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Meditations over the Mersey

With competitive dialogue now underway on the Mersey Gateway, shortlisted bidders are embarking on a robust re-examination of the bridge design, which could see aspirations for an iconic cable-stayed structure abandoned in favour of a cheaper, more austere alternative.

Client Halton Borough Council opened competitive dialogue with its three shortlisted bidders in February and is keen to see innovation that will ensure greater value for money.

Bidders are now scrutinising every aspect of the £600M project, which involves constructing a 1km long bridge with three lanes in each direction over the Mersey Estuary along with 8.5km of associated new and upgraded link roads.

During development of the bridge’s specimen design in 2006, the Commission for Architecture and the Built Environment called for the gateway to be a “symbolic landmark” and urged Halton to “ensure that the fi nal structure will be of the highest possible quality in order to produce an elegant and distinctive bridge”.

Having survived the 2010 Comprehensive Spending Review, the project did, however, suff er from a renewed desire for cost savings, in much the same way that other public works schemes did.

What is a gateway structure?

Halton confirmed in February it intended to drop future proofing on the bridge in the form of a double deck to accommodate light rail in a bid to save money, and is now likely to be more open to a less iconic design delivered at a cheaper price.

“What is a gateway structure?” asks one source close to the project. “That’s what the bidders will be asking of themselves and the client.”

The source says the competitive dialogue stage is being used to find out what areas the client is “pushable” on and what is “non-negotiable” with regards to the bridge design.

During the development of the specimen design for Halton by consultant Gifford (now Ramboll) a three tower, four span cable-stayed bridge was selected as the best option, because it was the cheapest and had the least negative environmental impact.

Whatever new alternative design is developed, restrictions on the bridge remain the same - it can have a maximum of three towers in the estuary, to protect river fl ow and wildlife, as well as for navigational purposes, and they must be no taller than 150m because of planes using nearby John Lennon airport.

So how different could the structure look?

Bridge types such as box girder and arch bridges that were previously ruled out could now be reconsidered.

“The abandonment of the double deck opens up many possibilities for the designers,” says Arup associate director Steve Kite, who worked on the crossing design for the Vinci and Laing O’Rourke joint venture.

“It doesn’t have to be a cable supported structure,” adds the source close to the project. “An arch bridge or box girder bridge could be used.”

Both had been ruled out because of their need for too many piers in the estuary. But an arch bridge could be ruled back in according the same source.

Although this option has not been thoroughly investigated in the past, such construction is popular in China with spans now frequently exceeding 500m in length. As a result it could now be built with few enough piers to meet the restrictions.

A box girder bridge could also now be considered, he says, potentially creating large cost savings for the client. This option was considered by Gifford during the design development but ruled out because it would require nine piers in the estuary.

But with box girders now frequently being built at spans of up to 300m, again in China and also Brazil, it’s potentially back on the table. This would be cheaper than a cable-stayed structure because there is no need for complex and expensive towers and cables.

If a box girder could be designed to span a 1km crossing with just three piers, it would also avoid the height restriction.

However, Kite believes the focus for the bidders will not be on changing the type of bridge but instead on fi nding a way to build a cable-stayed structure even more cost effectively.

“The main issue for the bidders is to integrate the constructability of their bridge into the design process,” says Kite.

He says the bidders will need to decide whether they want to rely on using a hover barge over installing temporary works. Kite says a hover barge potentially offers the greater savings.

Introducing stiffness into the structure

It would allow the design of larger deck sections giving much greater freedom in selecting the cable arrangement, which in turn can help overcome a new issue over stiffness within the structure introduced by the decision to drop the double deck.

A single deck, although cheaper, is more slender and less stiff , meaning designers will need to introduce stiffness elsewhere.

The longer deck sections give more space for designers to alter the cable arrangements - for example, stiffness can be increased by overlapping the cables from different towers on the deck.

But this comes with its own diffi culties, primarily because the more complicated the arrangement, the more detailed design is required, which ultimately adds to design costs, according to Kite.

Construction will also be limited by the low tidal range in the vessel, he says, and the barges will be expensive.

All these shortlisted teams will be looking at ways to deal with the issue of stiffness.

Bidders could reduce the bridge to three spans with two towers and anchor more of the cables into land abutments - the cables for the middle tower on the specimen design are only anchored to the bridge deck - to introduce more stiffness into the structure.

But this again remains diffi cult because reducing the number of towers means the tower height would likely need to be increased - while still working to the stringent height restrictions.

In addition, deviating too far from the specimen design could pose more problems - the client is expecting a cable-supported structure so any major changes will be at the bidders’ risk, says Kite.

Uppermost in designers’ thoughts will be the recent experiences on the Forth Replacement Crossing. In a two horse race the perceived underdog - Forth Crossing Bridge Constructors - surprised many because there was little scope for changing the bridge design - but still managed to come in £260M cheaper than the client was expecting by, among other things, using innovative foundations.

It remains to be seen how brave and motivated by savings the bidders, and the client, feel when a winning bid is revealed in December.

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