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Cold steel for a Cold War

STEEL

Wolverhampton's RAF Cosford is to be the site of the world's fi rst museum dedicated to telling the story of Cold War aviation. The twisting structure supported on huge steel sections will house 20 warplanes but is also designed to create a backdrop in keeping with the threatening era it celebrates. Joanna Booth reports

RATHER like the Cold War itself, the new museum at RAF Cosford looks completely different depending upon the angle from which you are looking at it. Walls start as vertical faces at one end and twist in opposite directions to a 30-degree pitch at the other. If you stare too hard, a sneaking suspicion forms that the whole thing is some sort of optical illusion.

'It is quite hard to get your head around, ' admits Steven Cheshire, project manager for the main contractor, Galliford Try. Computer modelling was used extensively at design stage but to really get to grips with how the building fitted together the team had to make a scale model of the finished structure ? even filling it with tiny versions of the planes that would be displayed inside.

The architects, Feilden Clegg Bradley, designed the building around the theme of the Cold War's opposing ideologies.

The steel sections are fixed in a pattern that slowly rotates from a fixed line, creating two triangular spaces that meet at a fault line that symbolises the clash of east and west.

Galliford Try won the £9.8 million contract to build the structure and started enabling works in November 2004.

The sandstone underneath the site meant that huge concrete strip foundations were enough to support the load.

The frame of the structure is entirely of steel and must support not only its own weight but also the load of seven of the exhibited planes that will be hung f rom the ceiling. The building cent res along a straight spine, at the apex of which is the major box roof t russ. Each end of this is suppor ted by a st retch of ver t ical wall.

These are made from substantial 'I' sections and take most of the building's weight. Pivoting off the main roof truss are hollow tri-fold steel sections, which swing out from the vertical ? where they meet the spine walls ? to 30 degree angles at the other ends.

The shapes these make are technically described as hyperbolic paraboloids, but to the eye the finished structure resembles a steel tent viewed through a distorted fairground mirror.

The roof follows the curving line of the huge trusses that form the building's frame. Half way along the 135 m length, the f loor level drops by 5 m, opening up extra glazed space at the base of the building that increases light levels. At its widest point the building measures 60 m ? only 10 m more than the wingspan of the largest plane it will house.

Along with the architect and Michael Barclay Partnerships, the structural engineers, steelwork contractor SHStructures has been involved in the project since the design stage. The firm specialises in the design, fabrication and erection of complex structures and has worked on projects such as B i rm i ng h am's Bu l l r i ng , L ondon's Eve l i n a Children's Hospital and the huge Anish Kapoor Marsyas installation in the Tate Modern's turbine hall. The work the company contributed on the detailed design proved invaluable. 'Without them we would have been st ruggling, ' Mr Cheshire adm its.

'It's a technically challenging project, ' adds Tim Burton, SHStructures sales and marketing manager.

'It's an asymmetric portal frame. Surfaces curve in two direct ions but are formed by st raight members.' As a result, the f lat structural decking hits the tubular members at constantly changing angles. The SHStructures solution was to attach saddles to the steel tubes to provide a f lat surface to which to fit the decking.

The project uses 1,340 saddles, all mounted at slightly different angles and at different centres on each truss.

'We used a computer modelling package called Tekla Xsteel, ' Mr Bu r ton explains. 'We've just come second in the software company's worldwide modelling competition for our work on the Welsh Assembly building.' A mock-up of a 9 m bay was fabricated in the company's works to ensure that the system would function satisfactorily. Instead of leaving the saddle surface f lat, SHStructures f itted a central stud to each one, allowing the roofers to fix slats straight on to each saddle.

The large trusses, each a different size and shape, were fabricated at the factory in North Yorkshire to their full lengths, and then split up for transport. On-site welded connect ions were replaced with bolted splices as par t of a value engineering exercise, significantly reducing costs.

'There were big tolerance issues with the bolted pipe f lange details, ' says Mr Burton. 'Some truss sections have 12 interfaces, each with 16 individual bolt holes to align. If even one was out of position the whole thing wouldn't fit together.' Time spent on the shop f loor making sure these were correct paid off, and the entire frame went up in 10 weeks. 'It's like a big Meccano set, ' says Mr Cheshire.

The 631 tonnes of steelwork were transported to site in sections and erected using four cranes at any one time. The centre of gravity for each lif t had been calculated by computer sof tware before reaching site. This saved t ime as lif ts were oriented correctly from the off.

To create an industrial, utilitarian feel, the steel sections were left exposed and unpainted. The members were shotblasted to limit the development of mill scale and soon rusted to a dark orange colour. At low level, where the public could come into contact with them, they are laquered for safety reasons, but no other t reatments will be applied.

'This will have no adverse effects on the long-term structural integrity of the building, ' says Mr Burton. 'We've not seen untreated steel of this type used before in a structure. There is a special type of weathering steel, used on the Angel of the North, but this is only available in plate and a limited number of sections. It would also have blown the budget. Within the closed environment of the hangar the steel we have used will be f ine.' Apart from the vertical spine walls, which can be clad from mast climbers, all roofing, glazing and cladding materials have to be installed by roped access, owing to the difficult angles of access. A vapour barrier to make the structure airtight is installed beneath the cladding sections. The open ends are covered in a translucent plastic fabric curtain to lower light levels and allow the atmosphere inside to be controlled.

Init ial const ruct ion will be complete in Apr il next year but the Galliford Try team will have to return in August when the aircraft are being installed.

'The fabric end walls have to be removed to get the planes in but need to be re-erected so the wind doesn't get through, ' Mr Cheshire explains. 'It's gusty here and there is a worry that if the wall is down and the wind gets up one of the planes ? or even the building itself ? might take off.'