Norway building first bridge of new type

Photo: Arild Solberg /  Norwegian Public Roads Administration (NPRA) Bridge being built across Gudbrandsdalslågen river at Harpe.

Photo: Arild Solberg / Norwegian Public Roads Administration (NPRA)
Bridge being built across Gudbrandsdalslågen river at Harpe.

M. Michael Brady
Asker, Norway

In Norway, the E6 highway of the International E-road network is the country’s longest, stretching from Svinnesund at the Swedish border in the south to Kirkenes in the far north, a distance of 1,634 miles, comparable with the 1,700-mile length of the Alaska Highway in North America. It now is being upgraded in stages, and along it eight ongoing road projects are scheduled for completion in 2017. The E6 road projects include many new bridges, of which one at Harpe in Gudbrandsdalen is of a type of bridge not built before in Scandinavia.

Its structural classification is extradosed, a loanword in English as well as Norwegian from the conjugation of the French verb extradosser, which in free translation means “to make the exterior curve of an arch.”

Extradosed is the ultimately sensible technical moniker, as it connotes a structure that employs the principle of the classic arch. An arch bridge functions by transferring its weight and the loads it carries to a combination of vertical downward force on and horizontal thrust into abutments on either side. An extradosed bridge functions by transferring its weight and the loads it carries to a combination of vertical downward force on its towers and tension in the cables strung from them.

In short, an extradosed bridge is an arch bridge without the arches, a design distinguished by its combination of the oldest and newest of bridge technologies. The earliest arch bridges were built in Antiquity by the Greeks, among them the Arkadiko Bridge, probably built in the 12th century BC and still in use, located close to the modern road from Tiryns to Epidauros. The extradosed technology is the newest in bridge building. The first extradosed bridge was the Autoroute 87 road bridge opened in 1987 at Lhez in the Hautes-Pyrenées Department of France.

Today there are an estimated 70 extradosed bridges of various sizes worldwide. Structurae, the international database for civil and structural engineering public works, lists only 35 extradosed bridges, of which three are in North America. The Pearl Harbor Memorial Bridge carries Interstate 95 over the Quinnipac River at New Haven, Connecticut. In Vancouver, B.C., the Golden Ears road bridge and the North Arm bicycle, pedestrian, and light rail bridge span the Frazer River.

An extradosed bridge is more expensive to build than an equivalent girder or suspension bridge. But in many environments it’s esthetically more pleasing because it doesn’t have tall towers, the major reason for it being chosen for the Harpe bridge.

When Statens Vegvesen (“Norwegian Public Roads Administration”) started designing the Harpe bridge, its engineers found that there were no Norwegian or international sets of design rules for extradosed bridges. Being sensible and not wishing to reinvent the wheel, they went to France to find the design rules used in building the first extradosed bridge on Autoroute 87 there. So in design as well as in name, there’s a French connection in the building of the Harpe bridge, scheduled for completion in the spring of 2016.

Further reading: “Stor interesse for uvanlig bru på E6 i Gudbrandsdalen” (Great interest in unusual bridge on E6 in Gudbrandsdal valley), Statens vegvesen (Norwegian Public Roads Administration) press release November 19, 2015, free PDF downloadable at: www.vegvesen.no/Europaveg/e6biriotta/Nyhetsarkiv/stor-interesse-for-uvanlig-bru-p%C3%A5-e6-i-gudbrandsdalen (in Norwegian only).

This article originally appeared in the Jan. 22, 2016, issue of the Norwegian American Weekly. To subscribe, visit SUBSCRIBE or call us at (206) 784-4617.

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