A bizarre and beautiful new artwork is testing engineers' mettle, discovers Bernadette Redfern.
Scaling up a sculptor's model can't be too hard, can it? After all, every town in the country has a larger than life bronze of some civic hero, king, soldier or saint.
But the astonishing spiky seaurchin of a structure now under construction and destined for Manchester's Commonwealth Stadium has had engineers and fabricators scratching their heads.
Titled 'B of the Bang' (see box), the 55m tall steel sculpture was inspired by an exploding firework, claims its designer, artist Tom Hetherwick. One hundred and eighty vicious looking spikes jut from a tightly packed nucleus. The prickly 165t mass is wildly out of equilibrium, canted over at 30infinity from the vertical, and is precariously supported on just five insubstantial points.
Working out how the structure actually performs was not as hard as its appearance would suggest, admits structural designer Ron Packman of consultant Packman Lucas. 'The structure is not as highly stressed as you would imagine - it's basically a fivelegged tripod, ' he says.
These five points will be tied down to steel plates, which in turn, are anchored to substantial foundations, already in place and waiting. Contractor Thyssen Construction has installed 22, 15m deep, 600mm diameter reinforced concrete piles with a 2m deep C40 concrete capping layer.
The supporting spikes are being fabricated in beefy 40mm thick steel - the other 175 have wall thicknesses varying from 4mm to 10mm - and will be solid over the bottom few metres. They are designed to take care of windinduced torsion and rotation forces, as well as dead loads.
Working out how to build B of the Bang has proved the greater challenge. Each of the spikes tapers over its length to a 50mm diameter point. At the hub each one is theoretically 7m in diameter.
But in practice, the convergence of so many spikes on one node means only a few actually originate in the sculpture's centre. Building up the sculpture one spike at a time presents fabricators with a geometric puzzle of impressive complexity.
Construction of such a structure has been left to the expertise of main contractor Westbury Structures, which in December last year roped in Sheffield-based specialist Aker Kvaerner Heavy Engineering to tackle the hub and Abacus, based in Sutton in Ashfield, to manufacture the spikes. The whole structure is being produced in weathering steel supplied by Corus.
'We got involved early on to advise on whether we thought this could actually be built, ' says Westbury managing director Kevin Cumberland. The firm's IT wizard Glenn Fletcher modelled the structure in three dimensions, assigning the ends of each spike a set of X, Y and Z co-ordinates.
Fletcher then transposed the design from AutoCAD design software into the steel detailing package Xsteel for analysis of joints in the congested hub.
Westbury's key problem was what should go at the very heart of the sculpture - what the spikes should grow out from. 'We tried a variety of centres including a 1m diameter sphere, a 0.5m diameter sphere and a peanut shaped centre, but in the end, we decided not to put anything in the centre at all, ' says technical director Richard Hawkes.
By welding two of the spikes together it would be possible to build out, adding spike on spike and weld on weld.
To determine the position and geometry of each welded connection, Westbury went back to the Xsteel model and virtually dismantled the structure, slicing the spikes longitudinally, unwrapping and flattening them out. For each spike, the curvilinear geometry of the connecting surface was printed out at life size and taken to Aker Kvaerner's workshop. Here a steel section was cut to create the welding surface using the print-out as a template.
For added complexity, contrary to appearances, the spikes do not have a true circular section, but an 18-sided polygonal - or octdecagonal - section. The taper on the spikes is being achieved by folding flat sheet steel around on itself. The computer model, however, assumed that the spikes were conical so each profile has to be meticulously checked before the template is used.
At only 4mm-10mm thick, welding the steel does not pose a problem. And welds retain the characteristics of the surrounding steel, so there are no complications caused by overwelding joints as new spikes are added. At the moment there are 35 spikes attached, with the number increasing fast as Aker Kvaerner is working 24 hours a day on it.
Packman is confident that, as the last of the 180 spikes are joined, there will still be just enough space for welders and their torches to squeeze in and make the connection. Completion of the core is due in early April.
Construction is in two phases. The hub, made up of 180 spike base sections, is welded in Sheffield, while Abacus is producing 7m long spike end sections.
Only when the hub has reached its destination on the Commonwealth Stadium's main approach will it be equipped with its array of thorns. Even so, it will be an exceptionally large load, measuring 5m high by 5m wide and 14m long and weighing a hefty 71t.
It will be transported in late April with a police escort along the M1, M62 and the M60. On arrival in Manchester it will be hoisted up, on its side, onto an 8m high trestle so that its five legs can be attached.
Two of these legs will then be secured to steel plates with a pin connection. Using a 700t crane, it will then be lifted and rotated about the two pin joints, landing its remaining three feet on the ground. These will be welded in place.
Finally, a team will move in to fasten the remaining spikes using a 12 bolt connection.
These will be covered over with a welded plate to provide the seamless finish specified by the artist.
What's in a name?
B of the Bang was born out of a design competition held by Manchester City Council in 2001. Artist Tom Hetherwick, who had teamed up with structural engineer Ron Packman to dream up a winning idea, was on holiday in Mexico when inspiration struck - an exploding firework caught in time. He called Packman at 2am UK time to share his brainwave.
The sculpture's name is about more than fireworks. It is as much a celebration of athletics and the Commonwealth Games, which were hosted by Manchester in 2002. In a television interview that year, sprinter Linford Christie said he had been working on his start with athlete Kris Akabusi.
Akabusi told Christie that he was starting on the bang of the starting pistol but that he needed to get away from the blocks on the B of the bang.