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Sliding - A recipe for success

Bridges - A new footbridge has been hauled into place with the help of an unusual ingredient. Andrew Bolton reports

Cook 'n Bake is a name familiar to South African cooks.

But the proprietory cooking oil has found another use away from the kitchen. In the UK it been used to smooth the jacking across railway lines of a massive bridge forming part of Arsenal Football Club's new stadium in north London.

'We used it to reduce the coefficient of friction, ' says Simon Fryer, a senior engineer at UK consultant Buro Happold. Apparently Cook 'n Bake is the best lubricant on the market when it comes to sliding heavy objects into position.

The bridge had to slide across three temporary supports plus Teflon coated pads mounted on jacks - placed on the bridge's permanent pier and abutments - during a strand jacking operation last month. Liberal quantities of the oil reduced the coefficient of friction to below the 2.5% to 3% figure achieved using the Teflon alone, making for a much smoother operation.

When complete, the bridge will be able to ferry 16,525 spectators every eight minutes above Drayton Park's busy twoplatform commuter railway station and over a parallel freight line.

But designing the bridge was not as straightforward as might be expected. The crossing has to avoid a series of underground and above ground obstacles while maintaining a relatively shallow gradient.

Above ground, engineers had to squeeze down the height of the deck, while maintaining a 5.88m clearance for the railway's overhead electrical lines. In addition, the bridge must connect with the local Drayton Park road at grade so that emergency vehicles can use it to reach the stadium.

Underground obstacles also posed a significant constraint on the positioning of the three temporary supports and the permanent pier for the bridge.

Two shallow London Underground Victoria Line tunnels plus a Thames Water sewer cut across the site, restricting the location of piled foundations.

Drayton Park commuter station was a further obstacle.

The bridge is a 105m by 90m by 35m wide structure skewed at 22.5infinity at its western end and 45infinity at the other. During the jacking operation three temporary steel towers were used to support it as it made its way across the 60m gap between the permanent pier at its western end and the eastern abutment.

Parapets comprise trusses made of 100t of tubular steel, all produced by specialist steelwork subcontractor Watson Steel.

The trusses' top chord comprise double booms welded to pairs of vertical struts, which converge at the 1m by 1m box girder forming the bottom chord.

Parallel inclined struts then connect the junction points at the top and bottom chords to create a lighter looking structure than a more conventional truss made up of fewer, but chunkier members.

Watson started delivering the steel to site in sections just after Christmas, before assembling it off line in a gap in the stadium's outer wall left by main contractor Sir Robert McAlpine.

Ahead of the slide, the structure was fitted with a steel 'nose' or guide and the front was raised 9.5m above the ground to line it up with the tops of the Teflon pad supports before the launch began.

The bridge had to follow a curved launch trajectory in the vertical plane, so it could achieve the railway clearances needed. Jacks mounted on the temporary and permanent works also allowed contractors to raise or lower the trajectory, guiding it to the next support.

At the same time, the single permanent pier and three temporary supports were erected along the bridge alignment.

The temporary supports were needed to steady the deck as it made its way across the gap between the permanent pier at the western end of the structure and the eastern abutment.

Specialist contractor ALE Lastra then fitted two sets of strand jacks to each side of the deck, anchoring the jacking cables to the permanent pier immediately to the east of the structure. With everything in place and a 41 hour track possession ready, the spray-on cooking oil came into action and jacking began.

In all, three weekend possessions were needed to position the bridge.

Removal of the temporary supports and snagging took place late August, when final traces of Cook 'n Bake were also removed from the underside of the deck.

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