, configured to normally carry westbound traffic on weekday mornings and eastbound traffic at other times. Use of the reversible express lanes was restricted to HOV traffic, except for vehicles traveling to and from Mercer Island. With a total of five traffic lanes and three full-sized shoulders, the Third Lake Washington Bridge was the widest floating bridge in the world, until the completion of the new Evergreen Point Floating Bridge in 2016.

4. Nordhordland Bridge, Norway

The Nordhordland Bridge is a combined cable-stayed and pontoon bridge which crosses Salhusfjorden between Klauvaneset and the island of Flatøy in Vestland county, Norway. It is 1,614 meters long, of which the pontoon section is 1,246 meters long. The floating section is a steel box girder bridge with ten pontoons, which because of the fjord’s depth are not laterally anchored. The roadway sits on an orthotropic deck. The pontoons and the cable-stayed bridge are built in concrete, with the main span being supported with 48 cables. The fjord end of the main span is supported by a 30-metre (98 ft) deep foundation, where the two bridges meet. From there and for 414 metres (1,358 ft), the roadwall has a 5.7 per cent gradient on a viaduct anchored to the pontoon bridge.

5. William R. Bennett Bridge, British Columbia, Canada

The Okanagan Lake Bridge was a three-lane, 650-metre long floating bridge in British Columbia, Canada. It crossed Okanagan Lake, connecting the Westside area to Kelowna on the lake’s eastern side. Taller boats such as sailboats were able to pass under the lift span which was located at the east end of the bridge. Extending 1,060 metres long in total, the bridge includes a 690-metre string of floating pontoons supporting an elevated deck. There are two 54-m transition spans, and a fixed west approach structure. The latter is 277- metre long and includes an elevated span to allow for an 18-m high by 44-m wide marine passage. Floating bridges can pitch, roll, yaw, move up and down, and respond dynamically to waves and wind. The Canadian Highway Bridge Design Code (CHBDC) is written for fixed bridges, therefore other codes, particularly the Norwegian Code, were used to help establish the design criteria for vessel collision, pontoon stability, allowable movements and so on. World specialists in floating bridge design, Aas-Jakobsen and Johs Holt of Denmark, were subconsultants.

6. Yumemeai Bridge, Osaka, Japan

The Yumemai Bridge is a floating moveable bridge in Konohana District, Osaka, Japan. It spans the North Waterway connecting the man-made islands Yumeshima and Maishima of Osaka Port. It spans the North Waterway connecting the man-made islands Yumeshima and Maishima of Osaka Port. It comprises a floating bridge over the waterway, transitional girder bridges on both ends of the floating bridge, and approach bridges on Yumeshima and Maishima. The bridge is supported on two large steel pontoons. Ordinarily, the bridge allows a navigation passage width of 135m. In the event that the main waterway is out of service, the bridge is swung by tugboats to widen the passage width to 200m or more, enabling the passage of larger vessels.

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