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Stepping into the future

Graham Parkhouse has a passion for highly efficient 'diluted' structures where each member is arranged to make the maximum contribution to the overall strength. Mike Winney reports on his latest project.

Graham Parkhouse is patenting the concept of a tubular multiple cable bridge for very long spans. The plan is not only to top Akashi-Kaikyo's near 2km world record for a conventional suspension bridge but also to create multi-span structures stepping between floating piers across waterways such as the English Channel.

If the idea sounds familiar then think back to the 1980s.

Parkhouse, then with Mott MacDonald, was an entrant in this magazine's Image of the Bridge of the Future competition.

Shortly afterwards, his almost botanical concept of a featherweight strut for use in space fabricated using repeated cellular dilution of material featured on a cover. At the time he had the idea of a multi-cable long span bridge but was not clear about how the structure could be made sufficiently stiff to be stable.

Since then Parkhouse has spent several years at Surrey University researching an 'integrated approach to theory of structures'. Now he is with WS Atkins' structural integrity division. 'We advise on ailing structures in the nuclear, offshore, civil and mechanical engineering industries, ' he says.

Parkhouse has also worried away at developing the multicable bridge idea, turning it into something that might actually become a real bridge. 'You can't patent something you can't build, ' he quips.

And Parkhouse thinks he now has the answer to buildability.

Wrap the main longitudinal cables in a double helix of ropes to form a tubular mesh and tighten up the helix wraps to create a waisted shape net structure. The helical paths of the outer wrapping would approximate to a geodesic form, taking the shortest path around the main cables.

A bridge deck can then be assembled inside the tube and hung from the helix ropes. This deck could be very light as it is only required to span between the individual ropes. It does not have to be a stiff beam like a conventional suspension bridge deck because the stiffness of the structure derives from the multiple main cables and constraint provided by the helical ropes.

'The nice thing about this is that you can't see the bridge. You can have a very efficient structural depth but not have an eyesore.'

Horizontal wind forces and vertical loading from traffic is all transferred directly to the cable net. 'You've got strength and stiffness in the net, ' says Parkhouse.

He envisages that the main cables would be similar to but of smaller diameter than normal suspension bridge cables. There would be about 12 of them, not necessarily all the same size.

They would be spun conventionally but across saddles set on pear-shaped collar structures forming the bridge piers. Multiple spans are possible and preferred as a normal arrangement, since the concept is intended for long crossings.

Cable ends would have to be deviated over the first and last piers to anchorages formed in the ground. But the intermediate pier substructures could be pontoons, envisages Parkhouse, because of the inherent stiffness of the structure.

Wrapping the relatively small diameter helical ropes is likely to require the development of novel travelling ring frames which would ride on some or all of the main cable catenaries.

Parkhouse envisages that when these ropes are tightened the mid span diameter of the net would be about half that at the piers.

Precisely how these geodetic ropes would interact with the main cables during this tensioning operation has yet to be worked out. A wood and wire model spanning about 1m was built earlier this year. With the geodetic ropes at radio aerial wire scale they simply slip along the main cables until they find stability.

Contemplating the abrasion resulting from that operation on a full scale structure is something else. Rubbing strips would be needed to avoid damage to the main cables' long term weather protection. And once the ropes are in their geodesic position some sort of rope-tocable connector or clamp appears to be necessary.

In service the geometry will change according to factors such as the load on the deck, wind and temperature. Hence there will be movement and friction between the geodesic ropes and the cables which would determine the dynamic behaviour of the bridge. Its static form would be a series of long humps with the high points at the middle of each span.

Erection of the deck units might require some sort of overhead ropeway to feed them along within the net. Lifting large assemblies into place from barges would be ruled out because they cannot be pulled up through the invert of the cable and rope net. Once the deck is in position and distributing its point loads to the helical ropes it is possible that the lower main cables could be de-tensioned and removed since they become redundant.

These are things that can best be resolved in relation to a real project. No one would put money into huge long span bridge based a totally untried concept and Parkhouse hopes to start with something modest - perhaps a medium to long span footbridge.

Sponsorship is being sought to help 'work out the practicalities and find out how economic a bridging system it is.' The civil engineering department at Surrey University has already confirmed its support.

First full size structures are likely to be built using conventional materials rather than composites and carbon fibre cable. A characteristic of the design would be very flat and consequently highly stressed cables, which would make special materials an attractive proposition.

But Parkhouse is wary of taking a radical approach to bridge design and complicating it with the unknowns of relatively new materials all in one step.

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