Project-in-Focus: Designing Suspension Bridges on the Sea to Sky Trail
March 13, 2018 |
Have you ever hiked or biked near Whistler, B.C.? If so, you have likely been on the Sea to Sky Trail, a 180 km multi-use trail open year-round for all types of activities, including running, hiking, biking, and even snowshoeing. The trail, part of the Trans-Canada Trail, starts at sea-level in Squamish, goes through Whistler and ends at the community of D’Arcy. In 2015, the Resort Municipality of Whistler (RMOW) retained Klohn Crippen Berger (KCB) to design and manage the construction of two new pedestrian suspension bridges on the trail, over the Cheakamus River near Whistler. One of the RMOW’s goals is to develop a world-class trail network to enhance the experience of trail users. Bridge aesthetics was therefore an important design factor.
Sea to Sky and Train Wreck Bridges
The first new bridge, the Sea to Sky Bridge, replaces a deteriorated forestry bridge that was closed to the public due to safety concerns. KCB designed a 55 m long suspension bridge for the new crossing. The main bridge cables were anchored into bedrock using drilled-in pairs of rock anchors for each cable. The bridge deck was built with rough cedar planks supported by steel crossbeams and cable hangers. Additional cables were strung along the bridge to act as a railing system. Different height towers inclined backwards from the river were designed to fit the constrained bridge site, creating an interesting and appealing structure.
A second suspension bridge, the Train Wreck Bridge, was built 2 km upstream of the Sea to Sky Bridge. Our team designed a 36 m long catenary cable suspension bridge with four main catenary cables. The two top cables act as handrails and the two bottom cables support the timber decking. Rough cedar was selected for the decking invoking the impression of railway ties. Great care was taken in every design detail to ensure a clean and attractive design.
The Train Wreck Bridge allows safe access to one of Whistler’s popular attractions, a historic train derailment site featuring graffiti-covered train cars. For years, the only access to the site was by a dangerous route along CN Rail tracks. The new bridge allows easy access to the site from the Sea to Sky Trail and other popular hiking and biking routes in the area.
Design Challenges and Solutions
The Sea to Sky Bridge site presented the following design challenges:
- A forestry road on the south side of the river reduced the space available to build the bridge abutment.
- The bedrock for anchoring the bridge cables was irregular and unreliable on both sides.
- The river was relatively wide at the selected bridge location.
Consequently, both towers were inclined backwards to allow for a steeper backstay angle for the main cables, allowing them to be anchored closer to the ends of the bridge. This created room for the forestry road to remain in service and ensured that the cables at the north side could be anchored into bedrock.
Galvanized steel was used for the bridge components to ensure a long design life and minimize maintenance requirements. Locally-sourced cedar used for the decking contributed to the rustic appeal of the bridge.
The Train Wreck Bridge site presented the following design challenges:
- The site was accessible only by foot, so materials and equipment for the bridge required helicopter fly-in.
- The bridge components needed to be within the weight limit of available helicopters.
- The new bridge site was over a deep canyon, making back-and-forth crossing of the river impractical before the bridge was installed.
KCB designed the bridge abutments using relatively light steel components to facilitate assembly without heavy equipment. The rock anchors used to secure the abutments and anchor the main bridge cables were sized so that they could be installed using a mini air-track drill and air compressor small enough to be helicoptered to site.
The final construction cost of the Train Wreck Bridge was well within the RMOW’s budget. The bridge’s design is a remarkable combination of engineering efficiency and aesthetic design, and shows that with a little skill and imagination, bridge designs can be both economical and beautiful.
Environmental, Economic and Social Benefits
Sustainability was considered in all aspects of the project. The final bridge designs have a very small footprint, minimizing the amount of tree clearing required at the bridge sites and adjacent paths. Significant effort was made to eliminate the need for concrete poured near the environmentally sensitive Cheakamus River, by using rock anchors and built-up steel abutments instead. Locally-sourced natural material, such as cedar decking was used whenever possible, and durable components such as galvanized steel was used to reduce future replacement requirements. This allowed the designs to blend seamlessly into the surroundings. In addition, the playful inclusion of train wheels and track elements for the Train Wreck Bridge creates a standout design, which is as much of an attraction as the colourful train wreck cars that it leads to.
The bridges, which opened to the public in 2016, have become favourite destinations for many, including school children on field trips, photographers, tourists, and locals alike. They provide long-term safe and convenient access to some of Whistler’s most important outdoor recreation assets. They also contribute to the development of tourism and the local economy by linking key trail segments and providing access to nature for all trail users to enjoy. Our team is proud to have participated in this rewarding project that will directly benefit British Columbians for decades to come.For further information about this project, please contact us at firstname.lastname@example.org or 604.669.3800.