20 February 2025
The partial collapse of the Carola Bridge in Dresden in the autumn of 2024 has fuelled the discussion about the safety of bridges in Germany. How are bridges inspected in this country? Are the measures sufficient? And what will help deal with the renovation backlog?
Dresden, 11 September 2024: At 2:51 am, a number 7 tram crosses the Carola Bridge, which connects Dresden’s old town with its new counterpart. At 2:58 am, a white van drives over the carriageway that runs parallel to the section dedicated to trams. Suddenly there’s a deafening bang, and a huge fountain of water explodes out of the Elbe into the night sky. A 100-metre section of the bridge which the number 7 tram has crossed just a few minutes before plunges into the river.
This unexpected collapse sends a shock wave through Germany, and the experts are stunned. On the one hand, there is relief that, by some miracle, no one has been injured; on the other, the question arises as to how this could have happened.
“Hands-on” main inspection
Like every bridge in Germany, prior to its collapse the Carola Bridge had been subjected to an elaborate main inspection every six years. In these inspections, structural inspection engineers take a close look at the entire bridge, opening every manhole cover and scrutinising hard-to-reach structural elements as closely as possible. The bridge is then awarded a grade, and further measures are planned based on the assessment.
Between the main inspections, there is a stripped-down version every three years, during which, for example, the carriageway joint and the foundations are examined. Once a year, the bridge is inspected for obvious damage and defects. “Given the low number of incidents at around 130,000 bridges in Germany, this inspection cycle seems to be vindicated,” Dieter Westkamp, head of Technology and Society at the Association of German Engineers, told the Tagesschau. Manfred Curbach, senior professor at the Institute of Concrete Structures at the Technical University (TU) Dresden, also considers the test cycle and process to be sufficient. “Our German system is still one of the safest in the world,” says the bridge expert. "Above all, because, in this country, if there are serious deficiencies, there’s an immediate response which doesn’t consider the economic consequences. This wasn’t the case with the Morandi Bridge in Genoa, for instance.” When deformations were discovered on the Rahmede viaduct in North Rhine-Westphalia on 2 December 2021, for example, the bridge was closed on the same day - even though this entailed major detours for tens of thousands of motorists and haulage companies.
Cause of the collapse: stress corrosion cracking
But why didn’t the proven mechanisms work in the case of the Carola Bridge? The last main inspection in 2023 revealed a need for renovation, but no imminent danger. TU Dresden's report on the collapse in December 2024 brought some clarity. The main cause was what is known as stress corrosion cracking, which, according to expert Steffen Marx, started while the bridge was still under construction.
This phenomenon has been known for several decades and occurs in certain prestressing steels used in structures in both eastern and western Germany. After reunification, all the affected structures were intensively examined and guidelines issued on how to proceed with them. Some were subsequently demolished, others reinforced. The problem with the Carola Bridge was that the wide cracks which experience has shown to indicate a dilapidated bridge did not occur in advance of the collapse. “The Carola Bridge was the very first case in which a bridge built using this prestressing steel didn’t show any of the typical warning signs,” says Mr. Curbach.
Making cracks audible
According to Mr. Marx, it therefore follows that the collapse of the bridge would not have been foreseeable using conventional methods. “The only way forward is to use a sound emission method to actually hear the damage,” the expert explained. The remaining A and B bridge sections have been permanently monitored using this noise emission method since the end of 2024. This makes it possible to detect when prestressing steels in the bridge develop cracks, further weakening the load-bearing capacity.
“The collapse of the Carola Bridge has heightened awareness of the problem, and that’s a good thing,” says Mr. Curbach. As a result, 19 bridges of the same design in Saxony have been subjected to a special inspection and equipped with monitoring systems. The Elbe bridge in Bad Schandau has been closed as a consequence of this special inspection. In the meantime, it has become clear that there is no alternative except to rebuild, and this also applies to the remaining sections of the Carola Bridge.
© Adobe StockWhat nobody thought possible or could have predicted: Parts of the Carola Bridge in Dresden collapsed.
© Adobe StockIncreased lorry traffic is a major stress test for bridges. One 40-tonne lorry puts as much strain on a bridge as 30,000 cars.
When bridges age
Bridges built using this special prestressing steel are by no means the only causes for concern in Germany. More than half of the bridges in the country date from before 1985. Many are even older, approaching the end of their life cycle, and need to be replaced. Since many older bridges are not designed for today’s traffic situation, their service life is being further shortened by the amplification of structural fatigue. A major problem is the increase in lorry traffic. After all, a single 40-tonne lorry puts as much strain on a bridge as 30,000 cars. If defects are discovered, the affected bridge is therefore often initially closed to lorries to relieve the strain.
Alleviating the renovation backlog
Federal Transport Minister Volker Wissing announced a package of measures for faster bridge renovation back in 2022. Of the approximately 40,000 bridges under the responsibility of the federal government, 4,000 are to be renovated within ten years. Instead of around 200 as before, 400 bridges are to be renovated annually and the funds for this increased to 2.5 billion euros per year from 2026.
However, in January 2024, the Federal Audit Office expressed strong doubts as to whether this restructuring schedule could actually be adhered to. “If we’re going to renovate twice as many bridges, we also need twice the specialist staff – and we simply don’t have them,” says Mr. Curbach. To achieve the defined goal, other, much more efficient procedures are therefore needed, “from the way contracts are awarded to the construction companies to the techniques used to renovate bridges”.
Deploying carbon to ward off collapse?
One approach would be to make greater use of carbon concrete, in the development of which Mr. Curbach and his colleagues in Dresden are significantly involved. In this method, mats or rods made of carbon fibre are inserted between the concrete layers. “Carbon concrete is lighter, six times stronger and much more durable than reinforced concrete, because carbon, unlike steel, doesn’t rust,” Mr. Curbach says, outlining the benefits. Last but not least, it can be installed more quickly, which speeds up the renovation. One disadvantage is that steel is currently still considerably cheaper than carbon. But since its corrosion resistance and higher strength mean that less building material is needed, it can make up lost ground when it comes to the total costs, with a reduction of 50 to 80 percent in the amount of concrete used compared to cases in which only reinforced concrete is used. And less concrete is also better for the climate; after all, cement production is responsible for almost eight percent of global CO2 emissions.
The first small bridge made of carbon concrete has spanned the Kuppritzer Wasser in Wurschen in the Bautzen district since 2021. Mr. Curbach and his team have already used their novel material to reinforce motorway bridges in Hesse and Bavaria. But the problem here is that all these construction projects require individual approval, as carbon concrete is not yet approved as a standard building material in Germany. Mr. Curbach highlights the problem that, all too often, the decision-makers in the relevant authorities give preference to familiar methods for bridge renovation. “For a long time, we’ve done too little, turning a blind eye to wear and tear. This means that we now need to innovate; if we don’t, we’ll never manage this huge task.”
About Manfred Curbach
Manfred Curbach is a senior professor and former director of the Institute of Concrete Structures at TU Dresden. He and his team are significantly involved in the invention and development of carbon concrete.
