Elegance of form is as much a priority for TGV Mediterrane bridge designers as structural efficiency. A good example currently under construction is an unusual multispan steel structure carrying the railway 324.6m across the wide Canal de Donzre south of Valence. To produce a 'softer' outline, two conventional bowstring arches are linked at their crowns by a curved steel section. The bridge lies in a relatively flat part of the landscape of the Rhne valley and its designers believe the third 'arch', reaching a maximum 34m height, will minimise visual intrusion.
'The superior arc has been designed by the architect for effect,' says Ghislan Van Tieghem, Belgian site manager for steelwork contractor Victor Buyck which is currently erecting the bridge superstructure.
'But since the architect has chosen this approach the engineer has taken advantage of the upper hoop to use it for stability, putting in small diagonal struts from the upper arch,' adds Tieghem, Architect was SNCF's Marc Mimram and the structural work was by VOM at SNCF and BEG Engineering in Lige, Belgium.
In operation the two arches take static load and the upper one comes into play only when there are trains on the bridge. Dynamic loads are relatively high as for all the TGV structures on which trains may pass at 300km/h in each direction, with the infamous Rhne valley 'mistral' blowing at the same time. A less frequent southern wind can gust up to 120km/h.
The bridge has two steel side spans 51.9m long and two 110m long main spans supported by the arch. Unusually for bowstring arch designs these are being erected bottom upwards. Steel decks go in first, supported by two temporary piers under each span, and then the arch is erected above and the hangers added last.
'We can't use the conventional method because the arches are not vertical, but incline inwards by just over 6,' explains Tieghem. That means the up to 14.8m long hangers cannot hang vertically during erection, firstly because they would end at the wrong point for the deck and secondly because it would put unacceptable strains on them.
'We need to use a special frame to support the hangers at the correct angle when we make the erection' Tieghem says. A 'C' shaped frame attaches to the top of the steel arch and passes around it to hold the hanger a third of the way down; the exact position depends on the length of each hanger. The clamp is removed once the hanger is in place attached to the welded 'ear' on the arch at the top and the link on to the deck.
'It's a delicate operation because the tension in the hanger has to be exactly right,' says Tieghem. Accuracy of the steelwork has been critical - the hanger ears had to be positioned to within 10mm, for example.
The superstructure makes up some FFr110M (£11.3M) of the overall FFr165M (£16.9M) contract, with another FFr45M (£4.6M) for foundations, abutments and lower concrete pier sections built by French firm GFC, part of Bouygues.
Some FFr8M (£820,000 went to Intrafor which carried out foundation piling work and FFr2M (£205,000) for dredging works by DTP.
Foundation work did not progress easily after work began in August 1996. Says GFC's Pascal Bouclier: 'We had difficulties with the central cofferdam.' The bridge is supported on three main piers in the river, each with 12 bored piles 1.5m in diameter, and a 3m thick concrete pilecap slab above.
'But a sand lens in the canal bed meant the cofferdam was impossible to dewater on the central pier,' Bouclier goes on. 'We had to use underwater concrete to form the plug/pilecap. That cost us three to four months.'
He adds: 'Divers were needed to ensure the pilecap and piles connected well.'
The cap supports a 10m high 10m diameter concrete base pier formed with steel formwork lifted from steel cross beams at the top.
Above the concrete base piers, the bridge has V shaped steel piers at the sides. 'These extend the line of the arch downwards also for mainly architectural reasons,' says Tieghem. He adds that the feature means the pier 'wants to tip over' which meant supporting it during erection.
Steel for the deck and arches was fabricated by Buyck at its Belgian Wandelgem works and also France's Eiffel at works at Lauteburg near Strasburg. In both cases elements are shipped to Rotterdam by river and then by coaster around to Marseilles and up the Rhne. Elements vary in weight from 90t to a maximum 250t.
'We lift them in using a large floating crane, an 850t Demag cc2600 rigged with the shortest mast configuration,' says Tieghem. The crane barge, also fitted with a smaller 100t Demag as support, is hired from Belgian firm Sarens.
The single pier element was dropped on to a 'ring' of 127 steel bars positioned within the concrete using embedded steel plates and a template. A 1mm accuracy in three dimensions was needed.
Bolted initially to low torque, the 10m diameter of the pier element's 175mm thick bottom plate was grouted up before full torque was applied to lock it on to the concrete base. The smaller 50t wing of the V was welded on afterwards and deck sections followed.
Although not so heavy, other steel elements were more cumbersome, especially the long side span beams. These arrived as 51m long pieces divided longitudinally into 5m wide section to create the full 20m width of the bridge. Deck pieces of the bowstring were shorter, some nine pieces between 31m and 37m, a centre piece 10m wide and the side pieces 5m wide.
Welding of the second bowstring deck is currently in progress. The two temporary pier supports of four 1.2m diameter steel tubes driven 5m into the river bed were those used for the first deck, which were cut free and extended with new sections welded on.
The second arch will begin soon. 'The arches come in three pieces each side with the first two going in first,' says Tieghem. The central element is delivered 20mm over length which gives a small play for final adjustment, particularly to allow the 'ears' for the hangers to be positioned exactly right. Once everyone, including client SNCF's engineers, are happy, the element is cut and welded. 'But the tolerances are very tight,' adds Tieghem.
When it comes to the 'super-arch', Tieghem hopes that much of the contract's four month delay will have been clawed back. Time was lost in delivery of elements with damage sustained during transport, as well as from weather. Additional resources are being deployed.