Steelwork tenders are now being evaluated for construction of one of Scotland’s most complex sculptures. David Hayward reports from Falkirk.
It took a 20-strong team of structural engineers two years to design, cuttingedge 3D computer programmes had to be adapted, and hardware memory was boosted to four times normal, yet files for the £1M design still demanded a full half-hour to open.
The result is a design guide to building two iconic 30m high horses’ heads to stand as the focal point of a new urban park on the outskirts of Falkirk, central Scotland.
All that is needed now is an equally switched-on steelwork contractor to fabricate 600t ribs, skin, noses and eyes. They must be accurate to millimetres and be capable of offering an “internal experience” for 300,000 annual visitors to hopefully venture inside the £6M hollow horses.
Few outside the area have heard of these two Kelpies; named after mythical Celtic water horses. Yet when the sculptures — and the Helix urban park surrounding them — become reality in 2013, a 300ha narrow strip of brownfield wasteland, separating residential Falkirk from industry dominated Grangemouth, will be transformed into a major attraction.
The mane event
The Helix project is the £17M brainchild of British Waterways Scotland, Falkirk Council and the Central Scotland Forest Trust. Original plans for the Kelpies depicted the twin heads — one nodding downward and the other rearing up — to be working models, straddling a near sea lock linking the canal with the tidal river. But engineering reviews coupled with local public consultation ruled more in favour of static quality and the ability to explore the semi-transparent heads from the inside.
It is this requirement to provide a hollow see-through cathedral-like interior that headed the challenge list for the Kelpies’ designer. “With the sculptures located on an exposed wind-prone site, we would ideally have designed central support columns to counteract strong torsional forces,” explains consultant Atkins senior structural engineer Felicity Clement. “But with the centre to be left empty we had to place all vertical supports and cross bracing close to the Kelpies’ skin.”
Though none of the 7,000 individual structural components is the same, Atkins has strived to ensure buildability. Each frame’s up to 75m long, profiled tubular columns is formed of separate straight sections, with the 406mm diameter support “bent” to the required shape by short angled link pieces bolted between them.
The horses’ 42 ribs, supported by the columns, are horizontal flat plates up to 600mm wide and strengthened with T-shaped stiffeners. These maximum 25mm thick ribs are also formed in sections, with four bolted together to form each circumferential ring.
The clever bit is to cut and shape only the external edge of each flat plate, profiling it to the exact contour of the horse. Artist Andy Scott has already built two wire-framed, one tenth scale models, complete with steel skins, to create a miniature. From these models Atkins has laser-scanned their shapes to produce a 3D computer design.
To achieve the required profile, Scott formed each model’s skin with thousands of 50mm rectangular steel platelets, spot welded to the frame. And it was here that
Atkins faced further challenges. “Our original laser scans picked up each weld and plate edge, giving us a 3D cloud containing millions of points,” Clement recalls.
“Conventional design software was inadequate, so we had to go back to basics, mapping just a reduced number of control positions and using our own bolt-on programmes, plus state of the art 3D packages. Even so, we still had to process over 25,000 points per horse.”
The skin of the real sculptures will be formed of much larger plates, merged into irregular shaped 4m long flat stainless steel panels, again each one different. The 500 panels per horse will be bolted onto its rib and column frame, with the 6mm skin hand bent to shape.
Oversize bolt holes and small 40mm gaps between each panel will ease profiling. The winning tenderer for the Kelpies’ one year fabrication and erection will be presented with the 3D design models plus over 1,200 working drawings. The contractor has then about six months to fabricate components.
It is likely each Kelpie will be broken down into about 75 transportable sections which can be “skinned” either in the factory or on site.
Central to the new urban park will be its maze of footpaths feeding from some 16 existing communities into focal activity areas. It was while doodling sketches of these paths that engineers noted their interconnecting nodes resembled the shape of a double helix – the symbol for life’s basic DNA.
With its name established, the £43M Helix project will encompass much of life’s pleasure activities. A 200mm diameter lagoon will offer water attractions ranging from sailing to water polo. An events area will host outdoor markets and theatre workshops while, within hectares of woodland, deer, sparrow hawks and owls could be sighted.
The park will also feature some 80 new allotments, while the adjacent River Carron, its waters once poisoned by shipbuilding and ironworks but now cleaned up, attracts salmon and herons.
Civils works, apart from the Kelpies, include a 1km canal extension, two new locks, a canal tunnel beneath the M9 motorway and extensive services diversions. A £17M design and build contract for much of this work is also now being tendered.