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Structural hybrid

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MetalWorks Major Structures - Chuck Hoberman is the man behind the world's largest unfolding structure, which was used at last month's Winter Olympics

Damien Hirst may have blown a toy up to an enormous size and called it art, but his giant creature did not do anything. How much more satisfying then when inventor Chuck Hoberman saw one of his creations magnified and not only serving a purpose but shown on television screens to millions of people who had never heard of him.

Hoberman's fame so far has been restricted to his award-winning toy globes, the Hoberman Sphere and Mini Sphere. This same technology has now been transformed, with the assistance of engineer Buro Happold, into the world's largest unfolding structure. Still not familiar? Think of the medals plaza at the recent Winter Olympics in Salt Lake City, and Hoberman's was the structure at the centre of it.

This is not his first venture into largescale design.He designed a retractable dome for Expo 2000 in Hanover and has designed several 5.5m expanding and contracting spheres that are in museums around the world. But the Salt Lake City structure, at 11m tall and 22m in diameter, is in another league. Clever invention is admirable but whereas a toy that collapses unexpectedly could be amusing, similar behaviour in a large-scale structure would be unacceptable and dangerous. Between them Hoberman and Happold produced a structure that is reliable and fascinating. Its 96 aluminium panels retract to form a 1.8m-thick semicircular ring that frames the stage. They spiral upwards and outwards in a radial motion, revealing the stage behind.

It should perhaps not be described as a 'structure' at all. Buro Happold concludes that the arch is 'a kind of hybrid between a structure and a mechanism: at once stable and self-supporting yet capable of largescale movement'.

Hoberman described it in the following way: 'The conceptual design of the arch pioneers the overlap between mechanical and structural understanding as it is both a mobile mechanism and load-resisting structure. The final design is successful as it incorporates members as both operable machinery and load-carrying components.

The main struts, for example, that provide both the operational geometric symmetry and mechanical impulse, also provide the strength to carry panel loads. In the same way, the tension cables both pull the screen open and support the structural weight.'

The 96 panels of the arch each measure approximately 2.7m by 1.5m and have three-pinned joints. The primary members comprise aluminium box sections that form the rigid elements of the unfolding mechanism. So that retraction can occur, each beam type is offset in depth to form a layered system, and each successive ring of panels is laid on top of the previous ring.

The beams that face out towards the audience are clad to form a skin.

There is secondary framing that provides the outlines of the panels to which the sheathing or skin is applied. These panels vary in size up to 2m by 2m.When the arch is extended (closed), these panels form a 'lamella' structure whose members display a pattern of interlocking spirals.

Anything new can seem obvious in retrospect, once the objectives have been achieved. For Buro Happold, much of the work involved was to get to this stage, and to help make Hoberman's vision achievable, and prove that it was achievable.

The engineer developed a unique support system to cope with the complex geometry, the loading from both wind and structural weight plus the sheer size of the structure. Tension cables attach at the centre of the semi-circle with the primary role of sustaining the self weight of the screen.

Cable sheaves transfer the load to an independent steel arch structure, which also support the loads generated by the guided slots in the outer screen panel. The lowest panels at the base of the arch run along a sliding track which enables the complete system to retract and also partially supports the vertical and horizontal loads.

To show that this approach would work, Buro Happold used 3-D modelling techniques to run trial simulations.

With high winds in the opening days of the games, the value of these calculations was proved. Spectators who marvelled at the way these winter sports push the human physique to its limits will also have had some inkling of the fact that the way the awards structure folded and unfolded was not merely a piece of theatre. It also represented a triumph of human ingenuity equivalent to the athletes' achievements.

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