Entry ID 61

THE INSTITUTION OF STRUCTURAL ENGINEERS: STRUCTURAL AWARDS 2020

THE VIPER ELEVATED WOODLAND WALKWAY AT THE NEWT IN SOMERSET

1 INTRODUCTION

Nestled within a pristine stretch of forest

along the side of a hill lies The Viper, an

elevated woodland walkway which

smoothly slithers through the trees,

offering spectacular views of the

surrounding arboreal beauty, before

ending at the new Plant Museum

building of The Newt, a country estate

with magnificent gardens in the county

of Somerset.

This exceptional project showcases the

quality and stature of the structural

engineering profession and is

considered a worthy contender for the

ISE Structural Awards.

2 DESIGN, FABRICATION AND ERECTION

Using a comprehensive 3-D

cloud survey of the forest,

the route was planned such

that not a single tree would

being affected.

Mark Thomas and Henry

Fagan developed the design

concept during lengthy

deliberations. A detailed

finite element model was

used to analyse the structure

and check all stresses and

deflections.

Prokon Services, the Cape Town steel manufacturer appointed to fabricate the columns and walkway,

created a comprehensive Tekla model of the structure, which was scrutinised and tweaked by Henry,

Mark and Prokon before manufacture commenced.

All components were hot dip galvanised, epoxy coated, wrapped and shipped to The Newt for erection

by British Steel Contractor MJ Patch, who also fabricated the tripod foundations.

3 DESCRIPTION OF THE STRUCTURE

Foundations

Each column base is fixed to a

tripod supported on three steel

piles.

The tripods were manufactured

to individual bespoke dimensions

verified by the surveyed pile

positions after installation.

Columns

The columns are up to 11m long, fabricated in sections. To achieve the

required stiffness at their bases they were designed to be star-shaped in

plan, with 6 radial T-sections. The 6mm webs and 20mm thick flanges

taper from a 265mm column diameter below the walkway to 460mm at

the base.

Walkway Structure

The handrails and very delicate

stanchions and not part of the

spanning structure.

The main structural members are

beneath the walkway level,

comprising a central box section

plus two smaller side box sections,

all welded together from plates, with

the top and soffit plates profile cut

to the correct radius.

The central box is joined to the two

outer box sections with Y-shaped

ribs that are angled on plan.

4 SUSTAINABILITY

Minimal forest floor intervention

Steel piles were used rather than conventional concrete bases for the column foundations, their

positions carefully chosen to avoid all tree roots. Tripods above the ground were used to connect the

columns onto the piles, rather than conventional pile caps, thus avoiding the use of any concrete in the

forest.

Blending into environment

The walkway does not protrude above the trees and is not visible from the adjacent fields. Because of

its organic shape it blends into the forest, rather than making a dominating statement.

Minimal sections and weight

All sections were diligently optimised to minimise their weight.

The tapered columns use much less steel than a section with constant properties.

The stanchions, plasma cut from 12mm sheet, are only 14mm wide at the top and 42mm where welded

to the side box sections.

The combined inertia of the

central spine and two side box

sections (at a higher level) is

substantially more than the sum

of the three individual sections,

so they are connected to work

in unison.

Stanchions nesting

The stanchion curves were

developed with their outer edge

coinciding with the inside curve of

the adjacent stanchion when

cutting them from a sheet, thus

halving the cutting length and

saving material.

Cables

A substantial reduction in the weight of the columns

and walkway steel was achieved by introducing four

cables connecting the walkway to two ground

anchors installed in strategic positions. The cables

stiffen the walkway against horizontal deflections

both transversely and longitudinally and increase

the natural frequency of vibration.

Longevity

The steelwork was hot-dip galvanised and painted

with a stripe coat plus four coats of epoxy.

English oak was used for the handrails and the

runners onto which the Balau decking slats were

attached.

The structure is designed to require minimal

maintenance for many years.

Forest protection

The construction zones were cordoned off and the crew sensitised to eliminate degradation to any of the

flora or fauna in the forest.

End-of-Life

If The Viper were to be removed, the timber and steelwork can easily be dismantled and recycled,

restoring the forest to its original pristine state.

5 CREATIVITY AND INNOVATION

Tripods

Using tripods above ground level is a creative and novel solution for connecting each column onto three

piles.

Column shape

The star-shaped plan layout using six radial T-sections is an

innovative design for easily and economically fabricating elegant,

tapered columns that are strong and stiff at the bottom where the

moments are large, yet very delicate at the top.

Column junction

This star-shape facilitates bolting column sections together without

the customary flanges protruding around the perimeter, but rather

with bolts hidden between the T-sections.

Column base connection

An ingenious detail was devised for the connection of the base of

each column to the top of the tripod, allowing any possible

angular orientation between each column (built in Cape Town)

and tripod (fabricated in the UK). The column baseplate has

twelve slotted holes and the tripod top plate twenty-four round

holes. As the column is rotated around the common centre

point, bolts will always fit through both plates in an alternate set

of holes, ensuring that the column can be installed at its required

orientation.

Angled ribs

These provide a horizontal and vertical link

between the middle and outer box sections.

Rather than full triangulation, the alternate

angling looks more elegant, saves on material

and allows all the ribs to be identical on the

inside and outside of the curved spine beam.

Deck slats

Narrow transverse timber slats are used as

a walking surface. This allows varying gap

widths between them to accommodate

bends in the walkway without the need to

taper their plan width, which would have

been required if wider planks with fewer

gaps were used. Using identical narrow

slats is more economical, simplifies the

installation, reduces the risk of slipping and

provides an appealing visual texture.

6 ELEGANCE AND GOOD DETAILING

Stanchion and rib outer and inner curves

The stanchions and the angled ribs below have a continuous, flowing

outer curve from the handrail all the way around past the outer box

section to the central spine beam. The inner edge flows in a gentle curve

from the handrail down past the spine beam right up to the other handrail.

The top and bottom surfaces of the outer box sections are sloped to

follow these curves. This creates a clean, graceful appearance.

Delicate, repetitive stanchions

The stanchions are very

intricate, like the ribs of a

snake. Their repetitive use,

without any heavier members

in-between, creates a sense of

unity, consistency and serenity.

Walkway / museum deck

The walkway structure splits

as it unobtrusively morphs into

a deck in front of the

museum, with consistent

handrail details to provide

continuity.

Column junctions

The detail with the hidden bolts and no protruding flanges

provides a clean silhouette over the full height of the column.

Hidden handrail connections

Sculpted details

The steel

details were

painstakingly

crafted

throughout for

efficiency and

sculptural

beauty, as per

the adjacent

examples.

7 VALUE

The project cost was optimised by having the superstructure built in South Africa and the tripods

fabricated in the UK, to fit onto the installed piles.

The structural and architectural requirements are integrated, e.g. not with a structure first provided and

the handrails and stanchions then added, as often found on steel bridges. This integration creates a

minimalist aesthetic, with coincident economic benefits.

The main value of the project lies in the beauty that is achieved, since The Viper experience is likely to

draw many visitors to The Newt. At Kirstenbosch in Cape Town the popularity of the Boomslang, a

similar project, increased the visitor numbers such that the entire project cost was recovered within a

year.

8 CONSTRUCTABILITY

The walkway was detailed in 5m long

components to facilitate galvanising,

transportation and erection amongst the

trees using a small crane.

Transversely, the walkway was built as a

left, right and middle assembly.

The spine beam in the middle was first

erected onto the columns and the side

stanchion assemblies then added.

The entire walkway was pre-assembled in the workshop, 3 spans at a time.

With meticulous, systematic planning and excellent collaboration amongst the team members in both

hemispheres, everything fitted perfectly on site, like a Meccano set.

9 INTERESTING FACTS AND NUMBERS

Total length 130m

Width 1.4m

Number of spans 13

Typical span length 10m

Maximum height above ground 12m

Steel mass of bridge 31 tonnes, i.e. only 240 kg/m

Steel mass of columns 14 tonnes

Steel mass of tripods 12 tonnes

Total project value £ 2.4 million

The steelwork was mostly CNC plasma-cut from plates and welded together.

10 CONCLUSION

The Viper Elevated Walkway at The Newt is an excellent example of how a sensitive approach to the

delicate conditions on site, diligence during the design and construction processes and an abundance of

creativity can create something of amazing and enduring beauty. With predicted visitor numbers to The

Newt of up to 1500 people per day, the facility is bound to provide endless pleasure to many for years to

come.