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Whale of a bridge

The city of Hull is paying homage to its whaling past with a spectacular new swing bridge.

A world away from the traditional tension systems of normal competition winning bridges, a new footbridge will soon be opening up the dockside of Hull in more ways than one.

Sited close by the historic city centre, it will cross the River Hull, linking the city centre with a proposed new development on the east bank of the river.

The bridge's distinctive whale-like form will create a unique landmark for Hull, and the fact that it moves and the public can stand on it while it moves, will bring many an intrigued visitor to the area.

As an added attraction, a restaurant will be located at the hub of the movable structure.

The idea of a meeting place on the bridge was at the heart of the design. "The initial concept was more traditional with masts and cables. But half way through we had discussions and asked what would make it different?" explains Alan Baxter Associates project partner Jim Gardiner. "The concept of a hub and a meeting place was hit upon. Hull was the centre of the whaling industry and the concept of the whale was thought of."

At one end of the bridge is a circular drum structure made from a three-dimensional Vierendeel girder ring approximately 16m in diameter. This houses the restaurant and the turning mechanism which enables the bridge to rotate when large vessels need to pass.
From this ring springs a 35m long cantilevered steel spine, which arches up and over the river, allowing enough room for smaller boats to pass under without needing to open the bridge.

A massive 675t of concrete ballast is needed on the ring structure to balance the overturning moment from the huge cantilever. This is achieved by using concrete spanning between radial steel beams at the top and in the base of the ring structure. Concrete for the upper level is 450mm thick, while that on the lower level is 750mm thick.

Not only is 35m a long way to cantilever, but Alan Baxter Associates has agreed with the local authorities that 1,000 people can be allowed on the structure when it is moving very slowly.

When the bridge is shut, hydraulic wedges are placed under its tip to ensure that it is spanning from one side to another.

Careful coordination had to be given to services. Many moving bridges have to deal with electricity for street lighting, but not many have to provide the services required for a restaurant, which needs electricity, gas, water, drainage and telecoms. The centre of the restaurant hub is the only part of the bridge that does not move and so this had to be the point where drainage enters and exits the structure.

All services other than drainage are routed through a utility management chain system to allow unrestricted movement during full bridge rotation.

This is like an automatic winder where the services can unroll, extend and then fold back up as the bridge moves.

The bridge evolved from a design competition in 2006 for a "new concept" pedestrian bridge for Hull Citybuild which wanted to develop and regenerate the city's Eastern Banks.
Architect McDowell & Benedetti teamed up with engineer Alan Baxter Associates and contractor and M&E designer Qualter Hall to prepare the design, which is being funded by regional development agency Yorkshire Forward, Hull City Council and Hull Citybuild.

Contracts to design and build the £6.6M bridge were signed last month and piling work is due to start on site in February 2009.

DESIGN

The design team has achieved the bridge's unusual shape using a composite stressed skin and diagrid structure.
"The design evolved from a space frame," says Alan Baxter project engineer James Carstairs. "It's a three dimensional diagrid structure with steel plate added to it which acts as a stressed skin like a box girder." The steel plate is sized to the load it is carrying and its thickness varies between 6mm and 12mm. "The geometry would have been difficult in stressed skin alone," says Gardiner. "Here, the 3D steel truss forms the shape which is then clad in steel sheets."

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