86 precast deck panels
162 steel bearing cables / 162 post-tensioned cables
262-metre long bridge
The Terwillegar Park Footbridge in Edmonton, Alberta is the longest stressed-ribbon foot bridge in Canada. In fact, it ranks as the second-longest in the world.
As you start to understand the pre-stressing and post-tensioning of this unique concept, you will quickly see why so few bridges have been designed this way. Using steel cables to suspend the precast concrete decking, this footbridge required much more specialization than your typical girder or truss bridges.
The design complexities of a stressed-ribbon bridge, in the case of Terwillegar, stemmed largely from the environmental considerations associated with the project. To lessen the impact on the river valley trail system, a minimalistic bridge profile with only two piers in the middle of the river was imperative to preserving the natural views of the North Saskatchewan river.
The tension created by 86 deck panels and the compressive force held by 162 individual steel cables to accommodate a 262-metre long bridge, presented some serious challenges.
Stantec, the City of Edmonton’s Engineering Consultant, turned to Armtec Precast for the custom precast concrete design.
“Because we were involved in the pre-tender phase, we had influence over the precast design,” explains James Siffledeen, Business Development manager with Armtec. “We were able to identify the critical aspects for fabricating the deck panels, including the formwork challenge associated with the internal troughs, advised the City on the most efficient solution”
The trough formwork had to accommodate hundreds of individual projecting dowels, leaving a shear keyway accessible. Armtec’s internal production and formwork team carried out trials in order to create the best forming shape and stripping process to accommodate these specific needs.
“This (mock-up) verified that the specific sculptured shape of the precast panels (could) achieve the extremely tight placing and finishing tolerances of both the concrete and embedded steel and hardware,” Siffledeen continued about the success of the production process.
Each panel was cast on a sloped curve toward the middle of the bridge, and would require a minimum aging requirement of 180 days before post-tensioning.
In any case, Armtec Precast produces all concrete in a controlled setting, and in accordance with the Alberta Transportation specifications for durability. However, particular attention was paid to the curing the Terwillegar deck panels. For a bridge of this length, it will be absolutely essential to avoid future shrinkage and separation – something inevitable with the post-tensioning and the fluctuating Alberta climate.
Armtec worked closely with Graham Infrastructure, the General Contractor, on a plan to cast and store the decking off-site until they reached the specification. While this process may seem like it would have an adverse effect on the schedule and budget, the rest of the project components continued unaffected, and Armtec was actually able to gain time for the customer during the installation phase with flawless planning and on-time deliveries.
Once the precast deck panels were in place, and secured to the bearing cables by stainless steel pins, concrete was poured to join the elements. As the final step, the entire precast concrete deck was post-tensioned to become a monolithic structure.
The bridge, now open to the public since the fall of 2016, has been acclaimed by the ACI and the Alberta Construction magazine for its exemplary precast concrete design flexibility. It stands true to its natural surroundings, and is admired by Albertans on the daily.
“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.” Allan Bartman, General Supervisor, City of Edmonton. “Armtec did a superb job on the precast deck panels.” Manoj Medhekar Engineer of Record
*Source: National Precast Concrete Association / Precast Magazines / Precast Solutions Magazine / 2017 – Winter – “Progressive Engineering Afoot on Canadian Pedestrian Bridge.”