The Weekly Reflektion Week 47 / 2019

This week we are reflecting over the collapse of the Ponte Morandi bridge in Italy, and the relevance of the causes to other industries.

Degradation of materials in areas we cannot get to can lead to challenges. Do we make incorrect assumptions based on lack of data?

How do you evaluate how ‘robust’ something needs to be before it is ‘robust’ enough? 

Ponte Morandi was an important bridge, connecting Milan and Turin in the north and Rome and other major cities in the south and by-passing an increasingly congested Genoa. On the 14th August 2018, the bridge collapsed, killing 43 people who were crossing the bridge at the time.

The bridge was opened in 1967 in a post-war Italy in a difficult financial situation, with imported steel being expensive, and with no domestic steel production. What the country did have readily available were clays and river sediments necessary for concrete production.

Riccardo Morandi, an Italian civil engineer set his mind to a solution for a bridge design to by-pass Genoa. The bridge would cross east-west over a river valley, and an industrial area and be slightly more than 1 km long. Morandi came up with a design using minimal quantities of steel, and extensive use of concrete, stronger and lighter than equivalent bridges. It consisted of three towers, with 4 ‘stays’ connecting each tower to the roadway, transferring the weight of the bridge to the towers, and down to the foundations. These ‘stays’ consisted of steel cables surrounded by concrete. These were ‘pre-stressed’ by tensioning up the cables before pouring the concrete around the cables, which improved the performance of the ‘stays’ in tension and reduced any ‘sway’ of the bridge.

A schematic of a ‘stay’ with main steel cables, secondary steel cables, surrounded by pre-stressed concrete

As the steel cables were protected from the corrosive atmospheric environment of the salty Mediterranean air by the concrete, the pre-opening boast was that the bridge would not need any maintenance. The design, however, was not robust. If one of the bridge supports failed, there was no redundancy in the remaining structure to support the weight, and a section of the bridge would collapse. The design placed a premium on inspection and maintenance, but this risk was ignored as the steel was so well protected. Concrete structures were considered by designers and manufacturers to be ‘eternal’.

By the late 1970’s the concrete was visibly deteriorating. The concrete used turned out to be highly vulnerable to degradation, and the bridge designer inspected the bridge in 1979 and 1981, concluding that the bridge was already significantly degraded. The corrosion was hidden deep within the concrete making it difficult to evaluate.  Significant corrosion had been discovered on the ‘stays’ on one bridge support where 30% of the tendons had corroded away. This ‘stay’ had been strengthened with external cables. The ‘stays’ which later failed were not strengthened.

In 2017, an investigation using microphones was conducted, listening for the tone of the vibrations being produced by the steel cables. Smooth, predictable tones indicate steel cables in good condition, whereas discordant tones indicate damaged cables. The two southern ‘stays’ on the centre tower exhibited these discordant tones. These findings were not followed up, as Autostrada, the company managing the bridge, perceived no urgency in the conclusions.

The bridge collapsed when the southern stay on the central tower parted, the roadway tilted and began to break apart, as the weight of the entire span transferred to the northern stays, these failed, with the entire span then falling into the valley. The tower itself then collapsed. 

Italy’s new populist coalition government immediately blamed Autostrada for the collapse, who in turn blamed the government administration for delays in obtaining permission for the necessary maintenance. 

In the oil and gas industry, we also suffer from corrosion that is difficult to see for example, under the insulation on pipes on offshore platforms. Are there other areas where inspection is difficult, making it easier to assume things are OK?

New designs that bring obvious advantages are sometimes adopted without considering the new risks that they bring with them. Are we always critical enough?

We are encouraged by the authorities to design robust solutions. The definition of an acceptable level of robustness is not always straightforward. How do you evaluate your level of robustness and what do need to know before you can say this is robust enough?

Reflekt AS