The North Arm Bridge over the Frasier River in Vancouver, BC is a signature span cable stayed bridge built using precast segmental box girder construction. It has back spans of 456 feet and a main span of 590 feet - high enough to allow for marine traffic to pass below without obstruction. It is also 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 and results in both material and labor savings. For this project, the lower pylon in proportion to the span length allowed the designers to keep the bridge out of the glidepath 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 testing included fatigue, static strength and leak tightness tests. As an added measure, for each test an angular deviation was introduced at the end anchorages - making the test much more rigorous than usual.
The system chosen also allows for strand by strand installation. Each strand is stressed using an automatic stressing system (AMS) consisting of a specialized ram, pump, and computerized control box. Each successive strand in a stay cable is 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, 24 pairs of stay cable systems were installed and stressed – accounting for 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 and protect them from damage.
\The extradosed design has now been proven as a successful approach in North America as in other parts of the world. Additionally, the AMS system provided a great advantage in installation and quality control documentation.