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Economies of Scale in Hull: Scale Lane Bridge

In 2005 a team consisting of architects McDowell + Benedetti, engineer Alan Baxter Associates and main contractor Qualter Hall won an international competition to design a pedestrian swing bridge to span the river Hull. Eight years on, Scale Lane bridge has finally swung into action.

Positioned a few hundred yards north of where the river Hull spills into the mighty Humber, Scale Lane bridge was originally supposed to link a £100M regeneration scheme on the down-at-heel east bank called the Boom to the bustling and relatively prosperous old town on the west.

Backed by the then Regional Development Agency (RDA) Yorkshire Forward and Hull City Council, the Boom was due to open in 2009, but as James Carstairs, project engineer at Alan Baxter Associates explains, things didn’t go to plan: “The Boom sadly went bust and as it was the raison d’être of the bridge the whole scheme was put on hold,” says Carstairs.

Despite this hiatus and others, including floods in 2007 which led the structure being raised by 150mm, and a protracted period of what Carstairs describes as “financial jiggery pokery” – largely caused by the demise of Yorkshire Forward – eight years on the bridge, now backed by client Hull City Council, has finally opened.

The 57m long, £7M structure is constructed from around 200t of steel together with 700t of concrete to counterbalance the cantilever, which spans over the river. It resembles a big black comma or tadpole.

The Boom sadly went bust and as it was the raison d’être of the bridge the whole scheme was put on hold

James Carstairs

The heavy rounded end on the west bank is called the “drum” or “hub”. Sixteen metres in diameter, it houses the plant room and an as yet unoccupied café. Formed of a steel ring truss, the hub sits on 30m deep steel piles driven into the underlying chalk. On top of the piles a concrete ring beam supports a curved track on which 14, 400mm diameter wheels sit allowing the structure to “glide around” and span the gap between banks in two minutes.

Spinning out from the top of the hub’s viewing deck is a two-tier walkway, one side ramped, the other with steps. The two fuse into a single, gently curving 35m long deck on which people can stroll even when the bridge is moving; in fact, Carstairs says the bridge is engineered to take a live load of 1,000 people while opening, a weight they tested by placing 75t of water-filled bags on the bridge as part of the commissioning.

At the point the hub joins the cantilevered walkway – Carstairs calls this junction the “root” is a steel 3D truss. “There is a transition from the 3D truss at the root to a stressed-skin structure at the tip. In between the cantilever is a hybrid of the two,” says Carstairs.

That “stressed-skin structure” is in fact the bridge’s steel floor plates. Near the root the steel plates act purely as an envelope, but the more you progress down the walkway, the more structurally significant they become:

“At the root loads are entirely carried by the truss, not the plates,” says Carstairs, “but nearer the tip the stressed-skin does all the work.”

We made it as stiff as possible but the issue was that the tip has to ‘land’, so it can’t be too high or too low

James Carstairs

The 35m-long cantilever was, says Carstairs “quite a difficult structure to achieve”. “On plan it curves and it wants to twist; we took a lot of time to design a structure that addresses the various torsional and bending stresses and to establish how the stresses were shared between the stressed skin and the 3D truss,” he adds.

But while Baxter has designed the bridge to be “as stiff as possible” the engineers also had to allow for a significant amount of deflection: “People can walk on and off at the west bank as the bridge is moving. We don’t know how many people will be on it at any one time,” says Carstairs.

“We made it as stiff as possible but the issue was that the tip has to ‘land’, so it can’t be too high or too low”.

“To keep it aligned we have a wheel fixed to a hydraulic ram. As the ram extends it pushes itself up against a ramp on the east bank; it has the capacity to lift the tip up with a full design load of 1,000 people,” says Carstairs. 

Having already taken eight years to realise it may be many more years before the desired regeneration on the eastern shores is complete, but in the meantime at least the city’s residents have an interesting and, dare we say it, iconic way of getting there.

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