An Italian engineer investigating the collapse of the Polcevera viaduct has told New Civil Engineer about his early hypotheses into why the bridge may have collapsed, ruling out a previously-mooted theory.
University of Genoa associate professor Antonio Brencich said that he has already ruled out a geotechnical failure of the tower as a cause of the bridge’s collapse which killed 43 people after a 250m long section of the pre-stressed concrete, cable stayed bridge fell down.
It had previously been mooted that heavy rain could have washed out ground, undermining the tower’s foundations. However, Brencich said that he has already dismissed this theory, stating: “The only thing I am certain of is that it is not a geotechnical problem, because you see no rotation of the tower. It is piled and when it collapsed it was in summer rain, but in winter this is filled with water [river bed],” he said.
“I think the ‘lightning’ as it went down was probably the bridge hitting the electrified lines of the train below not from actual lightning.”
Brencich has been named on a six-person taskforce set up to investigate the collapse by Italy’s Ministry of Infrastructure and Transport. The taskforce has been given 30 days to report its findings to the government.
Brencich confirmed that corrosion was being investigated as a possible reason for the collapse. As revealed by New Civil Engineer last week, corrossion to the main stay cables has been found among the wreckage.
Although Brencich confirmed that evidence of corrosion could be seen on the bridge when driving over it, he stressed that this was surface corrosion and did not necessarily give an indication of the condition of the load bearing structure.
He said: “When you drove over it you could see widespread corrosion, but it was surface corrosion you could not see the condition of the cables. As an engineer I was not scared of the surface corrosion because I knew the load bearing structure was something different.”
He also said a phenomenon called “stress corrosion” (an accelerated form of corrosion when a material is subject to both tensile stress and a corrosive environment) could explain the fast deterioration of the main stay cables, whiIe adding that investigators are also looking into the lack of reinforcing steel within the tower structure.
“The reason that this is an unusual collapse, if you look at the column of the tower they have broken into relatively small pieces,” Brencich said. “If you look at the ruins and you remove the pre-stressing cables, what you see is a really low percentage of steel bars.
“When you see a collapse of a modern bridge you see a lot of reinforcement, but here there is very little. I would expect to see much more.”
The structural engineer said there had always been issues with the bridge and he first spoke out about it in 2016 to Italian engineering magazine Ingegneri.info.
Brencich said that the bridge had issues with concrete creep well into its working life. He said driving over it 30 years ago, there could be up to 8mm in variation across its surface.
“It was really uncomfortable and that was something related to creep,” he said. “If the bridge was like this 20 years after construction with such a strong evidence of creep then something had to be different to what was designed.”
He said it was difficult to tell whether this was due to a material or design flaw with the structure, but stressed that the bridge’s design was potentially dangerous as the structure of the tower, cables and deck were statically determinate – a structure with only one load path and no redundancy. This would mean that if one cable were to fail, the whole bridge would collapse.
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