A comprehensive and very accurate 3-D cloud survey of all the trees and their branches in the forest was done, so that the route of the walkway could be designed    and refined to meander through the wood without a single tree being affected

Since disturbance of the tree roots was also to be kept to a minimum, conventional concrete bases were not an option and the structure is thus supported on steel piles driven into the ground, with positions carefully chosen so as to miss all roots.

Each column is fixed onto a tripod, with  its three ends each supported on a pile. After installation the pile top positions were accurately surveyed again, and the tripods all manufacture to exact, bespoke dimensions, so that each column base would be in precisely the correct position and at the correct level, ready for the column to be fixed onto it.

SUPERSTRUCTURE STRUCTURAL CONCEPT

Since no diagonal members are allowed in the UK above deck level (to prevent climbing over the handrail), the handrails could not be used as the upper chord of a truss, as we have done on previous structures. The main structural members are thus all below the deck level, comprising a central box section plus two side box sections, with the stanchion ribs above not forming part of the main structure and thus designed as delicately as possible, only 15 mm wide at the top. All the box sections are welded up from plates profile cut to  the correct curves.

The central box is joined to the two outer box sections with diagonal ribs that are angled on plan, so at to provide horizontal stiffness.

The shapes of the stanchion ribs and the lower ribs have a continuous, even outer curve that flows  from the handrail all the way around to the central box section. The stanchion and lower rib  inner edge curve is also continuous until it becomes horizontal in the centre. The outer box sections have vertical sides but the top and bottom surfaces are sloped to follow the curved lines of the stanchions and ribs.

The superstructure was built in 3 sections: the central main box section with the lower ribs welded to it, and the left and the right balustrade sections. All components were made up in lengths of roughly 5 m, so that they could easily be lifted into their  final positions on site with a light crane.

Everything was pre-assembled in the workshop, then galvanised and painted, so that it would fit perfectly when bolted together on site, with only minor paint touch-ups required.

The Oak timber joists, Balau transverse decking slats and the Oak handrails were also pre- fabricated in sections, before being fitted in their exact positions on site.

COLUMNS

The columns are up to 11 meters long, and have to resist substantial moments at their bases. They also need to be stiff to reduce the horizontal walkway deflections. They are star shaped in plan, made of 6 radial T sections. They taper from a 265 mm diameter below the walkway to a diameter of up to 460 mm where they are bolted onto the tripod top plates.

An interesting detail was devised for this connection, to allow for any possible angular orientation between the column and the tripod. This was achieved by having 12 slotted holes in the column baseplate and 24 round holes in the tripod top plate. As the column is rotated around the common centre point (which also has a bolt for initial erection purposes, 12 bolts will always fit through the 2 plates.

The columns were made up in sections of max 6 m long and the detail where these sections are bolted together is also interesting, with no protrusion of the flange plates beyond the column edges and the bolts hidden between the T-sections.

COLLABORATION

The Architect and Structural Engineer have worked very closely together to sculpt all the details of the walkway deck and its supporting structure, to ensure that that every component is tidy and aesthetically optimal.

Furthermore, the entire walkway and all the columns were fabricated by a Cape Town steel fabricator before being shipped to the UK, achieving a considerable saving for our Client. A UK steel contractor fabricated the tripods supporting the columns and did the onsite erection of all the steelwork. The close collaboration achieved between the consultants and contractors in Cape Town and those in the UK has been one of the highlights of the project.

INTERESTING FACTS AND NUMBERS

Total length                           130 m

Width                                    1.4 m for most of the bridge, 3 m in one zone

No of spans                          12

No of columns                       12

Typical span length                10 m Max height above ground    12 m

Steel mass of bridge             31 tonnes Steel mass of columns 14 tonnes Steel mass of tripods 12 tonnes

Nearly all the steelwork was CNC plasma cut from plates and welded together

CONCLUSION

We feel that the Viper Walkway at Emily is an excellent example of how steel can be used to create something of amazing and enduring beauty. The facility is bound to provide a lot of pleasure to many people for a long time to come. Steel is a material with very special attributes and qualities, no matter where it is fabricated or where it is erected.

Project motivation editorials are provided by the project nominator. If any technical details, company names or product names are incorrect, please notify the SAISC so that the error can be corrected.

Project Team Role

Company

Nominator

Henry Fagan & Partners

Client/ Developer

Emily Estate (UK) Ltd

Architect

Mark Thomas Architects

Structural Engineer

Henry Fagan & Partners

Quantity Surveyor RSA

Bernard James & Partners

Quantity Surveyor UK

Synergy

Project Manager

Stonewood Design Architects

Main Contractor

Beard

Steelwork Contractor  RSA

Prokon Services

Steelwork Contractor  UK

MJ Patch & Co

Steel Erector   UK

MJ Patch & Co

Timber Work Contractor

HG Holliday (Pty) Ltd

Corrosion Protection
Consultant

KVB Associates

Corrosion Protection
Galvanising

Advanced Galvanising 

Corrosion Protection
Paintwork Contractor

MRH 

Photographer: competition

fotohaus

Photographer: other images

Henry Fagan & Partners

If you were a part of this project, and your company details are incorrect or missing – please notify the SAISC so that the error can be corrected.