The collapse of a section of the “Morandi Ponte” (more properly Polcévera viaduct) in Genoa caused 43 victims and over 500 displaced personswith 11 people who survived the tragic accident that took place at 11:36 on 14 August 2018. To cere was one of the stalls of the pile n ° 9 of the bridge that broke through the collapse of the entire central block. The section of the collapsed viaduct was then demolished in 2019. The processes for the 59 defendants began in 2022 and are currently still in progress.
The Polcévera viaduct was designed and built from 1963 to 1967 by Eng. Riccardo Morandihence the name “Ponte Morandi”. This highway bridge represented the final section of the 10, a fundamental connection between Italy and the south of France. The viaduct was built to climb over the Valle del Torrente Polcévera, to facilitate traffic in the city and improve connections with the commercial port. This makes us understand how important this infrastructure was for the city. But not only that: this connection was, and it is still strategic for our economy, because Genoa, together with Milan and Turin, completes the famous “industrial triangle” of our country. So it is clear from this that the disaster and its consequences has impacted considerably on the economy of the whole territory.
As the Morandi bridge was made
To really understand the reasons for the collapse, it is essential to understand the structure of the bridge, that is, as it was done and how the weights were distributed. The viaduct was long 1,102 meters And it was divided into two main parts: the first was made up of an impalcato (i.e. the support structure of the road surface of a bridge), it was long 484 m and was supported by 8 piles in reinforced concrete in variable height; The second was a rough bridge 618 meters long, where the road surface was supported by 3 pylons.
The struck bridge is a bridge where the deck is supported by inclined tie rods, called strokeswhich are anchored to the road surface and which flow into the highest part of a pylonalso called “antenna”, redistributing the forces on the foundations of the bridge. Morandi opted for this configuration because he had to literally overlook houses, sheds, the railway, and therefore did not have the opportunity to build many batteries. The struck bridge built by Morandi was based on a system, called to “Balanced Cavalletti”invented by him.
Morandi’s rough bridge at the time was truly innovative: observing the structure we note the harmony and lightness typical of Morandi’s buildings, conceiving a system that also guaranteed a strong “solidity” to the bridge.
The bridge consisted of three fundamental structures: the pylon, the stalls and the central supports. The external parts of the road, or better to say the deck, were supported by the stalls; There were 2 on each side. The internal parts of the deck were supported by the legs of the pylons and by hindered concrete supports in the shape of H. All loads were downloaded on the pylon and therefore on the foundations. We said there were three batteries: each battery was independent of the point of view of the loads, not surprisingly the collapse only affected the N 9 pile. The other two remained standing.
Because the Morandi bridge collapsed
The pylon that collapsed was the n ° 9the one in correspondence with the river bed. In the investigations and in the investigation reports, several hypotheses of collapse were initially advanced: one, for example, speaks of the assignment of the deck subjected to a strong solicitation, or even the assignment of the pylon itself. The most accredited hypothesis, however – also strengthened by the surveillance video of a company – is that of the structural failure of one of the stalls of pile 9. In fact, in a video released by the Genoa prosecutor’s office, the start of the collapse is perfectly noted.
What the fall of the Polcévera viaduct of Genoa triggered
At the time of the construction of the bridge it was mistakenly believed that the cement lasted forever. Let us remember that the bridge was Built in the 1960swhen this technology was very recent and all the properties of the materials were not perfectly known. Now we know that, with the passage of time, the cement that the tie rods literally drenched in water. So it happened that humidity and marine body They entered the pores of the cement by rusting the cables.
When a cable corrodes, in addition to losing its mechanical skills also increases in volume And it is precisely this increase in volume that caused cracks in the concrete, weakening the structure. Corrosion literally consumed the cables, weakening and greatly limiting the ability to support weights. At this point, when even a single strallo was no longer able to hold the weight, the steel cables from the cement coating have been paraded and the strallo broke. Then the weight of the vehicles and that of the deck itself triggered the collapse of the entire pile. Plausibly this is the dynamics of the inner.
As we said before, the weight of the deck was divided into 4 points. So when support came out, the system came out of its balance, collapsing on itself, because the structure was not redundant.
There structural redundancy It represents the ability of an infrastructure to redistribute the forces within its own system. In simple words, if a structural element fails this does not cause the collapse of the entire structure, because the weight is distributed on other supporting parts. But the Morandi bridge did not have this feature at all, which instead today is a norm. In fact, if we see the most modern struggled bridges you see well that they do not have a single strallo but have several.
What the investigation of the collapse established
To understand what happened that day, and therefore establish the responsibilities, the experts for preliminary investigations analyzed portions of the whole collapsed part. In particular they focused on Finding 132. Thanks to this, the investigators ascertained that 99% of the cables inside the stalls of pile 9 were corrodes and said that:
The steel was in a long -term generalized corrosive state, due to the presence of humidity and contemporary presence of aggressive elements such as sulfur, sulfur derivatives, and chlorides.
So the tragedy was imminent. Obviously there are still ongoing investigations and only time will be able to establish the causes with certainty. But one thing that investigators are quite certain is that maintenance was not made on that bridge and that therefore circulation had to be suspended. Even Eng. Morandi, who in 1979 had gone to see the conditions of the bridge, had observed the cracks on the strokes we were talking about earlier.
On that occasion he had said that bridges of this kind were inclined to deterioration due to the environment in which they are and admitted that, at the time of construction, all the notions on the durability of the materials had not yet been in possession. In short, he had launched him. As a partial solution in 1992, the stalls of pile 11 were replaced, while those of batteries 9 and 10 were not touched thus remaining for another 25 years. In more recent times other inspections also announced an important state of corrosion in the backgrounds of the bridge, but nothing has been done. On this obviously we refrain from making considerations, because the process has just begun and the responsibilities are yet to be established.
