The toll of traffic on German bridges | Science| In-depth reporting on science and technology | DW | 11.03.2015
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The toll of traffic on German bridges

Germany's roads and bridges take an export-driven beating every day. Especially around Duisburg, Europe's largest inland port, trucks drive over one of Europe's largest rivers - not without consequence.

What's good for GDP will eventually be a death knoll for a country's bridges.

Every single day, 11,000 semi-trucks carrying precious exports to be shipped along the Rhine to destinations the world over, make their way across the A40 autobahn bridge between Oberhausen and Duisburg.

That's in addition to about 90,000 other vehicles. This Tuesday, the bridge was closed for trucks, after being reduced from six lanes to two for all vehicles two weeks ago.

For the trucks alone, which weigh an average of 35 tons, traversing the 777-meter-long bridge yields a force greater than it was intended to withstand when it was opened in 1971.

Back then, the transport ministry of North-Rhine Westphalia (NRW), the federal state where the A40 bridge is located, assumed that 30,000 vehicles would cross per day.

"This bridge was supposed to last 100 years," Michael Heinze, structural engineer and self-proclaimed "bridge man" for today's NRW transport ministry, told DW.

The bridge barely lasted 30 years before a first intense stage of restoration was required in 2004.

"And now, here we are 10 years on," says Heinze, "and it desperately needs work again."

Damage to steel within the reinforced concrete was found with the help of x-rays

Damage to steel within the reinforced concrete was found with the help of x-rays

Compression and tension

So, how do the experts know when a bridge needs work?

"You don't really need scientific credentials to figure out that stress cracks mean danger," Heinze says, explaining that during a routine check of the bridge late last year serious fissures were observed.

However, those stress cracks weren't exactly visible to the naked eye. It was x-ray technology that exposed the damage.

And what work has to be done?

"It's not going to collapse anytime soon if trucks go across, no. But those cracks will grow and eventually increase the size of the vulnerable spots, thus decreasing the structural integrity of the bridge," Heinze says.

During a six-week closure of the bridge for semi-trucks, the steel where the stress fractures were found will be welded back together.

The cable-stayed bridge is made of reinforced concrete, which Heinze says was intended to "dissipate" the two main forces that every bridge on earth, regardless of its structural design, has to contend with: compression and tension.

The compression - or vertical - forces are absorbed by two massive pylons (see picture above).

The steel cables, which are attached to the pylons and directly to the roadway, are intended to spread out the forces that result from the tug of war between the weight of the traffic and the pylons, known as tension.

Due to the considerable width of the Rhine, a cable bridge is ideal.

The alternatives of truss or arch bridges could never offer the structural support needed.

In addition, the materials needed to make those kinds of bridges over the Rhine would be too expensive.

Suspension bridges, such as the Golden Gate Bridge that spans the channel between San Francisco Bay and the Pacific Ocean, would be extremely difficult to build over the Rhine, and completely unnecessary.

"For all the splendor of Duisburg and the Rhineland, this isn't California," says Heinze. "A suspension bridge needs to withstand immense tensional forces. The Golden Gate Bridge is anchored on both sides in solid rock - in the cliffs. You can't do that on the Rhine, unless you were to erect hideous concrete blocks on both sides of the river!"

The Golden Gate Bridge, offering one of the world's most stunning views, is anchored in rock

The Golden Gate Bridge, offering one of the world's most stunning views, is anchored in rock

Worth the wait, worth the work

When it comes to infrastructure problems facing Germany, in particular in the western part of the country, it's about a lot more than just one bridge over the Rhine.

Results from an ongoing assessment conducted by Strassen.NRW, the state road development and maintenance office of NRW, suggest 150 bridges in the state will have to be rebuilt, and 64 are due for repairs.

That's of the 229 bridges that have been assessed. A further 651 bridges have yet to be tested.

"The A40 bridge is just the beginning of a long story," says Bernd Löchter, of Strassen.NRW.

Löchter says the welding work being done on the A40 is a superficial solution.

"There's no way to repair this bridge indefinitely - we have to replace it," says Löchter, "and that's where the fun will begin."

The sarcasm meter would have exploded if it had been anywhere near Löchter when he said that. Fun? Replacing a bridge like this one will take 10 years, at least.

Just ask anyone driving in the vicinity of the A40 near Duisburg over the next six weeks if they think the repair work amounts to fun.

On Tuesday, the first day of the bridge closure, the average traffic jam on the A40, the A57, the A42 and the A3 (the exact detour for trucks trying to cross the Rhine over the A40) was 12 kilometers long.

That's at least an hour of nothing but the butt of the vehicle moving at a snail's pace right in front of you.

"This situation is absolutely unacceptable," said the NRW transportation minister, Michael Groschek, prefacing demands he posed to the federal government in Berlin to finally fork up funds for bridge reconstruction. "We need this, otherwise we will have no other choice but to paralyze parts of our economy."

Löchter, however, prefers a more positive spin: "The A40 bridge is closed because of safety concerns. Apart from the bombardments during the [Second World War], Germany has never really seen a bridge collapse catastrophe, and, ultimately, that is thanks to nothing other than the rigorous controls we carry out."

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