Article

B.C. to debut "extradosed" bridges

0 852 Infrastructure

by By Jean Sorensen

The new Canada Line bridge spanning the Fraser River’s northern arm from Vancouver into Richmond will lead North America in debuting a new bridging design known as ‘extradosed’ structures. “Extradosed is a new bridge form that is really a hybrid between a conventional cable-bridge and post-tension box girder bridge,” says project engineer Andrew Griezic for bridge engineering firm Buckland & Taylor Ltd. in North Vancouver, B.C.
B.C. to debut "extradosed" bridges

Canada Line Rapid Transit

Correspondent

The new Canada Line bridge spanning the Fraser River’s northern arm from Vancouver into Richmond will lead North America in debuting a new bridging design known as ‘extradosed’ structures. “Extradosed is a new bridge form that is really a hybrid between a conventional cable-bridge and post-tension box girder bridge,” says project engineer Andrew Griezic for bridge engineering firm Buckland & Taylor Ltd. in North Vancouver, B.C.

Both the Golden Ears and the North Fraser bridges, designed by Buckland & Taylor, are considered extradosed, but the North Fraser is closer in description to this specific building form than the Golden Ears structure. The Golden Ears bridge is scheduled to complete in 2009, while the Canada Line bridge is scheduled to finish in 2008, making it the first in Canada. Only one bridge in the U.S. uses the term extradosed and that is the Pearl Harbor Memorial Bridge in Connecticut. It has currently finished the design phase and started construction. However, it is part of a larger transportation development plan and is not slated to be complete until 2014.

Both the Pearl Harbor and Golden Ears bridges will handle vehicular traffic while the North Fraser bridge will handle only rapid transit cars from the Canada Line moving from downtown Vancouver to Richmond and the Vancouver International Airport. “This is the first transit bridge of this kind in North America,” says Griezic.

Extradosed bridges have been used in Europe since 1981 and in Asia in recent years for medium spans. They have gained in popularity in areas where a reduced footprint is required for their construction, although they are considered more expensive to construct. The decision to opt for the extradosed method of construction was prompted by the new bridge’s location, which is near a flight path of incoming aircraft to Vancouver International Airport.

An extradosed bridge uses shorter towers to support, via cable, the bridge’s deck segments. The effect of these cables, placed at lower angles, is two-fold. Not only do they support the bridge’s segments, with the cables transferring the weight to the towers and into the footings but the lower angle of the cables from the shorter tower provides a pre-stressing cable for the bridge deck. In the case of the North Arm bridge, the towers are only 21.4 metres above the level of the deck.

The North Arm bridge construction consists of extradosed, pre-cast concrete segmental box girders with a continuous deck superstructure of 562m. The 180m main span provides comfortable clearance to the 150m wide main navigation channel. The 139m side span easily clears the 57m wide, northern navigation channel

Currently piles are being driven to support the footing that will need to be constructed for the bridge construction to begin. Components of the bridge are being pre-cast of concrete with sections measuring 3.6 metres and weighing 70-75 tons at a facility near the foot of Fraser Street.

Once ready for assembly, these pieces will be barged to the site and lifted into place by crane as the bridge traverses the North Fraser. The process used will be a “balanced cantilever construction” method whereby the bridge’s superstructure is added one link at a time (like arms growing from a body) and moving from north to south side to balance the loading on the pier.

Since the work will be done from barges on the river, it will be impressive to watch.

Currently, work is slated to begin from the Richmond side however part of the bridge will cross over land on Mitchell Island, an industrial area, and “we have created an artificial island extension” by adding five metres to the island’s base, says Griezic. The bridge begins its southerly traverse over the Fraser River at the foot of Cambie Street in Vancouver.

Rodger Welch, who is project engineer for the Golden Ears bridge, says this Fraser Valley bridge design – although hybrid – “is really closer to the Alex Fraser bridge than the RAV line bridge” as it aligns itself more with the cable-supported sector of bridges.

However, because it does carry elements that conform to the engineering definition of extradose, it falls into the category. As such, it will be the first completed traffic bridge that is an extradosed bridge in North America. The Golden Ears bridge will cross over from the Maple Ridge area to Surrey, replacing a car ferry service now used on the river.

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