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.
Norway, which already boasts two of the world's longest floating bridges, is the one country most actively studying new pontoon bridge projects. Its E39 coastal highway, a long-range megaproject, will incorporate eight major water crossings. One of those crossings, Bjørnafjorden, is likely to be a pontoon bridge.
"We have recommended a floating bridge, but we have not decided whether it will be an end-anchored floating bridge (like Bergsøysund and Nordhordland) or if it will be a side-anchored floating bridge with moorings to the seabed [in the fashion of the floating bridges around Seattle]," says Eidem Mathias Egeland, project manager, fjord crossing project, Norwegian Public Roads Administration. "The big challenge for a side-anchored floating bridge here, compared to the ones around Seattle, is the depth. The deepest mooring on the Hood Canal Bridge is 120 meters below sea level, while the depth of the Bjørnafjord is over 600 m at its deepest."
NPRA has commissioned studies of both options. "Multiconsult, together with Johs Holt, NGI, Entail, Rambøll and Aker Solutions, conducted the study for the side-anchored floating bridge," says Egeland, "while Norconsult, Dr.techn.Olav Olsen, NGI, Aker Solutions and Aas-Jakobsen conducted the study for the end-anchored floating bridge. The next step will be further study of the two floating bridge concepts so that we can make a final decision between the two."
The four longest pontoon bridges in the U.S. are all located in the Seattle area. The oldest one, the Lacey Murrow Memorial Bridge, was based on a concept by visionary engineer Homer Hadley, who had developed his idea after building concrete barges during World War I. He first proposed his idea at a meeting of the American Society of Civil Engineers in 1921, but was rejected. Hadley went on to work for the Portland Cement Association. Eventually, he convinced the Washington State Dept. of Highways to try his idea. Completed in 1940, it was the first floating bridge to incorporate concrete pontoons.
The topographical features of two water bodies in the Seattle region favor floating bridges. "The depth of the water and length of the open water spans are WSDOT's main reason for building floating bridges," explains Steve Peer, media and construction communications manager, Washington State Dept. of Transportation. "Lake Washington is a deep lake with 200 ft of water on top of 150-200 ft of soft diatomaceous soil. Hood Canal has water depths in some places more than 300 ft. To cross Lake Washington and Hood Canal, bridges also need to span more than a mile. Support piers for a traditional bridge would need to be taller than a 40-story building and a suspension bridge like the Golden Gate bridge would need to be massive in scale to lift [suspend] the bridge out of the water. The bottom line: A floating bridge is the most practical bridge structurally and financially for our region's unique conditions."
The region's newest floating bridge, the Evergreen Point Bridge, is the world's longest floating bridge and opened in 2016. It replaced an earlier floating bridge and provided more traffic capacity. It was designed with the ability to add light rail to the bridge in the future. To overcome weather and maintenance issues, the design team raised the bridge deck 20 ft off the pontoons, thereby allowing complete access to all bridge systems from below. "What made the design a challenge is that a floating bridge tends to flex and twist due to live and environmental loads and thus the superstructure needed to have sufficient articulation so that it could allow for these anticipated movements," says Michael Abrahams, technical director, structures, WSP USA.
WSDOT expects to claim another milestone when it adds light rail service to the existing Homer Hadley Bridge on Lake Washington. The challenge for Sound Transit was how to install rails on a bridge that uses accordion joints to transition from a fixed structure to a floating structure that, due to water levels, rises and falls as much as 2 ft. Beginning with a consulting team led by what is now WSP, Andy Foan, director of the U.K.-based Andy Foan Ltd., devised the "curved element supported rail," a winged, curved platform at the two joints where the bridge transitions between fixed and floating.
"The WSP design team developed, analyzed and prototype-tested [at full scale] the curved-element design that met Sound Transit's light rail service performance objectives," says Abrahams. "It can be thought of as a continuously variable radius track panel that could be used in other applications, such as floating dockyards or track in glacial or seismic areas." Construction is expected to commence this summer and be completed by 2020.
Abrahams also helped design the only other sizable floating bridge in the U.S. outside Washington state, the Ford Island Bridge (aka the Admiral Clarey Bridge) in Hawaii. The Navy "required a bridge across Pearl Harbor to provide a low-level crossing that would minimize the visual impact on the nearby U.S.S. Arizona war memorial as well as provide a 650-ft navigation channel that could be traversed by an aircraft carrier," explains Abrahams. The 4,672-ft long bridge features a 930-ft-long pontoon section; it was completed in 1998.
Floating bridges are being looked at in several other countries that already have such structures. Officials from Nagasaki, Japan, are considering building a floating bridge on Omura Bay to offer additional access to an airport. And a second floating bridge across Lake Okanagan in Canada is a possible option.
One distinctive aspect of pontoon bridges is their maintenance needs. "They require more maintenance due to the floating nature of the bridge," says Nicolas Ryan, media relations adviser, SNC-Lavalin. "The joints will experience a larger degree of movements since the bridge is floating. The movements will cause more wear and tear of the joints during normal operation."
While permanent floating bridges have an established reputation, there are few of them, whereas temporary floating bridges are much more common, both for military maneuvers and civil emergencies. They were widely used by both Allied and Axis forces in World War II, most notably by American units to cross the Rhine River after retreating German forces had destroyed existing bridges.