Air View of the strait of Messina, that divides Sicily (on the right) from the Italian peninsula. Credit: Andrea Colantoni/Moment Open/Getty Images.
The publicly-owned company overseeing the project, Stretto di Messina, has approved a report outlining how the 2012 blueprint must be revised to meet new building codes, and to take in any new knowledge or technologies. The minister for infrastructure, Matteo Salvini, has vowed to start construction in 2024.
The bridge would span 3,300 metres between its two towers, nearly 60% more than the current longest span suspension bridge, the 2,000 metre Çanakkale in Turkey. The design would entail a 60 metre deck held by four suspension cables anchored to towers on either side of the Strait, each one nearly 400 metres high.
The update report was prepared by the contractor in charge of construction, the Eurolink consortium, and was given the thumbs-up by Stretto di Messina’s scientific committee, and by an additional expert advisory panel. Neither the update report nor the experts’ evalution are public yet, but Nature Italy talked to panel members and to other experts about the topics that the new design will have to consider.
“The update report does not contain substantial modifications to the old project, which is still reliable because it followed a very cautious approach,” says Giorgio Diana, an expert in long-span suspension bridges at the Polytechnic University of Milan who consulted for the 2011 project and now sits on the expert panel of the company. “The contractor proposed the use of slightly more resistant wires for the suspension cables and for active rather than passive mass dampers on the towers to counterbalance vibrations from winds and earthquakes. These are minimal changes, which however we will test again in a wind tunnel.”
Searching for the 1908 fault
Concerns about technical feasibility have been a running feature since the bridge was suggested. The Messina strait has a very high seismic risk, and in 1908 was hit by a magnitude 7.1 earthquake that killed more than 75,000 people, Europe’s highest earthquake toll. Its waters are notoriously choppy, and winds often blow at 120 km/h or more.
The seismic assessment for such an infrastructure follows a probabilistic approach. First, seismologists calculate how frequently earthquakes of different intensities can be expected to happen in that region. Then, regulators choose an acceptable risk level and engineers design accordingly. While ordinary buildings are designed to resist one-in-500 year earthquakes, the Messina bridge should withstand a one-in-2,000 year occurrence, explains Ezio Faccioli, a seismic engineer, who also consulted for the 2011 project and is a fellow panel member.
The various elements of a bridge respond differently to ground oscillations. “The bridge as a whole is very flexible and heavy and will experience relatively small forces, but the towers are tall and rigid and will experience much higher forces,” Faccioli explains.
Based on the earthquake records in an area spanning 300 kilometres from the crossing site, scientists concluded that the ‘design earthquake’ (the most intense one the structure should withstand), would be similar to the one that presumably shook the area in 1908.
A more precise approach to seismic risk is to identify the source of the design earthquake and use numerical methods to simulate ground motion, taking into account how different rocks and soils attenuate or amplify the seismic waves. “In the last ten years there has been progress in physics-based simulations and a sharp increase in computing capabilities, and this approach could be expanded in the transition to the executive project,” Faccioli explains. Such methods can rely on studies of the 1908 fault led by Gianluca Valensise, a geologist at the National Institute of Geophysical and Vulcanology2. “The [1908] deformation pattern indicated a fault formed by the stretching of the Earth's crust, inclined at a small angle with respect to the ground and located between 3 and 18 kilometres below the surface,” Valensise explains. But other geometries and positions for the 1908 fault have been proposed in the last decade and may be investigated during the executive design phase.
Knowing which kind of fault generated the 1908 earthquake is crucial also because similar faults caused the earthquakes in Central Italy in 2016, when record ground accelerations for the country were measured. “We could use those sequences to stress test the design,” says Faccioli.
Dispersing the wind
The biggest risk from wind is flutter instability, which happens when the bridge’s own vibrations and the movements of the air reinforce each other. The forces on the deck gradually increase at each oscillation and they can cause the structure to collapse.
The longer the span of the bridge, the lower the wind speed that can cause instability. “This led to an innovative aerodynamics design for the Messina bridge deck, which comprises three separate boxes,” explains Daniele Rocchi, an expert in the aeroelastic of suspension and cable-stayed bridges at the Polytechnic University of Milan. The voids between the boxes disperse the wind, decreasing the forces that cause flutter instability. The two lateral boxes will be for car traffic, and the central one for the railway. Designed for Messina, this solution has already been employed in other suspension bridges such as Çannakale, inaugurated in 2022.
The three-box design for Messina was tested in wind tunnels around the world, including at the Polytechnic University of Milan, showing that flutter instability emerges with winds of nearly 300 kilometres per hour3. This speed is higher and less likely to occur than the design wind, which is around 216 kilometers per hour and is predicted to happen on average every 2000 years, as estimated through a statistical analysis of data collected by the anemometers at the crossing site4.
A Eurolink representative says that it would take eight months to develop an executive project, which is needed to start construction. Experts say that starting work in 2024 is unlikely.
