Have you heard about the European city with a lot of old docks, a tough rough reputation and a damp climate that, as part of its regeneration plans, put up an exhibition building clad in titanium? The mind, of course immediately jumps to Bilbao and Gehry's Guggenheim, the first use of titanium cladding in Europe.
But there is another city that fits the bill.
Glasgow has not one but two titanium-clad buildings as part of its science centre, designed by BDP. In these buildings BDP has taken steps to avoid the staining that affected the Guggenheim by using a greater degree of protection during construction. It also has a different surface treatment, which it is confident gives a more consistent result.
Why titanium? Alistair Elder of BDP says: 'One of the nice things was the visual aspect, the durability, the connection with the science centre. It seemed appropriate.' Not only did the architect like the 'space-age' nature of the material - it was also keen on its subtle and imperfect reflectivity.
Cost was not a deterrent either. 'Initially we thought titanium would be expensive, ' said Elder, 'but it was not. It only costs 5 per cent more than stainless steel'.Although titanium costs about £35/m 2on coil, compared with £25/m 2for stainless steel, this is relatively insignificant when compared with the overall cladding cost of about £375/m 2.One reason that the architect went for the softer, more organic form of titanium cladding was because of the rounded, organic shapes of the buildings. The IMAX cinema, which was the first to be completed, is a kind of squashed-egg shape, whereas the exhibition building is more of an elongated dome.
Although they are evidently both of the same family, they are mathematically, and hence structurally, very different.
On the IMAX cinema, BDP accepted a suggestion from Mero, the primary subcontractor for the design, supply and installation of superstructures and cladding as a single envelope. Mero's experience suggested the use of a steel space frame instead of the more conventional structure put forward by the structural engineer WA Fairhurst. 'The space frame was a nice solution, ' said Elder. 'Mero worked out all the elements on the computer and when it came to site it was a bit like a Meccano set. We could be sure that what we got was exactly what we wanted.'
Mero's project manager, Kevin Burke, believes that a space frame was the only solution that would allow the 'box' of the IMAX cinema to fit within the external envelope that the architect had designed. But Elder disagrees. 'One of the problems was that the corners of the space frame had to be 'cut out' to get the box in, ' he says. 'A space frame is usually about 1m deep - that is one of the disadvantages. You have to either take floor area away or increase the external area.'
This is one of the reasons why BDP rejected the idea of a space frame on the main exhibition building. The geometry of the building is based on a section of a torus tilted at an angle. 'This is how we reduced the double curvature, ' says Elder. The structural solution is a diagrid which, by spanning the two directions, can control the forces and create a rigid structure.
'Mero's idea, ' says Elder, 'was to connect the structure with bolts rather than welding it together. This gave us the advantage of no flange connections. All the bolts are hidden.
We got the seamless structure we were looking for without the disadvantages of welding.'
The diagrid means that all the members could be curved to the same radius.
When it came to the cladding, BDP was keen to expose the diagrid on the outside.
The pattern therefore follows the structure, with four tiles within each 'square'. Made by local manufacturer Gilmour Ecometal, the diamond-shaped tiles are about 4.5m square, with two radii induced during manufacture. The geometry means that there are a significant number of repeats across the surface of the building. A pneumatic rig lifted the tiles into place.
On the IMAX the approach was different.
Since the shape varies at every point, the titanium cladding was built up as a series of strips, manufactured on site for precision.
These shingles are applied over a bituminous waterproof membrane, and held in place by stainless steel clips. Beneath the membrane are steel liner trays, filled with insulation, which slot into place between steel purlins attached to the space frame structure underneath.
Mero had two teams working from both sides at once from the bottom up, using rubber ladders to accommodate the curved surface.
When the teams met at the top, the cladding fitted to an accuracy of less than 3mm.
Although Mero was concerned by the 'pillowy' look of the material on the finished IMAX, BDP was delighted. 'It is almost like a foil, ' says Elder. 'We liked the material to express itself. It pillows and the wind can move it.'
Titanium cladding can be stiffened by using a panel backed with aluminium. But as well as denying the essential nature of the material, it would not have been an option in Glasgow because both the buildings' skins have a double curvature.
The titanium was manufactured in the USand then sent to Japan for texturing. This is a relatively new treatment, which gives a more consistent finish than the pickling and wire brushing that was used on the Guggenheim. Pieces from each coil were numbered and positioned adjacent to each other to minimise the effect of any colour variation.
Other problems at the Guggenheim were caused by failure to protect the surface during construction and the existence of sharp points on the building that concentrated rainwater run-off, says Elder.
Glasgow has no such sharp points, and the titanium was protected with a film until construction work was complete. The material will still need cleaning about once a year, and Elder is not prissily perfectionist. He is, for example, unconcerned about the patina of handprints building up at the entrances.
Whereas the Guggenheim, as a magnificent one-off extravaganza, may have discouraged others from thinking of using titanium for cladding, Glasgow shows that it is a reasonable material to use where something different is wanted but the budget is not limitless. Several architects are already specifying or considering the material; and Elder raises one intriguing possibility. Overall, at Glasgow, the weight saving with titanium over stainless steel was about one tonne.
This provided little advantage structurally - one would not want a building that was incapable of being reclad with a heavier material. But, Elder points out, it can be a major advantage for buildings with moving parts. Now there is something to get the imagination racing.