Architect and Builder November/December 2015 | Page 69

BRIDGE CATEGORY WINNER:
Kirstenbosch Canopy Walkway (“The Boomslang”)
In praising this outstanding structure, the judges said that it was a
true team effort. “The end result is an efficient, flowing structure that
provides the visitor with a truly memorable experience,” they said.
The brief could be summarised in a few words: to design
a pedestrian bridge between the tree tops of the Arboretum at
Kirstenbosch. The project, however, was indeed a challenging
one and is the ‘story’ of how well the whole project team worked
together to realise the brief.
The walkway was designed to be ‘organic’ and blend unobtrusively
into the forest, while minimising damage to the trees. To achieve
this, a delicate structure, carefully located between the trees, and
finished in a colour that would blend in was required. “Steel was
clearly the material of choice, to satisfy these criteria,” the project
team said.
To minimise disruption and damage to the vegetation, a
comprehensive survey of the area was undertaken. Columns were
located relative to trees and were not necessarily placed at the most
efficient structural support positions. Where it was not possible to
avoid a particular tree, it was either allowed to pass through the
structure, or was tied back to ensure that it experienced minimum
interference from the structure.
The spine of the structure, a single tube section, forms the
bottom chord of a truss. The box section handrails double as the
top chords of the left and right trusses. Ribs cut from 8mm plate at
1m centres serve both as stanchions and as the vertical elements
of the trusses. The ribs are made up of three parts bolted together
below the deck. This ensures efficient use of material, facilitates
handling in the confined areas between trees and limits the use of
heavy equipment in this sensitive area.
All components were pre-assembled in the workshop and, when
installed on site, everything fitted perfectly, with no on-site cutting
or welding being required.

Steel Awards

Since being opened to the public, the Boomslang has proven to
be extremely popular. The increase in gate income from more visitors
allowed the capital costs of the bridge to be recovered within one
year of it being opened.
This fact influenced the judges who said: “The project not only
met the professional team’s requirements and was aesthetically and
practically outstanding, but it also increased the numbers of visitors
to the Gardens by over 30%, thus making it a resounding commercial
success. It thoroughly deserved the Bridge category award.”
Client: SANBI (South African National Biodiversity Institute)
Architect: Mark Thomas Architects / Christopher Bisset
Structural Engineer: Henry Fagan & Partners
Quantity Surveyor: Bernard James & Partners
Main Contractor: Slingsby & Gaidien Construction
Steelwork Contractor, detailers: Prokon Services
Land Surveyor: Stern & Ekermans
Galvanizer: Advanced Galvanizing
Corrosion Protection Consultant: Koning Vadas Blom Associates

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