ITALY is still reeling from the collapse of a large portion of a bridge in Genoa last week, killing at least 43 people. Italian prime minister Giuseppe Conte declared a 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.
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Harrowing footage and tales from survivors are at odds with the precision and reliability we have come to expect from modern engineering. It has left many asking what went wrong – and if the same thing could happen to other bridges.
Completed in 1967, the Genoese bridge was made primarily of pre-stressed concrete, which is strengthened by high-tension steel cables running through it. It is part of a motorway that connects the city to local ports and hooks up the Italian and French rivieras. On 14 August, a massive tower and a 200-metre section of road collapsed onto railway lines, a river and a warehouse 45 metres below, taking dozens of vehicles with them.
More than 400 people were evacuated, 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. At the time, work was under way to firm up the bridge’s foundations, and there was torrential rain. Some engineers have suggested that 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, UK.
High-strength concrete normally has a pH of about 12. At this highly alkali 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.
“Concrete is fantastic and can last many years, but if it’s poor quality, the tensioning steel corrodes”
It is hard to tell if the steel inside concrete is corroding 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 similar bridge in London, Jackson and his colleagues attached acoustic sensors to the bridge that detected when strands from the steel cable later 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 Jackson. This meant that extra steel cables had to be added 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 this year said that a third of the country’s road bridges are in need of repair, with about 7 per cent being at risk of eventual collapse. And a report from Germany’s Federal Highway Research Institute last year found that more than 12 per cent of Germany’s road bridges were in bad condition.
In the US, a recent report determined that 54,000 of the 613,000 bridges surveyed were structurally deficient. These bridges are crossed 174 million times each day.
This article appeared in print under the headline “Genoan bridge collapse”
Article amended on 22 August 2018
We previously said that Paul Jackson was involved with the refurbishment of the Genoese bridge. This has now been corrected.