Transportation Award of Excellence
Nancy Pauw Bridge
T
he aquamarine, glacier-fed Bow River traces its way through Canada’s Rocky Mountains and the town of Banff, Alta., in one of the world’s first national parks, which is visited by more than four million people annually. Today, the new Nancy Pauw Bridge spans the river right next to the town, fulfilling a 109-year dream. Responding to demand for natural materials, StructureCraft designed and built an unusually slender 80-m clear span out of sustainable timber for this footbridge. The shallow high-thrust arch was designed to integrate well with its natural setting, without detracting from the scenery, and is expected to carry up to 10,000 pedestrians daily. More than a century in the planning Banff has long promoted walking and cycling. Across the river from the town centre are hot springs, natural attractions, the Banff Springs Hotel and a golf course. As early as 1914, town planners had it in mind to locate a footbridge in Central Park, immediately adjacent to the town centre, to enable better connections. While this wish did not come true for more than 100 years, it was not for lack of trying. In 2007, for example, an attempt was made to create a crossing, but the technical challenges were 24
CANADIAN CONSULTING ENGINEER
deemed too difficult and the project was abandoned. A new opportunity recently arose, however, from a private donation offered by the Wim and Nancy Pauw Foundation. The new bridge is thus named after the late Nancy Pauw, a long-time Banff resident and hiking/cycling enthusiast. An unprecedented design Numerous constraints demanded a very low-profile design. StructureCraft pondered the challenge for some time before designing the slender, shallow arch. Such a shallow arch had never been attempted before. With a 5% maximum slope at the abutments and the required clearances, it could only have a rise-to-span ratio of about 1:20. Challenges included non-linear behaviour, potential for snap-through buckling, large abutment thrusts, susceptibility to unsymmetrical loading and difficulty with vibration characteristics. The solution was unprecedented, particularly in timber, in its high reliance on the stiffness of supports at each end, both laterally and rotationally—laterally because a shallow arch creates enormous thrusts. A small lateral deformation at the support would result in large deformations and even the collapse of the span; and rotationally because the extremely slender timber arch is very sensitive to snap-through buckling and needed stiffening at the ends.
“It’s gorgeous, with art, technique, skill and design all coming together.” – Jury
The dense sedimentary soils would need to be able to receive the forces with very small deformation. Large-diameter piles and thick pilecaps were key components of StructureCraft’s solution. Extensive non-linear soil analysis was conducted to ensure the structure would be achievable. Esthetics were also top-of-mind. The tapered arch design, with its slender glulam beams and weathering steel ‘haunches’ that stiffen the arch at the supports (much like the stone arches of the past), addresses the technical challenges and creates natural beauty. Slender footbridges are very susceptible to excessive vibrations. To address both walking and jogging, StructureCraft designed a special tuned mass damper. This involved a simple carriage with weathering steel plates suspended by splayed cables from the bridge. It was finetuned on site and accelerometers confirmed performance, both before and after installation of the dampers. Bridge decking consists of spaced Douglas Fir timbers. They were prestressed into 1-m wide removable panels using galvanized rods and rubber spacers. September/October 2023
PHOTO BY PAU L Z I Z K A, COU RT E SY ST RUC T U R EC R A F T.
StructureCraft
