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Aiming for a flying finish

Bridges Rion Antirion

Construction of Greece's huge RionAntirion Bridge is gathering pace in the hope of completing in time for the 2005 Olympics. Adrian Greeman reports from the Gulf of Corinth.

Like giant butterflies emerging from the chrysalis, the RionAntirion bridge in Greece began spreading the fanned wings of its cable stayed deck this summer. When fully extended they will fly over the Gulf of Corinth in three 560m main spans and two side spans of 286m.

Rion-Antirion bridge will provide a four lane motorway link across the Gulf at its narrowest, western, end (NCEI 1 October 2002). With approach viaducts the structure will be 3km long.

By early August some 500m of the main deck already extended from the two completed northern pylons. Steel deck sections measuring 12.5m long are added on alternate sides of each pylon to keep the structure in balance, with four units attached weekly.

When the northern spans are complete lifting will begin at the southern two piers.

Deck units are lifted in by the Netherlands' enormous Taklift 7 floating barge crane. This is more often used on ship salvage and oil projects, but is now on long-term duty for the last phase of the bridge construction, now almost into its fifth and final year. Completion is due in November 2004.

Oil rig methods are appropriate for a bridge that began with the concreting, float-out and positioning of four giant gravity caissons with vast 90m diameter tapered bases. These sit on the seabed up to 60m down in the deep, fast moving waters of the Gulf and have been extended as piers to 50m above sea level.

From here they widen into heavily reinforced and post-tensioned square platforms, crowned with the pylons.

Last autumn, winter and spring saw slipforming of the earthquake-resistant pylons.

These consist of four 'arms' which converge to a central, steel anchorage tower. Concreting around the prefabricated steel boxes for these tower tops recently topped-off the tallest central pier at slightly less than 164m above sea level.

Taklift 7 began its main work lifting deck units in March, despite fierce, wintery weather conditions, says Gilles de Maublanc, project director for French contractor Vinci, which leads the seven firm FrancoGreek consortium building the project. 'We went straight from winter into summer at the end of May. High winds were stopping the lifts.'

The barge is rigged with a 175m long jib and fly extension for the tower tops and accurately positioned some 200t of carefully fabricated steel and associated tackle for the main stay cable anchorages. Originally, the steel boxes making up the anchorage towers were planned to be lifted by tower cranes, which were fixed to the caisson bases before they left the drydock and have climbed in sync with the casting of piers and pylons.

The tower boxes measure 7.6m long, 2.7m wide and 2.5m high and each weighs over 20t.

But hiring Taklift to speed up deck work and reduce risks also enabled the contractor to change its tower erection method. Where the tower cranes would have had to lift each box individually, the floating crane enabled half a dozen of the steel boxes to be welded into a single unit on shore, then lifted in together.

'The advantage is that less high up insitu welding is needed for this thick steel, ' says supervising engineer Peter Iley, from UK consultant FaberMaunsell. It also means only three lifts have been needed for the tallest towers and two for the lower 141.3m high M4 and M1 towers, 'which significantly reduces the risk of losing time because of high winds', adds de Maublanc.

Just one box, atop the highest M3 tower, had to be placed by tower crane equipped with a short heavy load jib - it was beyond Taklift's reach.

While the tower was completed in July, specialist cable stressing subcontractor Freysinnet was working in the lower tower boxes, threading strands for the bridge's stay cables.

Together with a team at deck level they form the cables using the now well established isotensioning system. This uses a first strand to control load, tension and length for the others as they are installed.

Steel for the deck units is supplied by UK subcontractor Cleveland Bridge. Despite some early delay with part of the first delivery last August, production has been going well. To assemble and paint units, Greek contractor Gefyra remodelled the yard on the northern Antirion shore where caisson bases were built in a large drydock. However, to cut down on the 2,000 man hours of on-site welding needed, Cleveland Bridge started assembling deck elements before shipping.

The dry dock itself has been modified to provide berthing for Taklift, which sails in before nightfall to lift a deck unit and leaves it suspended overnight.

'That means it is ready to go at 7am, ' says Iley. 'The winds tend to be quietest at that time.'

Taklift's huge capacity means it can lift a 350t finished deck section including heavy concrete pavement. The barge stays on station only long enough to transfer the box to a special temporary hooking frame devised by Vinci - two quick connection hooks grab temporary hooks on the deck element and hydraulic jacks position the new arrival in three dimensions. Each segment is tightened to its neighbour by about 1,000 bolts before cables are installed and the temporary hooking frame is moved on.

To guard against earthquakes and wind-induced movement the deck is also fitted with seismic 'fuses' and dampers. If the Peloponnese realises its worst seismic potential during the bridge's 125 year lifespan, fuses will break. Along with the capacity of the bridge piers to slide on the seabed, this freeing of the deck will give the bridge enough flexibility to cope with up to 2m of movement.

Because the deck is continuous over its entire 2,252m length a huge single expansion joint is needed where it joins the approach viaduct, says Iley. The joint, providing 4.4m of movement, will be supplied by German firm Maurer Sohn and positioned at the top of a hinged frame being made for the linking pierhead.

With progress on the bridge going well de Maublanc is now contemplating another question - can the deck be completed by August next year? The Greek government would like the runner with the Olympic flame to cross it. But it means pulling the schedule tighter - and finding at least a week for the inevitable disruption and media attention the Games will bring.

But then again it would be a fine advertisement.

Who's who

Contractors in the design/ construction consortium are led by French firm Vinci with J&P (Hellas), Elliniki Technodomiki, Volos Technical Company, Athena, Proodeftiki, and CI Sarantopoulos. Checking engineer for design is Buckland & Taylor of Vancouver in Canada.

Construction of the approach viaducts has been added into the contract since the beginning of the works. Design is by UK firm FaberMaunsell Overall construction cost is 750M ($750M) BOT, funded by the Gefyra concessionaire which is made up from the contractors. The construction and operating concession lasts 35 years.

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