The project kicked off in early 2012, with Mr Philip Leroux of Kirstenbosch approaching Architect Mark Thomas to design a pedestrian bridge between the treetops of the Arboretum at Kirstenbosch.  Mark requested that Henry Fagan & Partners be appointed as the structural engineers for the project.

Although the project budget was very limited, the entire project team were very excited, albeit rather apprehensive, about the prospect of adding a structure between the trees of this natural treasure.

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 colours that would blend in was needed.  Steel was clearly the material of choice, to satisfy these criteria.

GEOMETRIC REQUIREMENTS AND SURVEY

Since the walkway was to be built in an existing forest with a requirement to minimise disruption and damage to the vegetation, a comprehensive survey of the area was done.  This established not only ground levels but the position, height and canopy diameter for every tree.  Mark spent many hours carefully planning a route through and over the trees together with horticulturalist Adam Harrower. 

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 allowed to pass through the structure and was tied back to ensure that it experienced minimum interference from the structure.

The surveyor was required to accurately set out the position of each column, and the position and orientation of the two abutments on site, so that when steel was delivered to site, everything fitted. 

INTEGRATED DESIGN

Rather than adding the deck and hand-railing on top of a conventional structure comprised of trusses or beams, components were designed to be multi-functional, with balustrades and safety mesh being an integral part of the primary 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 1 m centers serve both as stanchions and as the vertical elements of the trusses. 

The ribs are made up of 3 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.

Two longitudinal angle rails, onto which the transverse walkway planking is fixed, also serve more than one purpose.  In the interim stage, when only the lower central portion of the walkway is erected, these angles serve as top chord members of a triangular truss, with the circular hollow section being the bottom chord.

The 8 mm rods forming the safety mesh contribute to the structure as truss diagonals.  Their gradient varies with span, being steeper near the columns where shear forces are highest, and shallower at mid-span.  The curves thus introduced soften the appearance and give the structure an organic feel.

ARCHITECT / ENGINEER COLLABORATION

This project was more like crafting a sculpture than designing a structure.  To develop a sensitive and appropriate design given the practical constraints of the site required that Mark and Henry collaborate very closely from the outset. 

The shape of every component of the bridge was carefully tweaked and adjusted over many meetings until a solution which performed as required structurally, without excess material and which was considered aesthetically optimal, was achieved.

DESIGN AND ANALYSIS

Hand calculations and a finite element analysis of simplified straight sections of the bridge were initially used to obtain the member sizes for the tender.  The tender specifications stipulated that all the elements of the bridge be defined in a 3-dimensional model developed by the steel manufacturer using a suitable detailing package.  Prokon Services, who fabricated the steelwork chose to use Tekla for the detailing.

The Tekla model would serve two purposes:

  • Firstly, it accurately defined the shape, orientation and position of each component of the bridge for manufacturing purposes.
  • Secondly, the file was converted and imported into the structural engineering finite element package Strand 7, setting up the geometry for a detailed analysis of the structure. The stresses in each component of the bridge could be checked under design load combinations.

An ultimate limit state design was applied to the bridge, with deflections and frequency response being checked.  Based on the FE Analysis, supplementary members were required, in particular where the section is wider.

To achieve the clean uncluttered aesthetic, and limit the impact on the forest, a slender structure was required.  Movement was thus always going to be significant.  At a late stage of the design process, the deck was raised to clear the tree canopy, optimizing the visitor experience.  The column lengths thus increased, from a maximum of 9 m to the current 12 m maximum length, more than doubling the calculated column deflections.

Conventional cross-bracing was designed to limit deflections and movement.  A dynamic finite element analysis indicated that this would reduce movements by approximately 70%.  This bracing was not installed. The approach being to first monitor actual bridge behaviour – and the response from the public.  Subsequently, cable stays have been introduced which limit movement somewhat, though less than cross-bracing would have done. Interestingly, feedback from many visitors indicated that movement added an extra dimension of excitement, enhancing the experience.

CONSTRUCTION

Once the structure had been fully drawn out, but before manufacturing commenced, a further detailing iteration was required, to ensure that all the transition curves were smooth.  To accommodate the changing horizontal and vertical curves, each portion of the central tube, the box sections at the handrails and the angles supporting the deck had to be rolled to the correct radius. 

The stanchions (single 8 mm plate) and central transverse frame sections (double 8 mm plate) were laser cut from large steel sheets to create the perfectly smooth curved edges one sees on site.

All components were pre-assembled in the steel yard at precisely the correct angles and slopes, with two adjacent sections being connected at a time, to check the geometry and ensure that the required smooth transitions between the curves were achieved. When installed on site everything fitted perfectly, with no on-site cutting or welding being required.

Tight controls were implemented on site to ensure that a similar level of care was taken during construction to that taken with the design.  Plant size and access were restricted and designated areas were allocated for hoarding, to protect the natural vegetation.

Since being opened to the public, the Boomslang has proven to be extremely popular.  Kirstenbosch saw an increase in visitor numbers of 31.6% from the 2013/14 to the 2014/15 financial year (829,668 to 1,091,438), and attribute this largely to the Boomslang which opened to the public in May 2014.

There is no additional fee to walk on the Boomslang; only the standard Kirstenbosch garden entry fee.  However, 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.