Those clever architects at Feilden Clegg Bradley didn't take the easy route to their RAF museum pavilion in Hendon. Alex Smith divebombs on the challenges of cladding a semicircular roof in stainless steel and lining it with tensile fabric
The new RAF Museum pavilion at Hendon is a planespotter's delight. The museum is home to some of the most memorable planes ever built. In the pavilion's first exhibition – Milestones of Flight – a Harrier jumpjet hovers with intent near a Messerschmidt, while a Sopwith Camel jostles for wing space with the spanking new Eurofighter Typhoon.

It's not surprising that architect Feilden Clegg Bradley took its inspiration for the pavilion from the historic planes rather than the mundane hangar building that is the typical home of historic aircraft.

"We wanted to get away from a metal box," says architect James Risebero of Feilden Clegg Bradley. The design team has certainly done that. The semi-circular building resembles an oversized aircraft fuselage and even has riveted stainless steel panels to mimic the facade of modern aeroplanes. The interior, meanwhile, has a tensile fabric backdrop for the planes. These semi-translucent panels are a visual reference to vintage planes such as the Sopwith Camel, which was created by stretching fabric over a wooden frame.

Suspending the planes from the ceiling was just one of the technical challenges for the project team (see "How's it hanging?"). The team also wanted to flood the exhibition space with daylight without degrading some of the most fragile exhibits with UV rays. The specification of the fabric was tricky too. It had to offer 40% translucency but also be resistant to dirt where it was within reach of sticky fingers – fabric meshes required to diffuse light are also ideal for trapping dirt. And the exterior stainless steel panels presented specification challenges as the thin skin can ripple if not handled with care.

The barrel-shaped form of the building offers Feilden Clegg Bradley several advantages, says Risebero. "It's very simple and easy to read and an efficient way of enclosing space. It offered the maximum amount of space for the money we had. It's also a very stable structure from which to hang planes," he says. The structure of the pavilion comprises 13 steel arch trusses at 6 m centres, which sit atop 4 m high concrete fins. The barrel vault has an internal height of 14 m and a span of 26 m. The pavilion also houses mezzanine boxes and walkways, which provide smaller exhibition and audiovisual spaces as well as a cafe and toilets.

A central rooflight fitted with polycarbonate is one of the main sources of light in the pavilion. The others are the glazed ends of the pavilion and a low strip of glazing running along the west elevation. The rooflight has a UV-protective coating but the architect was still keen to avoid sunlight falling directly onto the fragile exhibits below. Feilden Clegg Bradley came up with a solution incorporated in a galvanised steel maintenance walkway. It has incorporated blades into the steel mesh walkway, which are angled to prevent direct sunlight from ever reaching the exhibits below.

Feilden Clegg Bradley relied on the specialist knowledge of structural fabric company Base Structures to specify the tensile fabric. The company took the architects' initial drawings and calculated the geometry required to realise the architect's designs. It requires a specialist like Base to incorporate double curvature geometry into the fabric – this is essential as it offers the fabric strength and ensures rigidity.

The tensile panels are hung between the columns and are fixed from steelwork at the apex of the arch to a horizontal cable that is fixed to the bottom end of the fabric. The cable can be tensioned once in position to reach the desired level of tautness in the fabric.

The architect wanted to specify one fabric for the interior but Base pointed out during the design process that a different finish would be required where the fabric is within reach of visitors. "The architect wanted a mesh fabric over the bulk of the interior, but we were concerned about the first floor walkway," says Mark Smith, director at Base. "If people touch it with sticky fingers it will make it uncleanable." The solution was to add a lacquer to the PVC-coated polyester fabric. The polyvinylidene flouride lacquer fills the holes in the mesh, and gives the fabric durability as well as making it easy to clean.

This seemingly simple process had a knock-on effect on the specification. The lacquered fabric stresses more than uncoated PVC fibre glass fabric so Base had to take this into account when cutting the panels. Welding the lacquered panel to the unlacquered panel is also tricky. Before the edges can be welded using microwaves, the lacquer must be ground off. Base is one of the few UK companies that can offer this service.

Between the fabric and the stainless steel panels are floodlights. These are hidden from view by the fabric but throw a soft diffused light onto the exhibits. There is also a sprinkler system in this space and above that is the built up roof, which provides the roof's waterproofing. This consists of insulation sandwiched between two metal decks. The decks rest on purlins suspended between the arch trusses.

Above the deck are spacer bars, a stiffening sheet and finally the stainless steel metal panels. As these panels only act as a rainscreen it means that the drainage is concealed and the building has a smooth, curved surface – like a plane. To keep the shape free of clutter, Feilden Clegg Bradley used "gargoyles" to throw rainwater clear of the facade into a gravel filled drainage system on the ground.

The installation of the panels was tricky. The architect specified a 1 mm thick stainless steel panel, which the main contractor reduced to 0.6 mm to save money. This meant that the installer had to be careful not to dimple the steel when riveting it in place (see "The secrets of stainless steel", below). To prevent excessive glare from the stainless steel, the architect specified a textured finish. This not only diffuses the reflection but also imbues the panel with more strength.

The finished building is a splendid contrast to the older sheds at Hendon, and Feilden Clegg Bradley have obviously enjoyed the exercise. It has been chosen to design another RAF museum at Cosford. This is a much bigger project, and will feature one of the biggest tensile structures in the country (see "Cold war at Cosford", page 06).

Following the successful collaboration at RAF Hendon, Feilden Clegg and Bradley will once again be working with Base Structures to realise the design and installation of the fabric panels. The completion of the next production from those magnificent men and their welding machines is expected in 2006.

Cold war at Cosford

Feilden Clegg Bradley is designing an even bigger pavilion at RAF Cosford to house planes from the cold war era. Initially the architect considered using ETFE fabric to enclose the whole 6200 m2 space but in the end opted for a standing seam roof on a structural metal deck and vertical cladding to the central spine wall. One of the main reasons for the change was the high light transmission of the ETFE foil cushions, which could expose the planes to excessive levels of UV light. A single layer fabric membrane made by Base Structures will be used for the end elevations. With a translucence of 6% this allows diffused daylight to enter the interior.

How’s it hanging?

Project engineer Buro Happold supplied the exhibit contractor, Unusual Rigging, with a summary of the load capacities of the aeroplane suspension points on the roof trusses. The contractor worked within these load limitations to design the cable supports for each plane. The support points were positioned at the nodes where the diagonal truss members meet the lower member of the arch trusses. At this point a steel box with a threaded hole was welded to the arch truss and a Starpoint eyebolt supplied by RUD was then screwed into the assembly. The load limitation of each support point – there are nine at about 3 m intervals on each arch truss – depends on their position on the arch. The support point that can take the greatest load is at the highest point of the arch truss. The specification of each eyebolt is the same even though they will take different loads. Unusual Rigging hung each plane from four or five support points. The RUD Starpoint eyebolt can adjust to the direction of the pull, which meant that the aeroplanes could be positioned virtually anywhere in the exhibition space, providing the loading capacities of the nodes were not exceeded.

The secrets of stainless steel

Stainless steel has appeared on the roofs of some notable buildings recently. Frank Gehry is a fan and has used it on his Experience Music Project in Seattle and at the Maggie’s cancer therapy centre in Dundee. Daniel Libeskind has also given London’s Holloway Road some polish in the form of a steel-clad extension to the London Metropolitan University. Stainless steel can be a difficult material to work with so when specifying, you should bear in mind the following:
  • When selecting steel panels specifiers should always pick a 316 grade of steel for external applications, according to Tim Childs, managing director of Rimex, which supplied the panels for Hendon. The other grade of panel used is 304 but this should only be used internally (except for swimming pools when 316 grade should be specified). The 316 grade has a higher nickel content than the 304 and it contains molybdenum, which the 304 grade does not. This makes the 316 grade more resistant to weathering, acid rain, pollution and other elements that can corrode stainless steel. When budgeting, specifiers should bear in mind that the 316 grade is 15 to 20% more expensive than the 304 grade.
  • Make sure you specify the correct finish. Both 316 and 304 grades of stainless steel are sold in 2B (dull) and BA (bright) finishes. The Hendon roof has a Pearl Silver BA finish. To reduce glare on the panels at Hendon, the stainless steel was rolled through a patterning machine. This not only reduces the reflectivity of the stainless steel, it also improves its strength by making it more rigid.
  • Specifiers should be aware of bi-metallic corrosion. Childs says that to avoid corrosion stainless steel should never come in contact with less noble metals such as aluminium. Seek advice from the British Stainless Steel Association
  • When working with stainless steel metal sheeting the contractor must make sure it installs the panels the right way round. No matter how highly polished the stainless steel, there will always be a grain in the metal. If the panels are not fixed in the same direction, the eye will be able to discern different tones of colour in the roof or facade.
  • When ordering stainless steel sheeting always make sure that your delivery is from the same batch of coiled steel as the colour will vary slightly from coil to coil.
  • Create a mock-up of your proposed stainless steel roof. Richard Rogers architect Kevin Gray said this was the most important thing he did to ensure the specification and installation of the curved stainless steel panels at Antwerp’s new law courts went without a hitch.
  • When designing the roof bear in mind that most steel panels come in widths of 1000 mm or 1250 mm though 1500 mm is possible, according to Childs.
  • Roofing