In the past two years, 13 bridges around the world have collapsed. While some of them were in countries that we might consider perhaps a little lax in their building codes, one was very old and another got hit by a high truck, there are still a perturbing number of incidents. Three of these have been in Italy.
Corrosion in conjunction with extreme weather is perhaps a factor in some of the failures, as has been hinted at in the Genoa collapse. However, another very similar bridge by the same engineer built in Venezuela has also collapsed although that might just be bad luck as it was hit by a wayward oil tanker.
There hasn’t been an official explanation for the Ponte Morandi collapse although a storm was raging at the time and there were reports that it suffered a lightning strike.
Dr Martin Fullekrug, Reader in the University of Bath’s Department of Electronic & Electrical Engineering, said that very often it is quoted that lightning can be as powerful as a nuclear power plant. Yet, it is normally not pointed out that this power only takes effect over an extremely short amount of time, typically just a few tens of a millionth of a second, such that the impact of lightning often remains quite limited.
“Lightning often has indirect effects, such as on consumer electronics when it strikes unprotected buildings. However, in the worst case, it can also have direct effects such as melting metal, ignite forest fires and kill people, animal herds and trees.
“Whilst it is perhaps not impossible to think that a lightning strike makes a contribution to such a collapse, it is probably very unlikely to happen. Lightning could potentially contribute to a critical fatigue of material. For example, the lightning generated heat could result in evaporating water to very high pressure and produce a subsequent crack or burst of critical support material, similar to the bark of a tree disintegrating after a lightning strike. In theory, in might be also possible that the lightning strikes a critical metal bolt such that its function becomes impaired. But again, any such kind of scenarios are rather speculative.”
Dr Fullekrug said the most important thing is to follow the guidelines for lightning protection as set out by the standardising institution in the respective country.
“These guidelines are normally based on the long term experience with lightning damage and take into account scientific evidence that is constantly updated, based on new knowledge that is generated. In this extreme case, a careful investigation of what might have happened seems to be particularly important to avoid similar scenarios in the future.”
Until there has been an investigation, of course, we shall not know the cause but there has been interesting discussions around the terrible affair, which are pertinent to all such structures.
The corrosion of tendons or reinforcement may have been a contributory factor to the Ponte Morandi collapse, says Ian Firth, Past President of The Institution of Structural Engineers, and a Structural Engineer specialising in bridges,
“It is too early to say what caused the tragic collapse, but as this reinforced and pre-stressed concrete bridge has been there for 50 years it is possible that corrosion of tendons or reinforcement may be a contributory factor. There are no obvious signs to say what specifically triggered the collapse at this time; the fact that there was reported to be a storm at the time may or may not be particularly relevant. In addition, ongoing work on the bridge may or may not be partly responsible for the collapse.
“The bridge is a very unusual design, very similar to its much larger cousin, the Lake Maracaibo bridge in Venezuela, also designed by Riccardo Morandi and completed six years earlier in 1962. The A-frame towers which support the concrete-encased stay cables combine with V-shaped supports below the deck to create a stiff arrangement which is not common in cable stayed bridges. This deals with potential unbalanced loads which arise due to the multi-span nature of the structure. As yet, there is no evidence to say whether any impact occurred; it is too early to say what triggered the collapse.”
Dr Mehdi Kashani, Associate Professor in Structural Mechanics at the University of Southampton also says that of course at this stage it is very difficult to make a solid judgement on the cause of the catastrophe.
The bridge was constructed using reinforced and pre-stressed concrete about 50 years ago. There are a large number of reinforced concrete bridges in Italy, Europe, USA, and Canada with the same age, which are suffering from corrosion of reinforcement and/or pre-stressing tendon.
“Recent research showed that corrosion of reinforcement changes the long-term behaviour of ageing reinforced concrete bridges. In addition, bridges are constantly subjected to cyclic dynamic loading due to highway traffic, wind and/or major/minor earthquake, which will result in fatigue damage in bridge components. It is reported that this bridge collapsed during a heavy storm. Therefore, dynamic wind loading, combined with additional loading due to on-going work on the bride, and reduced capacity due to corrosion and fatigue might be the cause of failure. However, there is a need for further detailed investigation to fully understand the cause of failure.
“The bridge engineering research community should take this seriously in their future research to improve the resilience of our infrastructure under extreme loading.”
Dr Maria Rosaria Marsico, senior lecturer in Structural Engineering at the University of Exeter, the Ponte Morandi was a beautiful expression of the engineering design.
“The viaduct includes three cable-stayed spans and a series of minor spans for a total length of about 1182m. The three largest spans consist of independent cable-stayed structures, each carried by an individual reinforced concrete pier and tower 90m high. The longest span – that collapsed – was about 210m long. The cable-stayed systems was characterised by the adoption of prestressed concrete stays, a common feature of bridges designed by R. Morandi in the sixties The viaduct was subject to maintenance work since it was built and in the nineties a complex intervention of repair was carried out involving the installation of conventional steel tendons which are flanking the existing concrete stays.”
As with the other commentators, Dr Demitrios Cotsovos, Associate Professor, Institute of Infrastructure and Environment, Heriot-Watt University, said one should consider all possibilities before drawing any conclusions. It is important to investigate in detail the reasons that have led to such a catastrophic collapse. Potentially, there are lessons to be learnt from such an event.
“Ageing infrastructure and its impact on structural integrity and safety should become of prime concern to structural engineers. The potential impact of the environment and extreme weather conditions (possibly associated with climate change) also needs to be assessed. The need for monitoring structural performance, understanding the causes of the exhibited collapse and developing reliable assessment methods is essential so that events like this be avoided in the future.”
Ending the conversation, Tim Ibell, FREng, Professor of Civil Engineering, University of Bath, summed up the art of bridges saying that while it is of no comfort to the Ponte Morandi victims and their families, the reality is that the beautiful, enormous bridge structures we see all over the world stand safely due to the extraordinary abilities of structural engineers. It is when we are hit by such tragedy, thankfully so rarely, that we realise just how important the work of these engineers really is.”
