What went wrong and are other bridges at risk? | Artificial intelligence
A large portion of a bridge in Genoa, Italy, collapsed on Tuesday killing at least 38 people.
The Italian prime minister Giuseppe Conte has declared a 12-month state of emergency in the region and has made €5 million available to help with the aftermath.
“These are unacceptable tragedies that should not happen in a modern society. This government will do everything to prevent such tragedies from happening again,” said Conte in a press conference.
The videos, photographs, and tales from survivors are harrowing, and seem out of place with the spectacular precision and reliability that many people expect from modern engineering. So how did it happen and are other bridges susceptible to the same fate?
The Genoese bridge was originally completed in 1967. It is made primarily from pre-stressed concrete – a form of concrete that has high tension steel cables running through it to improve its strength. It is part of the A10 toll motorway that helps traffic goods from local ports and connects the Italian Riviera to the south-east coast of France.
A massive tower and a 200-metre horizontal section both made of concrete collapsed on to railway lines, a river, and a warehouse 45 metres below, taking dozens of vehicles with it.
Over 400 people were evacuated from the area, including those who live in housing blocks under one of the pillars. Firefighters have been searching for survivors and bodies.
It is not yet known why the bridge collapsed. There was work to firm up the bridge’s foundations ongoing at the time, as well as torrential rainfall.
Some engineers have suggested the maintenance work may have been a factor in the collapse, whereas others are suggesting there were fundamental design or construction flaws.
“Concrete is a fantastic material that can last for many years, but if it’s poor quality it becomes porous, and the tensioning steel corrodes,” says Michael Byfield at the University of Southampton.
High strength concrete normally has a pH level of about 12. At this level steel embedded in the concrete won’t corrode. However, if the concrete mix contains too much water or drainage doesn’t function properly it can become porous, which results in the pH dropping over time.
Below a pH level of 9, the steel can corrode, weakening the structure of the bridge. This can also happen if cracks allow water to seep in.
“The bridge doesn’t have a lot of redundancies, so if one cable goes it could be enough to take the whole bridge down,” says Paul Jackson at engineering firm Ramboll, who helped with refurbishment of the bridge in the 1990s.
It’s hard to know when steel embedded in concrete starts to corrode and there is always an element of judgement, says Jackson. “The Genoa bridge is unusual because bridges tend to give more warning before collapsing, such as revealing cracks,” he says.
During a recent project on the Hammersmith Flyover, a bridge in London, they attached acoustic sensors to the bridge that heard when strands from the steel cable broke.
“Normally this happens so rarely you can’t be sure if your sensor is working, but with Hammersmith they kept pinging a lot,” says Jackon. This meant that extra steel cables had to be retrofitted to the exterior resulting in a lengthy and costly repair job.
Around Europe many bridges are in a poor state. A report in France from earlier in the year said that a third of the country’s road bridges are in need of repair, with around 7 per cent being more serious with an eventual risk of collapse.
And in Germany, a report from the Federal Highway Research Institute last year found 12.4 per cent of Germany’s road bridges were in bad condition, with around 12.5 per cent considered good.
In the US, a report earlier this year found that 54,000 of 613,000 bridges surveyed were structurally deficient. These bridges are crossed 174 million times each day.
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