To celebrate last year’s centennial anniversary of the Kirstenbosch National Botanical Garden in Cape Town, South Africa, the South African National Biodiversity Institute (SANBI) commissioned a 426-foot-long aerial walkway through the garden’s arboretum. The Kirstenbosch Centenary Tree Canopy Walkway takes visitors 40 feet up, through, and above a forest of approximately 450 indigenous species against the backdrop of Table Mountain’s eastern face.
The structure was designed by Mark Thomas, of local firm Mark Thomas Architects, in collaboration with local structural and civil engineering firm Henry Fagan & Partners and SANBI horticulturist Adam Harrower. “Often, an engineer would design the structure of a bridge, with the architect then adding practical requirements such as the walkway surface, handrails, safety mesh, and so on,” says Henry Fagan, principal partner at his namesake firm. “For this project, Mark and I worked together to integrate everything, so that many of the components serve both a structural and architectural purpose.”
The project team conducted extensive site surveys to ensure none of the 400-plus trees in the walkway’s path would have to be felled or trimmed during the installation of 19.6 tons of steel columns and abutments. “At one point, where a tree could not be avoided, a branch of a tree extends vertically through the walkway,” Fagan says.
Coined by local residents as the Boomslang, an indigenous tree snake, the walkway comprises a series of trusses connected by continuous wood handrails and a 420-foot-long steel tube spine that acts as the bottom chord of the trusses. The handrails are made from slender planks of Padauk, a sustainably harvested West African hardwood, and laminated onsite to accommodate the sinuous design contours. “One of our biggest challenges was the development of the eccentric curves,” Thomas says. “They appeared smooth at first on the computer model, but close up showed faceted straight lines.” By using wood instead of steel, the team reduced the visibility of the faceting.
Steel box sections encased in the handrails serve as the top chords of the walkway’s trusses in elevation view. Safety mesh woven from 8-millimeter-diameter rods between the handrails and decking acts as the truss web and varies in density based on the walkway’s local loads. In section view, the walkway’s horseshoe-shaped trusses consist of two curved steel ribs connected by a steel central transverse frame. The trusses vary between 4 feet, 6 inches and 10 feet in width.
All metal components were hot-dipped galvanized steel finished with two undercoats of epoxy, and two topcoats of polyurethane paint. Stained pine planking treated to H3 specifications, a South African wood treatment classification, forms the walkway deck.
Each walkway member and joint was defined in Xsteel, a BIM software program now known as Tekla Structures. “In terms of cost efficiency, many of the components are repeats,” Thomas says. “All the ribs are identical, for example.” The elevated walkway is in turn supported by steel columns, spaced about 39 feet on center, anchored into reinforced concrete footings and a foundation that sits 5 feet below grade.
With a design load of 5 kilopascals (104 pounds per square foot), which the team confirmed using the finite element analysis software Strand 7, the walkway can accommodate up to 1,100 visitors at once.
In total, the walkway comprises 63 prefabricated steel components: 21 20-foot-lengths of the central tube, and 31 pairs of curved ribs, and 10 columns, all manufactured by Prokon, in Blackheath, South Africa. Once the components were delivered to the site, everything but the steel columns were light enough to be moved around the gardens on a trailer and hoisted into place by a small crane, with Harrower’s assistance.
The designers programmed a lateral range of motion of up to 0.75 inches into the walkway to enhance the visitor experience of moving through the canopy. In fact, pedestrians may experience as much as 3 inches of movement in windy conditions, as per design intent. In October, the project team added bracing cables at two locations near the walkway’s midpoint to reduce the range of motion by 25 percent in response to visitor feedback that the amount of movement was disconcerting.
The walkway was completed this past May, almost 100 years to the day that the South African government established a botanical society to help build and promote Kirstenbosch.
“We were very aware that a large structure like this could become quite obtrusive if not carefully designed and built,” Thomas says. “Initially there were many critics, but all who have seen the bridge have given overwhelmingly positive reports.”