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04 Nov 2014
Converting a sculpture of two horses’ heads into a dramatic 30m-high landmark on the Falkirk skyline presented an unusual challenge for the engineers involved.
In 2006, Falkirk Council and British Waterways Scotland (later, Scottish Canals) were trying to resolve two related problems: how to regenerate derelict industrial land between Grangemouth and Falkirk, and how to overcome a low bridge carrying the M9 motorway over the River Carron that meant that canal traffic could only enter or leave the Forth and Clyde Canal at low tide.
The answer to the former was the new 350-hectare Helix urban park; the answer to the latter was a short extension to the canal with a new lock, with the two projects merging into one.
“A plan evolved to create two sculptures for the park, which would at the same time form an integral part of the lock mechanism, using the weight of the sculptures to displace the required amount of water to flood the central lock and lift boats into the canal network,” explains Felicity Starr, a senior mechanical and structural engineer with Atkins. The company has significant experience helping to create or upgrade memorable landmarks, from improving accessibility and safety at the Statue of Liberty to the Newbiggin Bay coastal scheme and, recently, the designs for Plymouth’s new History Centre.
Sketches were made of the sculptures, based on “kelpies” – the mystical equine creatures said to inhabit the lochs and pools of Scotland. The idea was a winner, but the sketches convinced everyone that engineers would be well advised to defer to a professional artist.
Andy Scott was a natural choice. He had already established a reputation for large public sculptures. He frequently sculpts horses: his 4.5m-tall Heavy Horse was already a popular landmark beside the M8 motorway near Glasgow. With one much smaller and simpler exception, he had always built his sculptures himself. Now he had to create two major structures with a design that could be fabricated and assembled at an acceptable cost. This was an altogether different challenge – and one where he needed engineering help.
The first creative stages were for Scott alone. He subtly shifted the brief away from mythological kelpies towards the heavy horses – the Clydesdales, Shires and Percherons – which would once have pulled boats along the Forth and Clyde canal.
His initial sketches were developed into small-scale clay models, and then a pair of 1:10 scale maquettes, created by hand-welding individually cut rectangular plates onto a wire frame. The mosaic effect created by the flow of the plates and the view through the openings captured the horses as if in motion.
These helped secure the lottery funding which enabled the whole Helix park development to go ahead. Scott was then commissioned to build a second set of maquettes, exactly as he wanted them to appear. Now came the period of collaboration and compromise.
At full scale, the open steel surface would be an invitation to adventurous visitors to climb the structure, so the engineers suggested that the bottom three metres should have a smooth, foothold-free surface.
Choice of material was also an area in which engineers provided a steer: a stainless steel surface supported by a structure in painted carbon steel would have a much longer life than the galvanised steel Scott usually works with. Engineers also recommended a glare-free milled finish for the stainless steel plates to minimise the risk of dazzling motorists on the nearby M9 motorway.
Atkins scanned Scott’s second set of maquettes to produce a 3D computer model of the surface. A file was created that reproduced each plate by recording the coordinates of three points on its surface.
The biggest engineering problem was how to replicate the surface, with its 9,000-plus plates, at full scale. A solution was to consolidate groups of plates, with laser-profiling to represent the separate sheets. These would then be bent in three dimensions to approximate the changes in angle between adjacent plates.
But how to avoid the costs and complications of shop-bending more than 900 sheets of steel in three dimensions?
“The answer was to erect the plates as flat sheets, and then use bolts to bend them to the shape of the underlying structure,” says Starr. “This required careful design of the support structure, both to locate the bolt positions in the right place to achieve the required curvature, and to take the bending loads.”
Long consideration was given to the bolts holding the skin to the structure. Besides taking the design loads, these bolts had to be tamper-proof to deter both vandals and metal thieves. They would also have to be vibration-resistant: wind tunnel tests on early models and computer simulations showed not only large loading from the high winds in the area, but also the likelihood of eddies causing high fluctuating loads.
The answer was borrowed from the railway industry: huck bolts, used in rail vehicles but rarely in construction. A huck bolt is a cross between a bolt and a rivet, consisting of a grooved pin and a smooth collar. After inserting the pin into a pre-drilled hole, the installation tool grips the “tail” of the pin and squeezes the collar over the grooves to form a tight joint.
“The biggest advantage of using huck bolts was that they allowed engineers to cope with the relatively large gaps between the panel and the brackets underneath before the tightening process,” says Starr. Panels could now be “pulled” using soft strops to a location close to the required shape, with the remaining gap closed up during bolt installation; this was essential to the panel-pulling system proposed. The bolts worked well, proving quick and simple to install.
During the design phase, the idea of using the sculptures as part of the lock mechanism was replaced with a proposal that they should be free-standing structures, enabling the public to go inside.
“From early work with the maquettes, Andy and Atkins had realised that the inside of the heads could be works of art in their own right, and this in turn required the internal supporting structure to be part of the sculpture,” says Starr. “In any event, the supporting structure would be partly visible through perforations in the skin, so it had to look right from the outside.”
Standard engineering structures are robust and geometric, but Scott wanted something that captured the form of the horse, recognising that there are no straight lines in nature. That meant setting out the columns and structural members to avoid equal spacing and prevent the lines of the structure beneath jarring with the flow of the skin plates.
The engineers and sculptor had to collaborate closely, poring over Scott’s books on equine anatomy to gain an understanding of the natural muscle form and to look at how this could feed into the steel design.
“These studies led to the back under the mane and the natural bulge under the throat being used to hide some of the additional supporting structure,” recalls Starr. “In places, curves were compromised by using straight pieces of steel which, when joined together, look like curves.”
The resulting supporting structure comes in two parts. The first is a structure based around twin-braced triangular trusses, interconnected by bracing frames to form an efficient and stiff primary structure. This supports a secondary frame following the internal surface of the skin, consisting of cladding rails carrying the brackets which pick up the thousands of fixing points for the stainless steel skin.
Particular attention was given to the details of ears, eyes and chin, where the surface curvature was greatest and the steelwork most complex, and to the horses’ manes.
The design for the whole structure was refined to ensure that it could be fabricated and assembled, and was put out to competitive tender. The £5 million contract was won by steel fabricator SH Structures.
A BIMsight building information model was used to exchange information between artist, engineer and fabricator. The model contained all the design detail, but fabrication and assembly drawings still needed to be prepared: nearly 2,000 were eventually produced.
SH Structures fabricated the structure in its own workshops, assembled into the largest pieces that could be carried by road.
The whole structure was far too large to test-assemble before delivery, but each adjacent frame was match-fitted to its neighbour before leaving the workshop to ensure a perfect fit on site. A computer program helped define the lifting points for each frame and the correct sling lengths, so that the frame would be at the right inclination for assembly from the crane.
“Erection on site was remarkably straightforward, with no heavy cranes or complex access systems required,” notes Starr. “All of the skin was added on site, most of it ‘in the air’ after the structure was erected: the exceptions were the more complex sections such as around the mouth, where the skin was added at ground level before lifting into place. The whole erection process was completed in 90 days.”
The Helix park, with The Kelpies as its centrepiece, was opened in April 2014 to international acclaim. Visitor numbers in the first two months were 150,000 – already half the number predicted for the first full year.
It has since been awarded the Saltire Society 2014 Civil Engineering Award, which recognises excellence in civil engineering in Scotland.
Based on an article that appeared in Ingenia, the magazine of the Royal Academy of Engineers in the UK.
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