Award of Excellence: St. Patrick’s Bridge
“The bridge is very elegant, very light and transparent, with an efficient structure that minimized its use of materials. It is also an innovative design that allowed much of the structure to be prefabricated. Despite the setbacks of the 2013 flood, it was completed on budget and for half the cost per square metre of other signature bridges in Calgary.” – Jury
From the October-November 2015 print edition, p. 51
St. Patrick’s Bridge in Calgary spans the Bow River in the city’s East Village. It links this downtown community that is undergoing revitalization with a park on St. Patrick’s Island and popular pedestrian paths and communities along the river to the north.
The bridge is a three-span, 182-metre long network arch structure, likened to stones “skipping across the water.” Given its slender and unique architecture, the design and construction posed several challenges. The designers also had to deal with damage from the Calgary floods of June 2013.
Slender open arches lean in
The result of an international design competition, the bridge was designed to be as efficient and light as possible. The design minimizes the total length of the structure and avoids long ramps at the approaches.
The bridge has three continuous steel arches forming a network arch system, with the post-tensioned concrete deck serving as the arch tie. The network cable arrangement permits the deck and arch to work together to resist asymmetric loading, thereby creating an efficient and streamlined structure.
On the two river spans the arches rise over the deck, leaning inwards to facilitate bracing. The open cable network gives views through the structure. On the island, the arches stay below the widened deck, providing a completely open viewing platform.
The structural scheme of a network tied arch allowed the arch and deck to function together as one large truss, using much more slender members than would be possible with a traditional arch and vertical hangers. The lightness of the structure allowed for much of it to be prefabricated, increasing the quality and reducing the need for construction on site. As a result the bridge cost per square metre is nearly half that of other signature bridges in Calgary.
Severe damage from Calgary Flood
The flood of June 2013 caused severe damage to the bridge while it was still under construction. The scaffolding supporting the deck was knocked out, causing considerable deck deflections. In addition to extensive concrete cracking, the steel reinforcing bars and post-tensioning strands had yielded.
As the post-tensioning strands spanning the deck’s full length had already been tensioned and grouted, it was necessary to remove the entire concrete deck. The south arch sections also had to be removed and reset since they were not fully welded into place before the flood and their position had shifted.
At this stage all the arches had to be erected and fully welded before the deck was complete. The goal was to transfer the full load of the deck to the arches before the next potential flood. Careful control streamlined the construction.
Despite the setbacks, the new bridge was delivered on budget and opened on October 20, 2014.
No in-water foundations or piers
This slender, efficient design reduces the use of building materials and minimizes the bridge’s environmental impact.
The bridge footprint also responds to the river hydraulics. The surface of the island is below the 100-year flood level, for example, so the bridge deck could not touch down on the island. Instead an access ramp is provided. Scouring of the soils around the interior piers in the event of a flood was a concern that was addressed by supporting the foundations on deep concrete drilled piles. With no in-water foundations or piers, the bridge was designed to limit the impact on the river.
The team used cutting-edge software to undertake a series of parametric studies of the arch geometry. Numerous studies were done for the geometry and layout, including the rise-to-span ratio of each arch, arch inclination, and hangar cable layout.
Oasys GSA software was used for the structural modelling, and Geometry Gym’s Smart Structural Interpreter was used for importing the parametric geometric model with the touch of a button.
Project name: St. Patrick’s Bridge, Calgary
Award-winning firm (prime consultant, architecture, structural/field engineering, project
management): WSP/Parsons Brinckerhoff, Calgary and RFR, France (Christian Rieser, P.Eng., Jean François Blassel, Michael D. McDonagh, P.Eng., Thomas Cooper, P.Eng., Chris Love, B.Tech, Navid Sasanian, P.Eng.)
Owner: Calgary Municipal Land Corporation
Other key players: Speirs + Major (lighting), Mulvey & Banani (electrical), Janet Rosenberg & Studio (landscape), Graham Construction (general contractor), Stantec (project manager)