By Kirk Stelsel
Terwillegar Park in Edmonton, Alberta, is a haven for the city’s outdoor enthusiasts and those who just want to spend a little time in nature. The park boasts a large off-leash area for dog owners; plenty of space and trails for cycling, running or cross-country skiing; and a launch point for the North Saskatchewan River.
One thing lacking, however, was a link between the park and the land on the north side of the river. To remedy that, the city built the Terwillegar Park Footbridge.
To maintain the pristine natural surroundings, the city of Edmonton selected a unique and rarely used bridge design. The stressed-ribbon bridge design uses steel cables suspended across the river between abutments to hold precast concrete panels that form the decking. The result is a form factor that resembles a rope bridge with natural slack on either side of two piers in the middle of the river.
“The concept design showed the least impact to the river valley and least imposing visual impact using an elegant and exciting shape with the efficiency and durability of precast/prestressed concrete,” said Allan Bartman, general supervisor for the city of Edmonton. “In addition to having the lowest cost and shortest schedule, the stressed-ribbon design, in public consultation meetings, was found to gain the highest approval and generated the most excitement due to its pleasing form.”
No easy feat
The city engaged Armtec, a precast manufacturer headquartered in Concord, Ontario, with a location in Edmonton, during the preliminary and design stages of the project. The result is a bridge comprised of 86 precast concrete deck panels. Each panel is cast with a trough where the panel sits on top of the 162 individual steel cables, which was no easy feat.
“The internal forming of the deck panel troughs presented some challenges,” said James Siffledeen, business development manager with Armtec. “The trough formwork had to accommodate hundreds of individual projecting dowels as well as create a shear keyway while still being removable in daily casting without binding or damaging the trough region. Armtec’s Production and Formwork Team carried out trials in order to create the ideal forming shape and stripping process.”
Each panel was cast to slope toward the middle and needed to meet extremely small tolerances. A second consideration was a required minimum “aging” time of 180 days before post-tensioning the entire bridge structure. Armtec worked with the general contractor to develop a casting and off-site storage plan to achieve these requirements. The precaster manufactured the panels with high performance concrete and moist cured them in accordance with Alberta Transportation specifications to ensure a long life cycle.
Once the precast deck panels were in place, suspended over the river by the cables and stainless steel pins, the contractor joined all three elements through closure pours. Then, the entire deck was post-tensioned. The resulting deck is only 18.3 inches thick, which allows for a mostly unobstructed view of the surrounding natural beauty. Thanks to on-time delivery of the precast panels and proper planning, construction of the bridge remained on time and on budget.
A rare sight
Now completed, the Terwillegar Park Footbridge is 860 feet long. It is only the second stressed-ribbon bridge to be completed in Canada and ranks as the second-longest in the world, standing only behind a bridge traversing Lake Hodges in San Diego County, Calif. The rarity of these bridges results from a number of factors.
“Stressed ribbons are a relatively new prestressed and post-tensioned concrete concept that is not well known,” Bartman said. “It is also challenging to design and requires more specialized analysis and design capabilities than typical girder or truss bridges.”
Bartman and his colleagues were happy with the use of precast and the unique design. They found the option to be the clear choice and one that received a very positive response in the public consultation meetings. Siffledeen was also pleased with the production process.
“It was smooth due to the front-end work and mockups carried out by Armtec’s team,” he said. “This verified that the formwork and casting process provided the specific sculptured shape of the precast panels and achieved the extremely tight placing and finishing tolerances of both the concrete and embedded steel and hardware.”
The bridge now stands as one more example of how precast concrete can enhance the world’s infrastructure while minimizing the effects on natural surroundings.
Kirk Stelsel is NPCA’s director of communication and marketing.