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Nature encapsulates the 485-acre Minnesota Zoo, the fifth largest in the U.S. Opened to the public in 1979, the Minnesota Zoo’s original monorail traversed a 1.25-mile loop through the landscape, within forest, above lakes and wetlands, and amongst zoo guests, expansive animal exhibits, and natural wildlife. The original structure for the monorail track was constructed of weathering steel consisting of a built-up box member spanning between wide flange columns that are embedded in concrete drilled pier foundations. This steel track weaved its way throughout the Minnesota Zoo, providing views to zoo exhibits and accessing undeveloped, forested parts of the campus that make up over half the property.
The zoo stopped monorail operations due to reduced demand, obsolescent components, and ultimately unsustainable operating costs. After decades of natural encroachment, nearby new development, and use as a support for modern data utilities, disassembling the enormous structure merely to eliminate an unsightly relic was unfavorable. The Minnesota Zoo’s mission to connect people, animals, and the natural world to save wildlife helped inspire the concept to reuse the existing structure as a platform for a walkway through the zoo and the natural world; the vision of the Treetop Trail was born.
The Minnesota Zoo recognized that reuse of the existing structure would guide decision making. A creative team was employed including structural design engineer Buro Happold, engineer of record Meyer, Borgman, and Johnson, and the Minnesota Zoo’s Director of Planning & Construction who is also an experienced structural engineer. The dynamic team, well versed in the intricacies of structure, was pivotal to the success of the Treetop Trail.
Access to the 1.25-miles of project site is complex. Spread between sensitive natural settings, local endangered wildlife, and dense campus development, the trail was only feasible as a reuse project where the site below could be minimally disturbed. Exhibits were required to remain occupied, all public areas were to remain open, and seasonal campus events had to be accommodated without disruption. The execution had to be respectful of the zoological animals’ home. Noise, dust, and vibrations affect feeding, stress, and natural breeding cycles in different ways for each species, so it was necessary to sequence construction to minimize impact.
Early in design, the project team recognized the need to limit disruptive and costly foundation interventions and minimize structural strengthening. To do this, the structural team identified five variables of the typical trail section that were tuned to produce the efficient final scheme:
• Self-weight of the trail system.
• Trail width.
• Required design live load.
• Guard rail height.
• Guard rail porosity.
Multiple structural systems were considered for the trail. A cast-in-place concrete option was ruled out due to complexities with placing concrete along the length of the trail. A precast concrete option was studied. The self-weight of this system resulted in significant strengthening of the existing structure and foundations, so it was eliminated. A fiber-reinforced plastic system was considered due to its high-strength, lightweight, and corrosion resistant characteristics. This system was eliminated due to the limited number of suppliers, challenges with future component repairs and replacement, and the high embodied carbon footprint of the product. Ultimately a weathering grade structural steel system with a secondary decking was selected. This approach was lightweight, low in embodied carbon, and resulted in decreased maintenance and life cycle costs for the Minnesota Zoo.
The Treetop Trail is not defined in either the International Building Code (IBC) or the American Association of State Highway and Transportation Officials Bridge Design Specifications (AASHTO). The structural team, in agreement with the local building code official, determined that the structure would be designed in accordance with the IBC and the American Society of Civil Engineers (ASCE) 7 Minimum Design Loads and Associated Criteria for Buildings or Other Structures (ASCE 7). AASHTO was referenced throughout the design for comparative purposes only. The main walkway areas of the trail were designed as “walkways and elevated platforms” with a live load of 60 pounds per square foot, and assembly and exit ramp areas were designed with a live load of 100 pounds per square foot per ASCE 7. With the structural system and live loading requirements determined, the structural team tuned the walkway width and guardrail system with the design team, settling on a typical trail width of 8 feet. At specific locations along the trail, 12-foot-wide “bump outs” were included to create special moments in the architectural design.
Despite limited existing drawings, a schedule of as-built foundation depths and typical rail cross sections were critical in evaluating the existing trail structure. Extensive field surveys were conducted to verify bearing conditions, identifying several unique and undocumented details. In addition to the field survey, a full condition assessment of the existing steel and welds was conducted, including visual and non-destructive testing. Welds were repaired where necessary prior to construction of the trail structure.
Structural analysis of the monorail structure was completed in SAP2000. To accurately approximate behavior, foundation boundary conditions were modeled as lateral translation and rotational springs based on a pier analysis done collaboratively with the project geotechnical engineers. Slide bearing conditions at the rail to column connections were modeled as link elements with non-linear springs to reflect a 0.10 friction coefficient prior to engaging the full column stiffnesses.
Although the structure was consistently designed with IBC as the governing code for load determination, the continuity and curving nature of the track required consideration of both longitudinal and transverse wind loads occurring simultaneously at different points along the trail. AASHTO wind skew coefficients were used as a basis for determining these components in conjunction with ASCE 7 wind pressures. The trail structure was designed as a Solid Freestanding Sign, and the railing was designed as an Open Sign. This approach was compared with AASHTO loading and determined to be a similar and reasonable solution.
The increased wind load of the reinforced structure and railing required additional lateral capacity. Initial designs focusing on strengthening the original lateral system required a significant number of columns and foundations to be reinforced. To avoid expensive and disruptive foundation retrofits, the design team explored and selected an approach utilizing eight new braced frames and selective column reinforcement. The braced frames were intentionally located to minimize additional thermal restraint in the system.
The existing structure was constructed as a continuous beam with no expansion joints, resulting in the potential for significant thermal movement. Expansion and contraction of the existing monorail beam was anticipated in analysis and observed as the structure was exposed to the harsh extremes of Minnesota’s frigid winters and hot summers. During the design phase, project team members monitored the existing monorail beam on the hottest and coldest days of the year, observing over 7 inches of movement of the monorail beam relative to bearing conditions, closely reflecting displacements predicted from the analytical model. Analysis of the structure focused on understanding existing thermal movements and minimizing additional restraint from reinforcement schemes. Worn existing bearing pads at slip connections were replaced with new material to refresh the original movement joints. This approach avoided more costly interventions, such as foundation reinforcements and new expansion joints.
In addition to extensive live load patterning and eight wind load directions, positive and negative temperature loads were considered, resulting in over 600 load combinations.
The limited self-weight and continuity of the structure highlighted vibration as a primary consideration in the early design phase. With limited opportunity for adding damping given the existing foundation constraints, the design team focused on understanding the behavior and working with the Minnesota Zoo to set expectations. While the trail may not be able to host 5k Fun Runs, acceleration due to walking excitations falls within the 5.0%g limit recommended by the American Institute of Steel Construction (AISC) Design Guide 11 for outdoor pedestrian bridges.
The design team collaborated early with the project general contractor, PCL Construction (PCL), to develop a modular construction approach for the typical trail sections. Approximately 420, 20-foot-long modules consisting of reinforcement, cantilevered walkway sections, and deck support angles were shop fabricated and shipped to the site. Even with the modular approach, active animal habitats and natural landscapes limited erection access to the trail. PCL developed a unique mobile erection platform to safely construct the trail using an existing monorail maintenance vehicle and a new trail vehicle specially made for construction. Construction was completed in two full passes along the trail loop. The first pass inspected and repaired existing welds, added reinforcement, and relocated utilities. The second pass pushed modular trail sections out from discrete access points along the trail. The steel sections were welded to the existing built-up box beam, and the composite decking made of recycled milk cartons was installed to complete the trail.
The structural design of the Treetop Trail required a deep understanding of the conditions and performance of the existing monorail track system, and creative intervention strategies to reinforce the structure while maintaining much of the original structural behavior. Intense collaboration with the full project team was required to meet the needs of the program, Snow Kreilich Architects’ design requirements, and the construction of the trail. The true success of the Treetop Trail, however, lies in its support of the Minnesota Zoo’s mission: “To connect people, animals, and the natural world to save wildlife.” This success extends beyond the technical accomplishments of the project, to the creation of a revitalized way to experience the Minnesota Zoo. ■
About the Authors
Thomas Root, PE (MN) is the Director of Planning & Construction at the Minnesota Zoo. Root directs and supports the activities of planning, construction, audio/visual systems, and exhibits throughout the Zoo with the goal of maintaining and improving the facility’s infrastructure, sustainability, functionality, and appearance.
Craig Huhtala, PE (MN), is a structural engineer at Meyer Borgman Johnson in Minneapolis, MN, who has worked extensively on existing building analysis and adaptive reuse projects, in addition to leading projects in the public, technology, and aviation sectors.
Will McDevitt, SE, PE, is an Associate structural engineer at Buro Happold in Chicago, IL. He is responsible for leading projects throughout the US and internationally and has worked on numerous new construction and adaptive reuse projects.