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Rescue mission

Building a cable stayed bridge is complex enough. Repairing one which was half destroyed by NATO bombing is even more difficult. Report and photographs by Adrian Greeman.

Novi Sad is well named.

For engineers at least, there is no more unhappy symbol of our war-filled times than the Serbian city's four bridges slumped in the Danube.

Most important was the Sloboda, a 351m central span single-plane cable stay road crossing. Both its steel pylons were hit in April 1999 by NATO Tomahawk missiles during the Balkan conflict.

To this day no one is sure if anyone was killed, although eight people were rescued from their cars by a heroic local tug captain.

As for the bridge itself, one of the 55m high pylons was destroyed; the other severely damaged at its base. With its support gone, classic plastic hinges formed in the steel box section main deck and most of it fell into the river.

The collapse caused further damage as the deck pulled in and dragged on the side spans - steel boxes supported on concrete piers.

Despite the damage, some of the structure can be salvaged, including the 25m high concrete base piers for the pylons.

And so, following clearance of the debris, the bridge is now being rebuilt using a E 35M (£22M) EU grant (see box).

'The bridge was examined and most of the substructure was intact, ' says Maurizio Ranalli, infrastructure programme manager for the European Agency for Reconstruction, which has funded and is overseeing the work.

The 1976 bridge's original designer, Professor Nikola Hajdin from Belgrade University, worked with Danish consultant Cowi on a pre-feasibility study and outline design, with further information contributed by French firm BCEOM's site investigation.

A contract was let to German contractor DSD Stahlbau, which began the repair on a design and build basis in July 2002.

Work started slowly but the pace has picked up since.

'The problem is analysing the structure, ' says DSD project manager Frank Minas. Although there are drawings of the bridge, the 'as built' details are limited.

The first task was to finish dismantling unusable and damaged parts of the bridge, particularly on the south bank where five pairs of cables on the tower were still connected to the deck.

To do this the bridge was supported on the riverside with a trestle erected to support the deck. Horizontal bracing was added for the piers. What was still standing had a de facto equilibrium, explains Minas, although 'no-one knew what the balancing forces were exactly'.

Minas did not want to risk the transient stability of the remains by dismantling them further, he says, particularly as it was unknown how close they or the remaining connections might be to failure. 'So we moved the whole as one piece, ' he says.

Jacks and winches on the horizontal bracing were used to pull back the steel deck, which had moved 1.8m towards the river. The deck slid over the half-intact bearing on the bridge's main pier; this had been prepared earlier by cutting away a bent section.

At the same time more jacks were applied at the base and top of the two concrete piers supporting the 120m length of side span. Each of these had tipped forwards about 300mm but the prestressed concrete bulk was intact.

Once back in position the pier foundations were strengthened by creating an additional concrete collar around the pilecap and boring extra piles.

Similar work was carried out on the opposite bank but here piers were in the river which meant creating cofferdams before inspecting the foundations and installing the jacks to tilt them back. The movement needed was much greater because the deck had been pulled forwards 5.5m;

pier heads had moved 3.5m.

One pier had to be demolished and rebuilt completely but the others were successfully rescued.

While all this work was under way new steel deck sections were being fabricated.

Of the 42 steel box segments of the bridge, 19 are sufficiently undamaged to be re-used and the rest must be fabricated anew.

Another eight of the steel and concrete top composite side span segments are also required.

The 200t units can be delivered by barge from a loading point 220km downstream, which means they can be lifted from barges in a classic bridge construction sequence using derrick cranes at the ends of the extending deck cantilevers.

Originally the whole bridge was to be done this way with the intact units dismantled and re-lifted in a straightforward re-erection.

'But instead we have supported the intact deck in place using clusters of heavy steel piles, ' says Minas. Sliding bearings on top allow the deck to be jacked backwards and forwards so that deck sections can be added in. Even when they sit finally over piers they can be lifted directly from the water and then be positioned.

A similar technique allowed a damaged mid section to be cut out and replaced on the south bank where the heavy segment under the pier was a mass of twisted steel.

Segments are now arriving and being placed, bolted and welded to an extending deck.

This will be supported at three points with clusters of four cables being made by BBR in Switzerland. BBR was the original manufacturer and has detailed knowledge of the bridge.

'Cables were originally preformed, ' says Minas, 'but this time they will be made strand by strand with an isotensioning system.'

Additional work under way includes welding extra stiffeners into the intact boxes. This is needed for the temporary works movements and also to bring the bridge up to modern European standards. It is expected to open next summer.

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