Canadian Consulting Engineer

Feature

Award of Excellence Bow River Bridge and Utility Crossing

In the resort town of Banff in the Rocky Mountains of Alberta, the Bow River Bridge and Utility Crossing is an iconic timber bridge.


In the resort town of Banff in the Rocky Mountains of Alberta, the Bow River Bridge and Utility Crossing is an iconic timber bridge.

For this new crossing Banff wanted a bridge that would be both functional and that would enhance the stunning mountain views and river setting.

The bridge creates an important community link, encouraging walking, jogging, and cycling in this recreational town. It also has to carry medium sized emergency vehicles.

Driving the agenda, however, was

a more pressing concern that existing sanitary pipes installed below the river at this location 50 years earlier could fail, which would spill raw sewage into the pristine Bow River. So the bridge also needed to carry new pipes, giving it an important dual use.

Timber was chosen partly because of its strong carbon sequestration environmental benefits. Another important part of the bridge’s sustainable design was reducing the use of materials. This was achieved through the shaping of the beams and use

of dampers. Further, the use of easily removable modular deck panels encourages the re-use and longevity

of the structure.

80-metre timber span and damping system

To avoid placing piers in the main stream of the environmentally sensitive river, the design features an 80-m clear span, which for a timber bridge is perhaps the longest of its kind in the world.

The primary structural system that Fast + Epp designed is simple. Propped by drilled piers located just outside the normal river channel, 40 m haunched glulam girders cantilever from either side to support a 34-m suspended span.

The bridge cross-section comprises twinned sets of glulam girders stepped to follow the flow of forces, which range in depth from 2.6 m at the piers to 0.9 m at the suspended span. The 4-m wide deck is made of pre-stressed solid timber panels. These are removable to provide access to the service pipes hidden below, and they can be simply replaced.

The span, with its extremely slender curved profile, created the primary structural design challenge for the bridge — its dynamic behaviour due to pedestrian excitation.

To address the potential liveliness of the bridge, an increase in the structural damping was required. While tuned mass dampers are commercially available, there was no room to conceal them below the deck. Instead, a completely unique, custom alternative was developed. Two cable-suspended masses were visually exposed (for honesty) as unique tuned-mass dampers beneath the bridge. These masses each have different fundamental frequencies, and address footstep and jogging excitation respectively.

Support for the

service pipes

Tension rods tie the propped cantilevers down to Rundlestone-faced concrete abutments at either end of the bridge. The north abutment also houses a new sanitary/water line 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 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 that left plenty of tolerance during erection.

The bridge’s visually minimal stainless cable guardrail system involves 135-m long continuous cables. A fine-tuned pretension analysis was done to ensure the system had adequate tension in the summer, and to avoid overtension in the winter.

Erection challenges led to prefabrication

The tight, remote site, huge structural elements and harsh winter weather all combined to make the bridge erection a challenge. The lifts were to be completed before the spring thaw and there was a fixed date for a large mobile crane to arrive. Consequently, there was a huge emphasis on the ease and accuracy of the bridge’s assembly in the field.

In order to accurately assemble and erect the bridge, the individual elements were prefabricated in the shop under controlled conditions and shipped to the site as a kit of parts. cce

Project name: Bow River

Pedestrian Bridge and Utility

Crossing, Banff, Alberta

Award-winning firm (structural

engineer): Fast + Epp, Vancouver

(Gerald Epp, P.Eng.)

Owner: Town of Banff

Client & contractor: StructureCraft Builders

Other key players: Tritech Group

(civil, design-build contractor);

Thurber Engineering (geotechnical);

PFS (landscape).