North Arm Bridge
October 1, 2008
By Canadian Consulting Engineer
The North Arm Bridge carries Vancouver's new Canada Line rapid transit system over the Fraser River to Richmond.
The North Arm Bridge carries Vancouver’s new Canada Line rapid transit system over the Fraser River to Richmond.
The greatest innovation of the project was the first use of the extradosed bridge form in North America. It looks like a cable-stayed bridge with short towers, but its structural behaviour is that of a prestressed concrete bridge with the prestressing tendons coming out of the top of the concrete box. Thus, the pylon tops are lower and the fatigue life of the tendons is extended.
The bridge form elegantly satisfies a number of conflicting requirements. It had to be high enough to clear two shipping channels, yet below the flight path approach to Vancouver Airport. The rail profile had to be low enough so that stations at each end of the bridge didn’t have to be raised. Shipping had to be maintained in the channels during construction. The bridge must be safe in earthquakes. A sidewalk and dedicated cycle path were incorporated. Environmental effects had to be mitigated. Above all, the bridge had to be economical and capable of fast construction.
The design/build contractor for the Canada Line, including the bridge, is RSL Joint Venture (Rizzani de Eccher/SNC-Lavalin). As their consultant, Buckland and Taylor designed the bridge and provided construction engineering services.
Extradosed bridge design
The extradosed form was modified to suit the local conditions. The main piers are “split” to allow balanced cantilever construction free of the river, while maintaining longitudinal flexibility to reduce the seismic effects. Unlike other extradosed bridges, the structure has a constant depth at the towers to maximize the shipping channel clearance. The pylons incorporate steel tension plates between concrete columns; the plates transfer tendon tensions between spans, but ensure that all the vertical load is taken by the concrete columns.
By placing one tower at the tip of Mitchell Island, only one pier is in the river, which minimizes the need for protection against ship impacts and has environmental benefits. By careful siting, the extradosed part of the bridge is straight, but once clear of the channels the bridge varies in depth and twists to suit the curving alignment. The flanking span geometry was relatively straightforward for the precasting and erection operations, but complex for design and erection engineering. The main span is 180 metres, and the maximum elevation is 25 metres.
Remembering the disaster of the Millennium Bridge in London, which closed two days after it was opened because of excessive vibrations, an analysis was done of the train/structure dynamic interaction to ensure the comfort of the train travellers as well as the pedestrians and cyclists. A model of the bridge, which has a sidewalk/bikeway on one side only, was tested in a wind tunnel to ensure it would be aerodynamically stable.
Soil densification is common in this area, but advanced geotechnical analysis showed that, at this site, densification could be avoided.
Great care was also taken with the aesthetics of the bridge, by keeping simple lines and parallel “cables.” The view from the bridge will be of interest to travelers, especially compared to a tunnel alternative!
The bridge was designed and completed in December 2007 ahead of the already accelerated schedule, and under the low bid budget. Trains are scheduled to start running in 2009.
The economy and fast schedule both resulted from maximizing the use of the technology applied for other parts of the elevated guideway, and from close co-operation with the contractor. The concrete box segments, for example, were designed to be cast in the same forms, and kept light for ease of handling.
Name of project: North Arm Bridge, Vancouver
Award-winning firm (bridge designer): Buckland & Taylor, North Vancouver (Andrew Griezic, P. Eng., Chris Scollard, P. Eng., Don Bergman, P. Eng., Hisham Ibrahim, P. Eng., Nik Cuperlovic, P. Eng., Steve Zhu, P. Eng., Tjen Tjhin, P. Eng., Dusan Radojevic, P. Eng., Mark Steunenberg, P. Eng., Dan Yang, P. Eng.
Owner: Canada Line Rapid Transit
Client (design-build contractor): RSL Joint Venture (SNC-Lavalin/ Rizzani de Eccher)
Other key players: RWDI (wind tunnel testing), Trow Associates (geotechnical), Northwest Hydraulic (hydraulics), PBA (electrical), ECL Envirowest (habitat), Triton Environmental (environmental), Levelton (materials)