Bow River Pedestrian Bridge, Banff
Set over the Bow River in the town of Banff (Canada's first national park), this slender 113m long timber bridge both serves pedestrian traffic and provides a new sanitary crossing replacement.
The 80m clear span - perhaps the longest of its kind for a timber bridge - is formed from two 40m tapered haunch girders, cantilevering from either side to support a central 34m suspended span. A replaceable modular timber deck sits atop twinned Glulam beams which grow to 1.9m deep.
A proprietary tuned mass damping system was implemented to help achieve the slender 4m wide deck section and to deal with vibrations from pedestrian loading.
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, and to allow for simple replacement, if required.
Tension rods tie the propped cantilevers down to Rundlestone-faced concrete abutments at either end of the bridge, and the north abutment also houses the new sanitary/water line pump station, a part of the design-build contract, eliminating the need for any additional above-grade structures.
The horizontal steel trussing provides both the diaphragm and support for the service pipes concealed just below the bridge deck. The bracing is configured such that only the timber chords are continuous, resulting in very little length expansion, one of the bonus features of wood.
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.
The historical Town of Banff, set in the beautiful Canadian Rockies, sought a new crossing which would not only be functional but enhance the stunning mountain and river setting.
While the primary behaviour is simple, the internal behaviour of the stepped beams is not, requiring finite element modeling which we performed in RFEM, and special grading and selection of the beam laminations. This required careful coordination for production of these massive timber elements.
The long span and slender profile of the bridge, while enhancing aesthetics and minimizing material, make it susceptible to both vertical and lateral excitation from human traffic on the bridge. Around 40-50 pedestrians crossing would be sufficient to cause lateral 'lock-in' and significant increases in resonant accelerations at this frequency.
While tuned mass dampers are commercially available, there was no room to conceal them below the deck. Instead, through much research and testing, an alternative was developed: two cable-suspended masses were visually exposed as unique tuned-mass dampers to address footstep and jogging excitation respectively. The dampers consist of carriages containing a series of plates (the “mass"), suspended with cables (the “spring"), and tuning is simply addressed with the addition or subtraction of plates. But it is crucial that the dampers be tuned to actual frequencies, which is why field testing is important.
Testing was carried out by installing 6 accelerometers at key points in the span, and using modal analysis software, both actual frequencies and mode shapes were extracted, as well as damping ratios. We compared these with the fundamental frequencies and mode shapes predicted from our RFEM models, and found fairly good correlation, and we updated the FE models to reflect the actual support stiffness conditions. Lateral accelerations due to lock-in, which were of slight concern during early design stages, were not observed to be an issue in service – partly due to the numbers of people commonly using the bridge. Even at the inauguration (>100 people crossing the bridge) there were no reports of adverse comments around bridge accelerations.
A parametric 3D model of the entire bridge in Autodesk Inventor was created early, allowing rapid investigation of a multitude of design decisions, providing visual feedback to both designer and client.
Fabrication and Installation
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.
We prefabricated individual elements 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.
The unique halving joint connecting the central span to the two cantilever spans uses neoprene bearing pads to transfer the compression across the joint. Long timber screws either side of the connection reinforce the joint to prevent splitting perpendicular to grain, and a steel drag strap atop the beams connect them and transfer axial loads.
The entire bridge superstructure was erected in 3 lifts over 2 days, with the heaviest assemblies weighing in at over 50 tonnes. At an over 60m reach, these lifts required a 500 tonne crane positioned carefully on the riverbank.