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.
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 ($46M) 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. '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 dismantle 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 to support the deck.
Horizontal bracing was added for the piers. What was still standing had a de facto equilibrium, says Minas, although 'no one knew what the balancing forces were'.
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 pulled back the steel deck, which had moved 1.8m towards the river. The deck slid over the half-intact bearing on the main pier which had been prepared 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 there 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, and lifted from barges 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 the original fabricator, BBR in Switzerland.
Additional work under way includes welding extra stiffeners into the intact boxes to bring the bridge up to modern European standards. It is expected to open next summer.
The great clean up
Destruction of the Slobodabridge has been a double problem internationally ever since April 1999.
First the wreckage blocked the River Danube, Europe's second largest after the Volga and an increasingly important trade link as the European Union expands eastwards. It is the biggest part of the Rhine-Main-Danube inland waterway barge system which stretches across eight countries from Rotterdam to the Black Sea.
Secondly Sloboda was the biggest road crossing in Novi Sad which is Serbia's second city, with a 400,000 population.
More importantly still, it is a crucial part of the Corridor Ten Pan-European link from Greece to the rest of the EU. The road carries significant Middle Eastern traffic to Europe and trucks from Bulgaria, Romania and Turkey pass by the dozens every hour.
But after years of sanctions and continuing political turmoil, Serbia could not afford to repair the bridge, especially after rebuilding a truck and rail bridge downstream.
It did manage a clearance contract with local firm Mostogradnja, the original builder, to stabilise the bridge, support the remaining structure and lift out distorted deck sections and fallen cables. But the river is still blocked because a temporary pontoon bridge is in place downstream and this can open only twice weekly for boats.
Repairing the bridge is a priority for the EU says Maurizio Ranalli, for European Agency for Reconstruction, and so funds are being made available.
A larger crossing was also mooted with a entirely new bridge.
But the EU remit is only to restore crucial links. Rebuild would also have incurred further costs of up to E 10M ($13M) because the old structure would have had to be demolished first.
Though most attention is focused on Novi Sad which blocks the crucial Danube route, Serbia has also been gradually rebuilding dozens of other bridges destroyed or damaged in the bombing.
Two more of these are in Novi Sad, but Belgrade itself also suffered damage: most significantly to two large bridges across the Sava river, one of the Danube's largest tributaries.
The first is a steel box section road bridge (BELOW) crossing to the south of the city, intended to form a connection for the longawaited Belgrade bypass which will take truck traffic from the Corridor Ten highway.
It is due to reopen this month.
The 583m long bridge, with a central span of 199m, was started in 1990 but construction was suspended after two years with the imposition of sanctions.
Contractor Mostogradnja, Serbia's major bridge firm, restarted the work after three years and steady progress brought the bridge to completion in 1999.
But two days before it was due to open, it was bombed. Much of the 4,400t of steel deck structure slumped into the river.
Two years ago the Roads Directorate was able to find enough money to begin a reconstruction.
First task was to lift the damaged steel deck from the river; this took a year because of need for close inspection and careful assessment. The fallen deck was then lifted with strand jacks mounted on tubular steel columns piled into the river bed.
About half the deck section has proved reusable and another half has to be remade in the contractor's steelworks to the east of Belgrade, now owned by US Steel. The sections were delivered by barge and made up into larger units onshore.
Only one carriageway of an eventual twin bridge has been completed, though the piers are wide enough for the expansion when it is needed and also is financially achievable.
The company is now looking just 500m downstream at a twin track rail bridge (LEFT) where it has begun driving piled supports to recover a downed trestle section.
At least two bombs hit the bridge which has further damaged two piers along although the spans remained intact.
The bridge carries lines into an important marshalling yard complex on the south side of the city.