An impressive first: We will shift a 1,000-meter-long bridge. © HOCHTIEF

Shifting bridges

HOCHTIEF engineers are eyeing a spectacular feat of engineering on the A45 autobahn.

Almost 1,000 meters in length and three times as heavy as the Eiffel Tower—can a bridge of these dimensions simply be shifted laterally 15 meters? How will they do it? Jan Felgendreher is working towards this achievement. He is leading the Lennetal Bridge rebuild project for the A45 autobahn in North Rhine-Westphalia. And will deliver a performance unprecedented in Germany: "There's nothing unusual about a bridge being first built and then shifted to its ultimate position. But a lateral shift for a bridge almost a kilometer in length is a first," says Felgendreher.

Working towards a German first
To avoid having to close the autobahn, the bridge superstructure is being erected on an additional row of piers and will be shifted sideways to its final position at a later date. All traffic will flow across this temporary bridge while the new bridge is being built alongside.

At the very end the culmination is planned, when 30,000 metric tons of steel and concrete will be moved 15 meters onto the new piers by hydraulically driven steel cables. "If all goes as planned, it will take a few hours. But we'll have to be very careful to prevent the concrete rupturing. Brute force won't work here," says Felgendreher.

The old has to go
Built in 1967, the old Lennetal Bridge has not even reached retirement age. And yet, it has to go and make way. "It's a shame, really. It's a very slim bridge, a real feat of engineering. But there's no alternative to demolition," says Felgendreher. The reason: At the time the autobahn was built, nobody could foresee today's volume of traffic. The A45—also known as the "Vacation Autobahn"—was originally intended to shorten the journey from the Ruhr region to the south. Nobody imagined that the then economically weak region would one day attract a lot of industry and, with it, an exploding volume of traffic.

Construction of the new Lennetal Bridge
The plan: The existing bridge is to be demolished and replaced by two new bridges, one for each direction of traffic. Phase 1: Before demolishing the old bridge, a temporary crossing will be built to carry the traffic. Phase 2: The old Lennetal Bridge will be demolished and replaced by two new bridges. For one of these, only the piers will be erected. Phase 3: In the culmination of the project, the 1,000 meters of roadway on the temporary crossing will be shifted sideways onto the newly erected piers. Never before in Germany has a bridge of this length been moved laterally.

The strain of heavy-goods traffic
Up to 80,000 vehicles a day cross the Lennetal Bridge. Of that figure, trucks make up 16 percent. In Germany as a whole, truck transport capacity increased more than threefold between 1967 and 2008. Alongside the growth in the number of trucks, gross vehicle weight has also increased. In 1956, its was 24 metric tons; today, it is 44. "The trucks cause the bridge to oscillate and accelerate wear and tear," says Felgendreher. A single 30-tonner with four axles does more damage than 100,000 normal cars. As a result, many bridges have developed tiny cracks in the concrete, through which saltwater penetrates to attack the structural steel within. Ultimately the rebar rusts through and the concrete begins to spall.

More in store
Known also as the "Queen of the Autobahns" on account of its sweeping curves and imposing viaducts, the Sauerland Line is especially badly affected. On the 100-kilometer stretch between Schwerte and Siegen alone, 32 major bridges must be replaced. With no alternative autobahn in the vicinity, a lengthy closure is out of the question. Hence, in a first in Germany's postwar history, the A45 is to be practically rebuilt while still carrying traffic. This "open-heart surgery" also poses a huge challenge for civil engineers.

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