THE INSTITUTION OF STRUCTURAL ENGINEERS: …
Transcript of THE INSTITUTION OF STRUCTURAL ENGINEERS: …
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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.