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Celebrating one year since it’s reopening, we spoke to associate principal, Fraser Reid about how his role unexpectedly took him out west to support the Minnesota Zoo reconnect people, animals and the natural world at the award-winning Minnesota Zoo Treetop Trail creating the world’s longest elevated pedestrian loop and demonstrating the power of adaptive reuse.

The newest addition to the Minnesota Zoo once stood as an abandoned monorail structure that was decommissioned in 2013. Reopening a decade later in July 2023, the structure is now 1.25-miles long Treetop Trail – the world’s longest elevated pedestrian loop.

º£½ÇÊÓƵ was engaged to provide structural engineering services that considered low embodied carbon options and could provide visitors with a year-long immersive experience. 

The Minnesota Zoo Treetop Trail is a prime example of how communities and the natural world can be connected through innovative adaptive reuse strategies. To further explain the Treetop Trail’s impact on visitors and the surrounding landscape, Fraser Reid discusses his involvement with the monumental project.

Tell us about the walkway. Any interesting facts that you can share about the project?

The monorail was decommissioned in 2013 due to high maintenance costs and lack of parts. At 1.25-miles long, the Treetop Trail is now the world’s longest elevated pedestrian loop. The Treetop Trail is emblematic of the zoo’s mission: to connect people, animals and the natural world to save wildlife.

Can you discuss any innovative or unique engineering solutions that were implemented in the construction of the walkway?

º£½ÇÊÓƵ reused the entire existing structure: beam, columns and foundations. Ultimately, the design worked within the constraints of existing structure, which was very sensitive to additional loading. We adopted a lightweight, modular steel decking system which:

• Allowed the trail to be as wide as possible, with minimal interventions to the existing structure
• Used Corten steel (or weathering steel), to match the existing structure
• Could be constructed ‘from above’, meaning modules were moved into place by an ingenious trolley system which was custom built to travel along the existing monorail. This solved the numerous access challenges during construction and allowed the zoo to maintain operations throughout construction

The existing structure was strengthened using a continuous top channel, embedded into the steel modules, and two angles, welded to the bottom of the beam. As well as providing additional capacity, the bottom angles helped brace the existing thin wall plates and stitch the original beam section back together where the original plates had buckled, and welds had failed. This clever approach allowed the structure to be upgraded for the new trail condition with the minimum level of intervention.

Can you describe the structural design considerations that were taken into account for building the elevated walkway?

As a reuse project, the key consideration from the outset was to understand the behavior and capability of the existing structure. The existing 1.25-mile-long monorail structure was constructed with no expansion joints hence thermal expansion and contraction was a major consideration in the design. However, significant locked-in forces were observed, which needed to be considered in the analysis. Foundations were modelled as springs to account for small movements and rotations, verified by geotechnical analysis.

How will the walkway integrate with existing infrastructure and landscape features?

As did the original monorail, the Treetop Trail connects the zoo with its surrounding natural habitat. The trail crosses two lakes and takes visitors into the ‘North 40’: a serenely peaceful wooded area, far from the main zoo exhibits. This provides an area of reflection and is emblematic of the zoo’s mission to ‘Connect people, animals and the natural world to save wildlife’.

How was the environmental impact assessed and mitigated during the planning and construction phases? What factors were considered in selecting the location for the elevated walkway?

By rehabilitating and reusing this abandoned structure, the team eliminated a huge amount of waste and emissions which would otherwise have been necessary for its demolition. Embodied carbon was used as a metric for comparing the different structural solutions. Ultimately, the selected steel-framed solution was the lowest embodied carbon option and also minimized the level of intervention needed to the existing structure.

How will the walkway enhance connectivity or improve access to key areas within the vicinity?

There are five main access points to the new trail, which include stairs, ramps and elevators, as well as 22 bump-out overlooks to enhance the viewing experience while walking the trail. All of the new access points were carefully integrated around the existing zoo exhibits and infrastructure.

Were there any community engagement or stakeholder consultation processes involved in the planning and design phases? How was it visiting the project for the first time after its completion? What aspect were you most excited to see?

º£½ÇÊÓƵ was so thrilled to join the zoo at the ribbon cutting event and see the trail open to the next generation of visitors. A key thing which stood out to me from the opening events was the emotional connection so many people have to the old monorail train. Seeing the smiles on the visitors’ faces: stepping onto the new trail and experiencing the zoo from this new vantage point was a wonderful experience and something I won’t forget.

Our innovative approach to revitalizing stranded assets like the Minnesota Zoo’s Treetop Trail has been recognized and lauded by several leading industry organizations including AIA Minnesota and American Public Works Association.