Bow River Pedestrian Bridge in Banff, Canada
Town of Banff, Canada
|Design-Build, General Contractor||
StructureCraft Builders Inc., Canada
This slender timber pedestrian bridge was constructed over the Bow River in Banff. The town of Banff is Canada’s first national park located in the beautiful Canadian Rockies. At 4 m wide and 113 m long, the bridge is complete with an 80 m clear span which is perhaps the longest of its kind for a timber bridge.
The structural and dynamic analysis was performed in RFEM by the Dlubal customer StructureCraft Builders Inc. located in Abbotsford, Canada.
Two 40 m tapered haunch girders cantilever from either side to support a 34 m suspended span. The bridge deck cross-section consists of twinned glulam girders. The girders are stepped to follow the flow of forces and range in depth from 2.6 m at the piers to 0.9 m at the suspended span. The horizontal truss bracing includes rectangular steel hollow sections arranged between the longitudinal girders.
Removable prestressed timber panels make up the 4 m wide deck to provide access to the service pipes hidden below.
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 over-tension in the winter.
The long span and slender profile of the bridge made it susceptible to both vertical and lateral excitation from human traffic. Without further consideration, around 40-50 pedestrians crossing would be enough to cause lateral “lock-in” and acceleration resonance.
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"). Tuning is simply addressed with the addition or subtraction of plates. However, it was crucial that the dampers were tuned to actual frequencies which required additional field testing.
Testing was carried out by installing 6 accelerometers at key points in the span. A modal analysis software determined actual frequencies, mode shapes as well as damping ratios.
These results were compared with the fundamental frequencies and mode shapes predicted from the RFEM models with accurate correlation. The FE models were then updated to reflect the actual support stiffness conditions. Lateral accelerations due to lockin which were a concern during early design stages, were not observed to be an issue in service. Even at the inauguration (>100 people crossing the bridge), there were no reports of adverse comments regarding bridge acceleration.
The town of Banff now includes a new pedestrian bridge crossing which is not only functional, but also enhances the stunning mountain and river setting.
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Programs Used for Structural Analysis
Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements
Dynamic analysis of natural frequencies and mode shapes of member, surface, and solid models