The historical Town of Banff, set in the beautiful Canadian Rockies, is one of the most visited tourist destinations in North America. The town sought a new crossing which would not only be functional, but would also enhance the stunning mountain and river setting. It is an important community link, encouraging walking, jogging, and cycling. The material choices are in keeping with those of the community, including timber as primary structure, stone, and well-detailed galvanized steel. Driving the agenda, however, was the more pressing concern that existing sanitary pipes installed below the river at this location some fifty years earlier could fail, spilling raw sewage into the pristine Bow River. So the bridge now needed to carry the new pipes as well as provide a secondary access for emergency vehicles. The project also had to meet stringent environmental requirements of Canada’s oldest national park. The design features an 80m clear span, which for a timber bridge is one of the longest of its kind in the world. This, with an extremely slender curved profile, creates the primary design challenge for the bridge: its dynamic behaviour due to pedestrian excitation.
Design
The primary structural system is simple: Propped by drilled piers located just outside the normal river channel, 40m haunched glulam girders cantilever from either side to support a 34m suspended span.
The bridge cross section comprises twinned sets of glulam girders stepped to follow the flow of forces, which range in depth from 2.6m at the piers to 0.9m at the suspended span.
The 4m wide deck is made of pre-stressed solid timber panels, removable to provide access to the service pipes hidden below.
The concrete abutments at either end of the crossing tie down the haunch ends, but also house the pump station, eliminating the need for any additional above-grade structures.
The horizontal steel trussing provides both the diaphragm and support for the service pipes just below the bridge deck. It is configured such that only the timber is continuous, resulting in very little length expansion.
The central drop span sits on neoprene bearing pads on notches in the receiving ends of the cantilevered glulam girders. This detail is achieved by using long screws which invisibly reinforce the notch,
forming an elegant connection which left plenty of tolerance during erection.
A visually minimal stainless cable guardrail system involving 135m long continuous cables, required fine-tuned pretension analysis to ensure adequate tension in the summer, and avoid overtension in the winter.
Durability
Durability was a topic of significant interest to the client, and great care was taken in detailing to ensure a 75 year design life.
Spacing between the paired glulams allows full ventilation, and the shingled heavy gauge flashing creates a strong drip edge protecting the beam faces.
All steel components are hot dip galvanized or stainless steel, and rubber spacers or grommets separate the two where they interface.
The guardrail system is anchored through the flashing to the beams in a unique way so that there are no penetrations.
Glulams are coated with a system which behaves like a breathable membrane and is easily re-coated.
Structural and Vibration Analysis
While the primary behaviour is simple, the internal behaviour of the stepped beams is not, requiring finite element modeling, and special grading and selection of the beam laminations.
The long span and slender profile of the bridge, while enhancing aesthetics and minimizing material (and, critically, erection weights), make it susceptible to both vertical and lateral excitation from human traffic on the bridge. Fundamental vertical frequencies are 1.5 Hz. (walking) and 3.3 Hz (jogging).
Through much research and testing, two cable-suspended masses were visually exposed (for honesty) as unique tuned-mass dampers to address footstep and jogging excitation respectively. Response reduction was verified through field-testing of actual frequencies and accelerations.
Fabrication and Erection
A parametric 3D model of the entire bridge was created early, allowing rapid investigation of a multitude of design decisions, providing visual feedback to both designer and client.
A tight site and harsh winter, coupled with a desire to complete the lifts before spring thaw, made ease and accuracy of assembly in the field critical. The main structural elements of the bridge were too large to be transported to the site; and fitting up the pieces over the river with a smaller crane would have presented significant environmental and safety challenges. Thus individual elements were prefabricated in the shop and shipped to site as a kit of parts. All cutting, drilling, sanding and finishing was performed indoors under controlled conditions so that members are permanently protected. Jigs were built to ensure accurate assembly of the main bridge components on the riverbanks. In all, the entire bridge superstructure was erected in 3 lifts over 2 days, with the heaviest assemblies weighing in at over 50 Tonnes.
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