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Spreading wings This weekend's television news will feature erection of Britain's tallest and most controversial sculpture. If nothing else, reports David Hayward, the Angel of the North is a mileston

Mike Wood removes his coat from what looks like a tailor's dummy in the corner of his Hartlepool office. 'This was our only contract drawing,' he says.

Late this Saturday night, a 108t ten-times scaled up version of his 'coat stand ' will travel on the back of a lorry slowly up the A1 to Gateshead. Two other lorries just behind will each carry vast 26m long steel wings, 50t apiece, to be bolted and welded to the shoulders.

This time next week, publican Eddie Smith will doubtless be enjoying considerably increased trade. His new found customers at the recently renamed Angel Inn will have been attracted by the dramatically changed view from the pub window.

Like it or loathe it, Britain's largest sculpture, the 20m high Angel of the North should by then be full frame.

This gateway to Gateshead will be standing where once a colliery bath house echoed to miners' voices. It is these men that the sculpture, in part, commemorates.

Mining, shipbuilding, steel, engineering - the now largely vanished legacies of the North East - are celebrated in this unpainted metal monument with its exoskeleton of heavy vertical ribbing over a skin of curved sheet steel.

Yet, to the multidisciplinary team that conceived, designed and built the £800,000 Angel, it also represents a modern civil engineering achievement - the unprecedented teamwork between artist and engineer.

The concept remains firmly that of renowned sculptor Antony Gormley. But translating his, at times, uncompromising vision into a statue 10 times larger than in the artist's mind, needed the skills - and lateral thinking - of both structural and fabrication engineers.

'Even if you don't like it, the engineering is extraordinary - like building the Forth Bridge without the banks either side.' This praise from Gormley himself will doubtless be long cherished by the Ove Arup engineers who helped steer him from plaster model to a structure just 10m shorter than the Colossus of Rhodes.

And it was they who designed him wings the width of a jumbo jet able to withstand, 'the wrong way on', the 160km/h wind speeds of an exposed Gateshead hill top.

Gormley's words will be remembered too by Mike Wood, contracts manager for Hartlepool Steel Fabrications. This is the company that, at the eleventh hour, remodelled the original costly cast steel design into a cheaper double-skin fabricated version, staving off possible cancellation of the entire project.

Whether history will thank Wood is largely in the hands of the 90,000 motorists who will pass the structure daily on the nearby A1 motorway. Whatever their verdict, it will probably not be coloured by the fact that the Angel is late - two months if you start from contract award date; two years if you go back to initial plans to see it erected to celebrate the North's 1996 Visual Arts Year.

This early delay was due in part, says client Gateshead Metropolitan Borough Council, to 'ongoing discussions with the artist'. Lifesize representations of himself are a Gormley speciality and the plaster cast now in Wood's office is a fairly accurate likeness, complete with somewhat pronounced calf muscles.

His demand for an 'empty' body, allowing the observer to spiritually occupy the space inside, suggested originally that the skin be structural. Thick vertical ribs, protruding through the skin and running full height, aid support and create a very visual exoskeleton highlighting the Angel's construction.

Scaling up this profile 10 times, and still achieving the correct shadows, took hours of 'discussion' between artist and engineer. The calves became too large, the neck too short.

'It is difficult for any artist to retain hands on control with a structure so large and it was like working with a particularly sensitive architect,' recalls Ove Arup associate Neil Carstairs. 'Some of his concepts made life easier, some much harder.'

By then, Wood had unpacked a wooden crate containing his sole contract drawing and the challenge moved on from concepts to skin.

Early ideas for a 20mm thick cast steel structural skin, profiled to a 1mm accuracy of Gormley's exact shape, had to be abandoned. Though moulding it in up to a dozen sections was possible, the tender prices of around £1M from the four bidders easily bust the client's tight £600,000 construction budget.

Fortunately, one of those bidders, HSF, submitted an alternative double- skin design exceeding the budget by just £50,000. 'If we built an inner structural tube we could attach a non-structural skin to it

formed of much more bendable 6mm sheet steel,' explained Wood to the client.

The result is exactly that: an inner tubular core with 20 rows of vertical ribs welded to it. Inset between ribs, the steel panelled skin provides the required exacting profile (see left).

The plaster cast was converted by photogrammetry into several thousand images. Virtual reality modelling, plus three dimensional computer aided design allowed exact dimensions for ribs and 280 skin sections to be downloaded by e-mail to the totally automated cutting equipment of nearby Teesside Profilers.

'After this paperless design exercise actual fabrication was relatively easy,' says Wood.

His prime challenge was the congested ankle section which Gormley insisted remain slender but at which main live load stresses are concentrated. The wind-induced moment at the ankle could be an impressive 12,000kNm, offering a bending stress of 175N/mm2.

Design stress at the wing-body junction is 250kN/mm2. And it is here, at dawn on Sunday, that engineers will be closely monitoring the two stage fixing mechanism. Initially, 88 bolts will secure overlapping link plates before they are welded together and hidden behind final sections of skin. Gormley's desire for seamless skin joints is, wherever possible, being honoured; even though most of the Angel's 33M annual watchers will only see the exposed relatively isolated structure from some distance away. But mass viewing was always part of the artist's gameplan.

'Art in the 20th century has been characterised by the individual's pursuit of his own freedom,' Gormley concludes. 'The point about this work it is that it has been built by a lot of people for a lot of people.'

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