Deoxyribonucleic lighting

It's not every day that Arup is brought in to analyse a light fitting. But the Helix is no ordinary luminaire. It doesn't contain a single lamp or length of flex, and the engineer was needed to find out if its extraordinary geometry could actually be built. It was dreamed up by Carpenter Lowings Architecture and Design to form the centrepiece of the Prince Consort Restaurant in London's refurbished Albert Hall, it is more sculpture than lamp.

"The Albert Hall could be seen as a stodgy building. We wanted to change that," says Luke Lowings, partner at Carpenter Lowings. "We wanted to do something between architecture and sculpture with light, something that looked as if it was floating."

The name "Helix" is derived from the form of what the press release describes as a "linear lighting device". Cables are stretched the 25 m length of the restaurant and anchored to each end wall. These cables twist to form an open spiral or helix. A series of metal struts fixed between the cables hold them apart to maintain the spiral. Just to add spice, the restaurant is curved – the Albert Hall is, after all, round – and the Helix has to follow this curve. A series of secondary cables are attached along the length of the outer curved wall near the ceiling and swoop down at a 45º angle to join the Helix. These pull it outwards to form the curve.

A round mirror is attached to each end of the struts holding the cables apart. Lights are concealed above the row of seats that runs along the length of the inner wall and directed at the mirrors. This light is reflected onto the tables and the walls of the restaurant, creating shadows. "It looks like a series of musical notes," says Lowing. "That was unintentional."

Arup was brought in to check that the unusual design would not come crashing to the ground in a heap of twisted cable. "Although Luke's concept was good it needed checking to make sure it would work as a full-scale structure," says Mike King, the associate at Arup who worked on the project. He found the design particularly satisfying. "The whole thing is perfectly balanced by its weight and geometry," he says. "The pureness of this geometry meant it had to be exactly in balance for it to work." This is because the weight of the cables, struts and mirrors is counterbalanced by the horizontal forces keeping the helix in a curved position.

King found this project made him re-examine some of his engineering preconceptions. "We had to think about it on a different scale. We initially looked at larger cables but found the loads were tiny, so the cables wouldn't hang straight. We had to retune our thinking and find the finest wires to ensure everything hangs dead straight and there are no wobbles." The cables forming the helix are 4 mm thick and the cables pulling the helix into a curve are 1.5 mm.

An engineering analysis was carried out using a computer model, and double-checked using a physical model. This showed the only problem was that the whole structure could potentially buckle – rather like a rope that is twisted until it snaps. This problem was solved by placing three very fine diagonal ties at each end of the helix.

The sculpture was made and installed by Stainless Steel Solutions of Doncaster. The steel struts were first attached to the radial suspension cables, and a temporary cable was fixed between the two end walls in the same curved position as the final helix. This had the effect of making the light fitting form a curve when viewed from above. The steel struts were fixed to the temporary cable, then helix cables were attached to the steel struts. Finally the mirrors were fixed to the struts and the temporary cable taken down.

For King, the Helix was an interesting project to work on. "It's a cross between architecture, art and craftsmanship. That was wonderful and something you wouldn't get on a large project."