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Structural connection

METALWORKS

The new truss bridge linking Gatwick Airport's new Pier Six borrows its concept from the human spine and ribs

If you have flown out of Gatwick recently you might just have noticed a daring bridge leading off from the North Terminal to a new satellite building. For some months it was a massively long and slightly twisted structure under assembly along the landside boundary.

Then all 2000 tonnes of it drove itself very slowly towards its final destination a mile away. Jacked up 22 metres on to its two intermediate supports and, with their connections to the buildings at each end installed, it now spans 198 metres between the exiting North Terminal and the new £100 million GMWdesigned satellite, Pier Six.

This new structure is by that engineerless architectural practice Wilkinson Eyre, which brought us the Gateshead Eye and, a string of innovative and quite beautiful bridges during the past decade.

The practice has worked with a number ofengineers; this time Arup - fresh, at the time, from sorting out the wobbling Millennium footbridge. Wilkinson Eyre associate director and lead architect Martin Knight explains: 'We looked at arches and cable stays and other alternatives but very early on we came to the conclusion that we would need a structure which was fully clad. That indicated a tube: either a structural tube or a truss on to which you hung cladding. The spine truss model won.'

In its final form, the profile of the Gatwick bridge is more than a little like a classic bending-moment diagram. That, Knight says, is not entirely accidental. 'It is because the form of the bridge has been driven by the structural demands. This was the best form in terms of performance and the impact of sightlines to the control tower.

[The top of the arch section is 34m above apron level. ] It should look like a structural diagram because that is what it is.We believe that you should be able to understand how structures work from looking at them.'

'The way the bridge works is simple and effective, ' explains Stephanos Samaras, lead engineer and associate director at Arup. 'It will be a large continuous frame fixed on piled foundations when it is complete, although it is a large, simply supported structure during its prefabrications and erection stages. The structural concept of supporting the external envelope and providing central segregation between arriving and departing passengers is similar to the concept of the human spine and ribs. A combined box steel-plated girder with a spatial truss provides the strength of the spine;

floor and roof beams cantilevering from the spine at either side support the floor deck with travelators, and the roof with services.

The external structural skin is completed with struts hidden within the glass wall.'

Roof and deck are joined by the steelwork supporting the canted glass wall, a unitised system from Schmidlin that has enough tolerance to cope with the deliberately warped geometry of the skin. The warping is a consequence of the shallow arched form of the upper longitudinal members, the fact that the deck remains straight and level, the unvarying 11infinity canting of the side walls and the decision not to vary the width of the roof.

It means that at mid-span the floor is slightly waisted, a bit less than a metre. The small child in you warms to the fact that there is a 10m gap in the travelators here to allow people to peer down as aircraft pass underneath.

Plane spotting does not get better than that, and this is the only place apart from Denver airport where you can do it - and Denver is a much shorter affair. The glazing, you hardly need reminding, is of the same security specification as anywhere else at Gatwick where people can watch the aircraft.

The bridge is currently being assembled, mostly from prefabricated elements. Knight says: 'As much as possible has been built in, even the glazing.' The structure currently sits on two transfer beams. These are at much the same location as the two future intermediate props - Y-shaped in elevation, A- frames in cross-section. The engineers had to design the whole ensemble for assembly as a deck segment of 163m with two 17m backspans - which will be removed for the drive to the site. Once jacked up in place and with these backspans re-installed, the whole structure will become a three-span continuous beam.

You worry about breaking glass, but this has all been thought through. The route from the airport boundary to the North Terminal has been devised and, Knight says, 'the axle load is less than a jumbo jet'.

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