Dressed to thrill

What has been revealed, however, looks like it is the product of a different civilization from the buildings around it – possibly a different life form. The curving, sculptural form of the building is covered with 15,000 silver discs that look like high-tech satellite dishes, projected on stalks from the ultramarine blue surface of the cladding. The sunlight reflects off the discs with a diffuse silvery gleam visible from miles away. As clouds pass across the sun the quality of the reflections change, so the building's surface appears constantly shimmering.

Given the level of secrecy surrounding this project, and its appearance, people could be forgiven for thinking it is a top-secret military installation. On the contrary, it is going to house Birmingham's Selfridges department store.

The three firms given the task of designing and constructing the skin of the building were architect Future Systems, engineer Arup and contractor Laing O'Rourke. "It was a real challenge to come up with something that matched Selfridges' aspirations," says Matthew Heywood, project architect at Future Systems. "It wanted something very distinctive so you didn't need a sign on the building saying 'Selfridges'." Arup's structural engineer Ed Clark hints at the practical problems of realising those aspirations: "It's a free-form shape that is constantly changing. How do you skin it?"

What is more, the team had to design and build this incredible cladding for the same cost as a conventional system. Retail specialist Benoy designed the entire Bull Ring centre for developer the Birmingham Alliance for a fixed price. Selfridges took a 35-year lease on the site at the Digbeth end of the scheme, and wanted to influence the design, but still had to work within that original budget.

Originally, Future Systems wanted to make the building transparent – but Selfridges vetoed this as it would make lighting the merchandise difficult. Windows would also occupy precious shelf space. The planners became concerned a mainly windowless facade would make the building look bulky, so Future Systems have made the store curve both in plan and section to soften its shape. In section, it sweeps out from the top of the building with a striking bullnose profile, tucks in again in the middle and bulges out at the bottom like a gown. In plan, two external corners are softened by curves. "What we wanted to do was create a uniform surface that we could wrap the whole building in. This would repeat all over and wrap around the roof too," says Heywood.

The gown analogy is apt: the idea for the uniform surface came from one. "Future Systems is always interested in finding images that relate to projects we're working on," says Heywood. In this case, the image was drawn from chainmail-effect dress by Paco Rabanne, the man who designed the costumes for Barbarella. Other influences also played a part, including the dimpled surface of the 16th-century Gesu Nuovo Church in Naples. "This is the most direct inspiration from architecture; the surface is covered in raised pyramids so you get shadow and texture so it's interesting," Heywood says. The curvaceous, textured cladding allayed the planners' concerns, and they embraced the concept with enthusiasm.

There was no way that this unique idea could have been realised without close collaboration between architect and engineer. Arup and Future Systems worked together intimately from an early stage. "The two went hand-in-hand, the technical influencing the aesthetic and vice versa," says David Gilpin, project manager for Arup. "It was an evolutionary process." Arup saw no reason, structurally, to change Future Systems' original shape, so all the work focused on developing the cladding system.

The initial idea was to use a panellised solution to recreate the chainmail dress. The problem was that every panel would have to be different to accommodate the building's curves, so the idea of identical triangular panels with large gaps between them was mooted. This would have been too tricky to build, so Future Systems came up with an idea. "We said, what if it was a circle? We wouldn't have to align a circle," says Heywood. This could project from the surface of the building. "We wanted to create a sense of depth so at ground level it didn't look flat and boring."

The team decided to blow the budget on the discs with a very cheap facade behind. The first idea was to use Kalzip cladding sheets supported on a steel frame. "We quickly realised Kalzip was not a good idea, because of the curves," says Clark.

The second idea was to go for a thick, self-supporting skin. The options that were explored included precast concrete panels, but this was dismissed, because of the large number of moulds needed. "Sprayed concrete was the next obvious step and seemed to satisfy all the constraints," says Clark. This would involve spraying concrete over a flexible mesh to form the create the shape of the building (see "How the cladding works" page 34).

Deciding on a suitable material and shape for the discs caused much head-scratching. The discs had to be strong enough to support a person in case someone attempted to climb the building, and to stay intact if a vehicle hit the facade. It would have to avoid dazzling drivers, and deter birds on the lookout for nesting sites. A convex disc was settled on as it dispersed reflections effectively and best achieved the curves of the building.

A whole range of materials was considered. These included glass, plastic, aluminium, enamelled steel, and vitreous china – the material used for making sanitaryware. "We spent a long time with Armitage Shanks," remembers Heywood. "They were initially sceptical but became much more interested when they realised the numbers involved." The appeal of vitreous china was that it could be fired to any colour, including a graduated shade, and it did not fade over time. The downside was it broke into glass-like shards.

Aluminium was a strong contender early on. However, there was an issue with the anodising process that colours and protects it. "You can't create the same beautiful colours externally as you can internally with anodised aluminium as ultraviolet light makes it fade – and it has to last 35 years," Heywood says. Powder coating was not a solution as that turned back to powder after 20 years. Natural anodised aluminium was a possibility as it provided an attractive, durable finish without the problems of colour change.

The only way to resolve the technical and aesthetic issues was to build a mock-up. "The mock-up was designed to test as many parameters as possible," says Gilpin. "It tested the technical and aesthetic issues. Two panels were built with different thicknesses of concrete, colours and disc material. For Heywood, the disc material issue was immediately resolved. "The beauty of the aluminium is that it reflects the colour of the sky, and will pick out colours in a very small area of the disc, for example a speck of colour reflecting from a bus going by. The vitreous china looked very flat, so this meant aluminium discs were definitely the way to go."

As 15,000 discs were needed, they had to be cheap to make. They also had to be easy to install, with a degree of adjustment in case a disc fixing was out of place. The discs were made by specialist cladding supplier James and Taylor, who helped Future Systems with their development. The 660 mm diameter outer cover was pressed into shape and the edge folded around to form a flange by spinning the disc and folding the edge around, like a potter creating an edge on a wheel. James and Taylor even had to source and recondition a machine used for polishing car hubcaps to burnish the discs to a mirror finish. Finally the discs were sent off to be anodised.

The background colour for the discs was important. Future Systems wanted a very rich deep blue, "The same colour as an Yves Klein painting," says Heywood. "The problem was putting this colour onto a building; this quickly became one of the biggest problems on this project as blue fades very quickly and it had to last 35 years." A company called Liquid Plastics came to the rescue with the right shade of blue, which it tested for ageing and guaranteed for 35 years.

The team could now put a tender package together for potential contractors. Only those firms capable of working with complex 3D models were considered. Finally Laing O'Rourke won the contract, and was also responsible for final detailing. Future Systems tried to make the job as easy as possible. "One of the biggest issues of the facade was the nightmare of setting out the discs," says Heywood. This was made simpler by having the same number of discs on E E each horizontal band around the building, but with different-sized gaps between them to accommodate variations in the building's profile. Every 10th disc was made a "control" disc and the others matched up to it.

Construction has gone smoothly although the facade has taken longer to build than planned. "As a facade system it was much more weather-dependent than most, as the temperature affected the spraying and curing of the concrete and the waterproof membrane," says Clark. Initially Arup employed someone to keep a close eye on the mix for the sprayed concrete to ensure it was right.

The cladding will be finished by June, the fit-out is under way and the store is due to open this September. The building is already a major talking point in the city, with some concern about its juxtaposition to St Martin's Church next door. Heywood says the planners were concerned about this, so Future Systems incorporated more glazing on that side – choosing glass panels with a yellow-fritted edge that would blend into the facade.

"I think the design is much more sympathetic to the church than the original Bull Ring," Heywood says. "You can see the church reflected in each disc, so it relates to it on the micro-scale too."

The local paper, the Evening Mail, which claims to have originally christened the local motorway interchange "Spaghetti Junction", is running a competition to nickname the new store. The current favourite is the Beehive. Whatever the winning entry is, and people think of the new building it is destined to become Birmingham's most famous landmark.