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UK engineers query claims 44t lorry triggered Italy bridge collapse

Polcevera

Claims that a 44t lorry travelling across the Polcevera Viaduct triggered its collapse have been queried by British engineering experts.

During an interview with Italian local media, lorry driver Giancarlo Lorenzetto suggested that the Genoan bridge - more commonly known as the Morandi bridge - collapsed as a direct result of his vehicle passing over it.  

Lorenzetto confirmed he was carrying 44t of steel in his lorry when the viaduct collapsed killing 43 people. The legal weight limit for vehicles travelling over bridges in Italy is 46t. 

And while official investigators have refused to rule out the lorry, UK engineering experts have aired their doubts. 

Independent bridge consultant Simon Bourne told New Civil Engineer that for the lorry to have caused the failure a huge number of the strands within the cables would have already had to have failed, resulting in a much lower load carrying capacity.  

Bourne said that the factor of safety applied to the bridge (a factor applied to increase the capacity of the structure) would have prevented it from collapsing under the weight of the 44t lorry unless there was serious damage to the bridge.  

According to historical drawings, there are 53 cables made up of 464 strands in each of the main cables. Each strand is then made up of seven wires making a total of 3,248 wires in each main cable.

“Assuming an original factor of safety was about 2.5 […] then two-thirds of the wires must have already been broken/corroded [for the lorry to have caused the collapse],” Bourne said.  

“Imagine that scenario, 2,300 wires already gone and only 1,200 wires left to carry any load. That’s the sort of condition the bridge must have been in for months beforehand.” 

He added: “The point is that it won’t have been the truck that caused the collapse - albeit it pushed the bridge over the edge, perhaps - but the fact that two-thirds of the wires were already broken.”

Cowi director Ian Firth agreed with Bourne saying that he doubted a single heavy vehicle would have been sufficient to trigger the collapse.

“The cable had probably expeienced greater tension earlier with a queue of vehicles on the bridge,” said Firth. “But of course it is possible that it was the straw that broke the camels back, perhaps if it travelled across the bridge in a convoy of other vehicles.”

In the days after the collapse New Civil Engineer revealed that corrosion had been found by investigators in the main stay cables as part of the examination of the wreckage, although the extent of the corrosion is still unknown.

The latest claims come days after bridge operator Autostrade confirmed that although tests showed degradation of the cables, it did not affect the load carrying capacity of the bridge.

“The inspection carried out by Spea on the totality of the cables through cores (in January 2016) and reflectometric tests (carried out in several phases in the period 2015-2017) had diagnosed an average reduction of section in the order of 20%, which did not affect any so the static nature of works as confirmed in the technical report of the project itself,” said Autostrade.

Plans are now being put forward by Autostrade to demolish and rebuild the bridge within eight months.

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