Designed to blend into the natural beauty of Canada’s first national park, the Bow River Pedestrian Bridge is a striking timber superstructure in Banff, Alberta.
Completed in June 2013, the bridge was designed and constructed by StructureCraft Builders, in collaboration with Fast and Epp Structural Engineers. StructureCraft is Fast and Epp’s affiliated design-build company.
“We worked closely with the Town of Banff to understand what they were trying to achieve,” says Gerald Epp, president of StructureCraft Builders and partner at Fast and Epp. “They wanted it to be complementary to the environment and beautiful but not overstated.”
It also had to serve as a utility pipe crossing to replace the aging sanitary pipes installed underneath the river. Slung underneath the bridge are two sanitary and one water line connecting the north side of town to the wastewater treatment plant on the south.
The goal to create a minimal and unimposing design emphasizing natural materials and environmental considerations led Fast and Epp to look at timber. The final design had to take into account wood’s unique properties while optimizing for both form and aesthetic.
“The use of timber is rare especially on a structure of this size,” Epp notes. “Normally a bridge that slender and spanning that far would be steel.”
The 113-metre long timber bridge features an 80-metre clear span, which is one of the longest of its kind. At only 4-metres wide, the slender bridge profile was one of the main design challenges.
Epp explains that with all wood structures, controlling vibrations is a key consideration. To achieve the slender profile for the timber bridge, two unique tuned mass dampers suspended beneath the bridge were designed to reduce vibrations from walking or jogging.
“If you walk over the bridge, you will find it to be lively. The dampers were a huge part of the design to control the vibrations,” Epp says. He explains that the system consists of different numbers of tuning mass plates that can be removed or added to essentially “interfere with the vibration to the bridge.”
The bridge is comprised of three distinct segments: two haunch glulam girders on either side that cantilever out to the centre to support a 34-metre suspended span. The central span was made as long as possible to provide minimal interference in the river channel, keeping the supporting drilled piers outside of the river channel during much of the year.
“The glulam beams were enormous pieces at 40 metres long and 2.5 metres deep that had to be shipped from the shop in the Lower Mainland to the site,” Epp says. “Just shipping the girders was a challenge.”
A tight, remote site and winter conditions also made the bridge erection a challenge.
“We had winter conditions when we were erecting it so we had to do as much prefabrication as possible in the shop so that when we got to site it would be just linking together all the pieces,” Epp says. “The prefabrication was a big part of the success.”
Pieces were assembled on the shoreline and the entire superstructure was erected in three lifts over two days, requiring a large mobile crane.
“We had to bring in a 510-tonne crane in order to do this enormous lift — at about 110,000 pounds,” Epp says, adding precautions also had to be taken to ensure the lifts were completed before spring thaw. “We were moving into mid-March so we were watching the temperature every day. We needed that solid frost on the river banks to do the lift.”
Located within the world renowned Banff National Park, demanding environmental constraints was a big challenge on this project for both the final structure and the construction process.
“Every single aspect of construction was controlled with extreme environmental restrictions,” Epp says.
The bridge features removable timber deck panels for ease of replacement and access to the service pipes running underneath them.
“A lot of care was taken in addressing durability. It has a high quality water based coating which is easily maintainable,” Epp notes. “It’s also a 75-year design life.”
Another unique feature is the custom steel guardrail system. It is anchored to the flashing and the flashing to the beams in such a way that there are no penetrations to the wood.
“The 450-feet long stainless steel cable is pre-tensioned — had to resist expansion and contraction of plus or minus 40 degrees Celsius,” Epp says.
The $6.8 million pedestrian bridge is the first new crossing of the Bow River in Banff since the vehicle bridge was built in the 1920s.
“There was only one road bridge crossing the river. Now people can easily walk over to the Banff Springs Hotel,” Epp says. He credits a great client relationship and a good project team for the successful outcome. “The town has seen an increased 60 per cent in the amount of foot traffic that crosses the river.”
The innovative use of wood and technical engineering excellence for this project has earned it two awards to date: a Wood WORKS! BC 2014 Wood Design Award and an ACEC-BC Award of Merit.
Cheryl Mah is managing editor of Construction Business.