North Arm Fraser River Crossing
A signature span cable-stayed bridge over the North Arm of the Fraser River in Vancouver, British Columbia, Canada became a part of the Canada Line Light Rail System. The bridge was built through precast segmental box girder construction with back spans of 456 feet, and a main span of 590 feet to allow for marine traffic to pass underneath without obstruction. This project incorporated bridge design and construction firsts for North America.
The North Arm Bridge is the first extradosed bridge in North America. An extradosed bridge can have the stays tensioned up to 60% of the strand’s capacity compared to only 45% on a traditional cable stayed bridge. By using a greater percentage of the strand’s capacity and anchoring the stays to a shorter, stiffer pylon, the design is more efficient resulting in both material and labor savings. Additionally, the lower pylon in proportion to the span length allowed the designers to keep the bridge out of the glide path clearance envelope of the adjacent Vancouver International Airport.
Early in the project and prior to erection of the bridge, a comprehensive testing program was conducted to qualify the materials and to confirm the performance of the anchorage system at the higher stress levels of an extradosed bridge. This included fatigue, static strength and leak tightness tests. As an added measure for each test, an angular deviation was introduced at the end anchorages resulting in much more rigorous testing than customary.
The system also allows for strand by strand installation where each strand is stressed using an automatic system consisting of a specialized ram, pump and computerized control box. The automatic stressing equipment is lightweight, handheld, reliable and user friendly. Each successive strand in a stay cable was stressed to a specifically calculated force that is slightly less than the previous strand so that when the last strand is stressed, all strands have the same force. In total, the bridge contains 24 pairs of stay cable anchorages and over 260,000 feet of stay cable strand.
Another first for North America is the use of the Gensui Dampers. These specialized dampers are made from a highly engineered rubber designed to absorb transverse vibration in the stay cables to protect them from damage.
The advanced stressing system was a great advantage in installation and quality control documentation. This bridge is one of the greatest milestones in construction and the extradosed design has now been proven as a successful approach in North America, along with other parts of the world.