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Innovative saddle system proposed for Mersey bridge

An innovative saddle system used by Spanish contractor FCC on the 300m-span Corgo Viaduct in Portugal could be used on the Mersey Gateway bridge after wowing engineers.

Nuno Castro, site engineer manager for the 2.79km-long Portugese project, told NCE he would like to see the system, designed by FCC’s Switzerland-headquartered subsidiary BBR, used on the scheme to link Runcorn and Widnes in Cheshire.

The saddle, designed and tested specifically for the Corgo Viaduct, allowed single cables to pass through pylons on the bridge and connect to the deck on both sides.

“I don’t know if this will be used on the Mersey Gateway but I hope so because it is a very good solution,” said Castro. “It is more efficient and better for maintenance as you can just change one string at a time and without closing the bridge.”

Castro added that the saddle system removed the need for cumbersome crossing and reinforcing of cables, allowing for slimmer, more attractive pylon designs.

He said the experience FCC has gained on the Corgo Viaduct would help it with the Mersey Gateway project, on which main construction work is due to start in the New Year.

“We have seen that we can manage this cable-stayed technology pretty well,” he said. “The experience of that - doing it for real on a very difficult project – will be of use on new projects.”

The Corgo Viaduct – part of a programme of work to create almost 200km of dual carriageway between Vila Real and the Spanish border – was not always going to be a cable-stayed bridge.

Built across a canyon containing the Corgo River near the town of Vila Real, the bridge had to be redesigned right at the start of the scheme.

“When we tried to start the foundations, we realised it was not possible to build the piers as planned,” said Castro.

The existing designs would have required a large amount of excavation in an environmentally sensitive area. So the end pier was moved back 60m to allow easier construction - but this meant the bridge span increased from 240m to 300m.

“This span length is very, very special,” said Castro. “We had to change from a concrete box girder to a cable-stayed bridge. This redesign took four or five months.

“Even with the changed pier location we still had to move a lot of earth. We took [a further] four months to get to start the foundations.”

When the groundworks were completed, the contractor had to build 41 piers, at a height of up to 130m.

“We had three different methods of construction, depending on height and access,” said Castro.

Traditional formwork was used for the shorter piers, with rebar prefabricated and lifted by crane to minimise the safety risk.

Middle-sized piers were built using formwork that relied on hydraulics rather than cranes.

To allow rapid, safe construction of the tallest piers, sliding formwork was used, moving upwards on a continuous basis. This had to be very closely managed.

“This formwork moves for itself, it doesn’t require dismantling, cleaning and resetting,” said Castro.

“But we had to work 24 hours a day, and could not afford any problems. We needed two generators, and guys on back-up just in case they were required.”

This work went smoothly, allowing the contractor to start on the main decks. Only two, 6m segments could be poured at a time due to the cable-stays and the need for balance.

“Extraordinary precision” was required when fixing the cables, according to Castro.

“It is not easy to install steel tubes at 200m in high wind and rain,” said Castro. “We had a good designer and very good site surveys. They did an extraordinary job.”

Plenty of planning ensured construction passed with the minimum of fuss, and the bridge is now open.

Readers' comments (1)

  • Suggest looking at the design of the Westgate Bridge in Melbourne - the cable stays of that bridge which was opened in 1978 also pass over the saddle.

    Richard Tomkins (41149146)
    London SW19 4PG

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