Calgary’s Peace Bridge

Iconic, world class and revolutionary — yet at the same time functional, universally accessible and well integrated with the community. That’s what the City of Calgary wanted in a new pedestrian and cyclist bridge that would improve access to the downtown core across the Bow River. The bridge site, however, did not provide the conditions normally necessary to achieve those goals.

From the outset there was a strong desire to clear span the sensitive trout habitat and deep water at this point in the river. But meeting this need was complicated because situated adjacent to the site is the city’s primary downtown heliport. One helicopter flight path crosses directly over the bridge and restricted the bridge’s height to only 6 metres. With a towering bridge expanse off the table, and riverbanks that didn’t provide sufficient height for other forms of suspension structures, it was a challenge to make the bridge a landmark.

For solutions, the city turned to famed Spanish architect Santiago Calatrava. His design was detailed by the structural engineers and designers from his Zurich office, with assistance from structural engineers at Stantec.

Calatrava’s design for Calgary’s Peace Bridge is a departure from his towering bridges in cities like Buenos Aires, Jerusalem and Seville that have contributed to his reputation as an iconic bridge designer. Instead, the team challenged the boundaries of design and structure in other ways to make one of the most recognizable structures in Alberta.

The result is a striking piece of sculpted steel. The structure has the geometric form of a helix developed over an oval cross section, 8 metres wide and stretching 126 metres across the river. It is a simple span structure with abutments that integrate into the embankments. The bridge provides a clear separation between the pedestrian and bicycle and other wheeled traffic, is barrier free, and incorporates four different types of lighting for nighttime use.

To achieve this landmark, however, several engineering challenges had to be overcome.

Engineering the complex geometry

Defining and analyzing the complex geometry of the structure required a very detailed analytical model. All the components of the steel structure are connected and form part of the structural system that resists the applied loads. Therefore all the steel components needed to be accurately included in the analytical finite element model to develop a true understanding of the performance of the structure. Frame elements were used to model the members of the helix, shell elements for the orthotropic deck, frame elements for the stiffeners below the deck, and shell elements for the end portals. The analytical model even included zero stiffness shell elements to model the potential response of the glass.

To create a more symmetrical appearance and provide sleek lines for the bridge’s canopy, the size of the top chord is minimal. Splitting critical structural component to enhance the aesthetic effect added further complications to the design. To meet the aesthetic challenge while ensuring structural integrity, the final design has multiple layers of steel plates that are connected to essentially create a compact solid steel section in each compression chord element. The result is a much sleeker design, without compromising strength.

Anchoring the forces from the helix and transferring those forces to the bearings at each abutment required substantial end portals. The top of the portal acts like a beam to transmit the thrust from the top chord into the curved elements of the portal. The top of the curved portal section could be considered as inclined struts, while the bottom curved portion could be considered as inclined tension ties that are balanced by the force from the tension chord. These inclined elements of the portal have out of plain bending that required thick steel sections with significant internal stiffeners.

Following its opening in March 2012, the Peace Bridge has quickly become a recognizable landmark in Calgary and has won several awards. It was named one of the world’s top 10 public spaces in 2012 by designboom, and it won the Canadian Institute of Steel Construction 2013 Engineering Award and 2013 Steel Edge Award.

Most importantly, however, the Peace Bridge is making getting to work and back easier and more sustainable for more than 6,000 Calgarians every day.cce

Eric Tromposch, P.Eng. is a vice president with Stantec, in the Calgary office.

Client-owner: City of Calgary

Designer, architect and engineer: Santiago Calatrava, LLC

Engineer of record, structural engineering, electrical engineering, construction management and transportation planning: Stantec Consulting (Eric Tromposch, P.Eng., Gerd Birkle, P.Eng.)

Other key players: Matrix Solutions (hydrotechnical);

Thurber Engineering (geotechnical), Acuren Group

(steel fabrication inspection)

Contractor: Graham Construction