Echoes of Victorian railway technology will mark out the new bridge over the M11 in Essex as one of the most unusual on the UK motorway network. NCE reports.
As earthworks begin on the £38M Stansted Airport slip roads project at Junction 8 on the M11, the last details of one of the key elements in the scheme are close to being finalised at structural engineer Gifford's Southampton offices.
To carry traffic from London across the motorway and into the airport a long ramp and bridge are needed, with the bridge itself on a tight 280m radius.
So acute is the combination of radius and skew that the structure can only be supported by two columns on the northern side and one at the south, where the longest span is effectively 50m.
'The illustrative design used 2.8m deep plate girders below the deck, ' reports Gifford associate Bib Wrigley.
'This not only looked heavy and clumsy, it meant higher approach ramps to maintain clearance above the motorway.'
Working in a partnering arrangement with main contractor May Gurney and 50:50 joint venture clients BAA Stansted and the Highways Agency, the Gifford team took an alternative approach. Victorian rail engineers, faced with similar problems of minimising embankment height while maximising head room below their underbridges, often chose to support their tracks off the bottom flange of the main bridge girders, rather than the top. These were normally dubbed 'through girder' bridges. Gifford decided to use the same concept - but with a twist.
'We went for box girders just 3m by 1.6m, which meant road deck and parapets could both be contained within the depths of the boxes, ' Wrigley says.
'Visually it's a great improvement. But the real benefit is the ability to reduce the size of the approach embankments, which will be more than 1m lower.'
The new approach also eliminated one of May Gurney's main worries, the complex erection of the curving plate girders on the original design. 'Potentially unstable, ' is Wrigley's verdict.
One practical problem with steel box girders is the maintenance of the inside faces. Safety regulations mean repainting work in enclosed spaces is complex and costly. Gifford has neatly avoided this problem by specifying corrosion resistance Cor-Ten steel up to 50m thick for the boxes - which will be painted on the outside only.
However, the basic asymmetry of the structure still remains.
One box girder will be made up of three sections, the other of two. To simplify both design and erection, the 140t radial cross girders will be pin jointed to the box girders.
Even since the slip roads project was officially opened by new roads minister David Jamieson in early July, some more key modifications to the original design have had to be made. 'Recent sheet pile driving tests have shown that the ground is hard and stony, especially to the west of the motorway, ' Wrigley reports.
'Original abutment design was sheet piling. But with the potential problems on the western side and a shortage of fill, the decision was taken to switch to an approach ramp on columns on the west. The eastern abutment will still be sheet piled.'
The western ramp will be supported on more predictable bored piles and connect to the steel bridge proper at a concrete crosshead some 5m above ground level. Although the through girder concept has been around for a long time, there is thought to be only one other such bridge on the UK motorway network.
'It's on the M3, near Winchester', says Wrigley. 'And I designed it 10 years ago, while I was working with Mott MacDonald.'