The Nancy Pauw Bridge, Banff
A Second Footbridge Over the Bow River
The Nancy Pauw Bridge is set in the heart of Banff, spanning the Bow River to connect Central Park to the Banff Recreation Grounds. The bridge creates an iconic connection between these two important elements of social infrastructure in the Town, in addition to being a destination in its own right – giving Banff residents and visitors alike a new way to experience the beauty of the Bow River and Rocky Mountain vistas.
Like its sister bridge downstream, the Nancy Pauw Bridge is an 80m clear span over the Bow River. The structure is an extremely shallow, pure arch, created with stepped Glulam girders and weathering steel haunches. The bridge was prefabricated and assembled into two sections, placed simultaneously.
On September 6, 2022, a 108-year-old vision was realized for the Town of Banff. Hundreds attended the grand opening of Banff’s new Nancy Pauw timber footbridge, crossing the Bow at Central Park. Gerald Epp of StructureCraft was invited to share with the attendees his joy in helping fulfill the vision, with the design and erection of this slender long span timber structure.
Challenges, Solutions & Analysis
The bridge needed to be a clear span to minimize impact on the river. It also needed to be low profile for user accessibility. The desire was for a bridge which was graceful, unobtrusive, and natural, fitting in with the beautiful surroundings and allowing unimpeded views while crossing. Could this be done elegantly with any material, let alone timber?
The only solution appeared to be a shallow arch. It was desired to create the natural form of a tapered arch. The first challenge needing investigation was soil conditions. The abutments StructureCraft created consist of pile caps and large diameter piles, socketed into the stiff soil. Tapered weathering steel “haunches” were anchored to the abutments both to add stiffness and to protect the timber from the river. Diagonal steel bracing links the two Glulam pairs, creating the diaphragm to resist lateral movements.
The well-thought through guardrail and decking system of the earlier downstream bridge had performed very well, so it became obvious to do the same again.
The most difficult aspect of slender bridge design is vibration performance. We found with the shallow arch design that it was difficult to predict the natural frequencies, and they were close together, even compounding each other. A central tuned mass damper was used, like the previous bridge, consisting of a simple mass of steel plates on a carriage suspended from cables stretched to four points on the girders. A unique feature of this design is that we were able to tune it to both walking (1.9 Hz 1st vertical) and jogging (2.4 Hz 1st torsional) frequencies. In the first case the mass moves vertically, and in the other it moves laterally, efficiently suppressing the large accelerations experienced initially in both modes. But it remains a somewhat “lively” bridge.
Fabrication and Installation
As with all longer span bridges, design must respect erection and fabrication considerations, and the site. Environmental impact assessments and approvals at numerous levels needed to be procured. All of these were managed under the design-build contract, and the client was very cooperative in assisting to ensure the critical timelines were met.
Piling was conducted in December, at low water but prior to deep freeze. Abutments were formed and poured in April, before water levels started to rise.
For spanning the river, the erection scheme chosen involved installing concurrently two – 40m long bridge sections, 32,000 kg each, with a central tight-fitting thrust hinge, which was later fixed using straps.
To minimize handling, the long tapered glulam pieces were fabricated and coated at the glulam plant and transported directly to site. They were assembled on shore into two half-bridge sections in preparation for erection.
By its nature the shallow arch design demands extremely tight tolerances. Small horizontal displacements create large vertical movements, and the bridge geometry was critically dependent on a tight fit. Erection of the bridge sections (with activation of arch thrust) was carried out in a matter of hours, and horizontal and vertical deflection measurements, even after set was achieved, were smaller than anticipated.