State of the art engineering has been used to create Gateshead's Angel of the North, Britain's largest sculpture.
Made from weathering steel, it is built in three parts - two wings and the body. Each contains a mix of up to five elements:
Ribs - the external skeleton cut from 50mm thick steel allows the structure that holds the Angel together to be integral to its appearance.
Skin - 6mm sheet steel bent and welded to ribs make up the body form.
Sacrificial ribs - these lie beneath the skin plates and help to shape them.
Diaphragms - looking like ribs on the surface, they are actually 50mm thick horizontal plates which go right through the body - up to six metres by three metres and weighing almost five tonnes. There are five of these - the most crucial in the chest which line up with the horizontal diaphragms on the wings to provide a solid anchor point.
Core - the unseen skeleton which runs from its feet to its chest to give extra support. This hollow steel tube - a series of cylinders and cones - mirrors the shape of the body. It is 30mm thick plate steel up to its knees and 15mm thick above. The lower body ribs are welded directly on to the core.
This revolutionary approach to the manufacture was devised by Hartlepool Steel Fabrications, which won the contract to build the Angel of the North. They worked closely with consulting engineers Ove Arup & Partners and Gateshead Council to make the design of the Angel a reality.
Hartlepool Steel is a family-owned firm which recently finished renovating Middlesbrough's Transporter Bridge. It has also worked on North Sea oil rigs and in the Teesside chemical industry.
The original body castings of the Angel by the sculptor, Antony Gormley OBE, were scanned by the Geomatics Department at Newcastle University and the precise co-ordinates plotted to create an electronic, 3D Virtual Reality Angel.
This data was converted into a three-dimensional CAD model by Grafton Software so that the computerised profiling machines used by Teesside Profilers were able to cut the main body into ribs following the exact curves of the artist's original castings. The ribs were then supplied to Hartlepool Steel for construction.
The foundations for the Angel are almost as impressive as the artwork itself.
One hundred and fifty tonnes of concrete were poured to form piles to root the sculpture into solid rock 20 metres below.
Windloads on the wing boxes were transmitted along the ribs, down the body and into the foundations, enabling it to withstand winds of more than 100 miles per hour.
A Cumbrian firm won the contract to install the foundations, Thomas Armstrong (Construction) Ltd, whose North East Regional Office in Catterick, North Yorkshire, oversaw the work.
They started by bulldozing away the hill next to the A1 at Gateshead. Then old mine workings were filled with grouting and eight holes drilled - each of them three quarters of a metre across - to take the steel reinforced concrete piles.
A concrete slab one and a half metres thick and covering an area 13 metres by 8 metres was then laid on top of the piles, with a plinth 5.3 metres high on which the Angel stands.
After a trial assembly on the ground, the body and two wings were transported overnight on three low loaders from Hartlepool to the site at a maximum of 15mph.
Cast in the plinth are 52 bolts - each three metres long. A 500 tonne crane lowered the 100 tonne body of the Angel carefully into position, lining up the holes in the footplate with the bolts. Nuts were then screwed on to keep the body in place.
Once the body was secure the 50 tonne wings were craned into position so that drilled 50mm steel plates in the wing lined up with plates on the body. Working from scaffolding, workers secured each wing with 88 bolts then welded the plates together.
Welders finished the job over the next three days by welding in the final skin plates. The hill was later replaced and re-turfed and the landscape in the surrounding area reinstated so it was ready for the official launch in June.