The problem of falling ice from suspension bridge cables is an interesting, if rare, conundrum.
From a structural point of view, most suspension bridges can easily cope with the additional load of snow or ice mass.
The problem lies in the melting phase, when large chunks of ice are prone to plummet and damage cars or pedestrians.
In 2009 ice fell from the two Severn crossings, resulting in smashed windscreens and scores of compensation claims from motorists.
Following the first closures, the Highways Agency commissioned consultants to urgently research incidents on cable-system bridges in Canada, Norway, Sweden, the United States and Japan.
But somewhere where there has been extensive research is on the Veterans’ Glass City Skyway, in Toledo, Ohio, a 2,682m long, 122m tall cable-stayed bridge, opened in 2007.
University of Toledo professor of civil engineering Douglas Nims has been leading a team attempting to rid the bridge of ice shedding, sponsored primarily by the Ohio Department of Transport. But it’s proving tricky.
“We’ve looked at putting chemicals on it, wiring it, heating it. We basically looked at every available technology, and none of them are really practical for the bridge. We’ve got 18,000 feet (6,000m) of stay, which is a lot of area to cover.”
Suspension ice 2
Engineers that designed the bridge said ice shedding would never be a problem. And yet since then, the region has had an average of two damaging ice storms a year, where sheets of ice of about 6mm thickness falls.
“It only ever happens at the beginning or the end of the winter, conditions have to be just right. Warm air comes up from Mexico, cold air comes down from Canada.”
But Ohio is not the only part of the US affected. The research has wide-reaching impacts, with more than 80% of US bridges located in or near areas that have had damaging ice storms, Nims says.
“Pretty much every bridge east of the Mississippi and north of Florida is subject to icing,” Nims says. ”In the middle of the country, east of the Rocky Mountains, there’s no water in the air so you don’t get icing.”
In 2007, sheets of ice up to 19mm thick and 1.83m long fell 66m from the Veterans’ Glass City Skyway.
A few lanes were closed and motorists reimbursed for damage to their cars. But Nims acknowledges this is far from a best practice solution.
“In London, I’m sure there’s bridges there that no matter what happens, you won’t shut them down. When we talk to colleagues in New York, you can’t close them either, unless it’s really extreme. But for bridges like ours, which is never really at full capacity, if you close a few lanes it’s okay. But if you close the whole bridge for six or eight hours, that’s when it becomes a problem.”
Suspension ice 1
So what is the best solution?
Traditionally, manual mechanical removal of ice has been the only option, with the often dangerous work reserved for hoisted workers. Or infrastructure could be erected to ’catch’ the ice.
Another solution is to use electricity to stimulate movement of the ice. One such method is an Electromagnetic Impulsive De-Icing System, which uses high current DC pulses run through a coil, leading to debonding and expulsion of ice. It was found effective for light to moderate ice accretions, tested for three years on the Storebaelt East Bridge in Denmark. Despite early success in the trial, a heavy 50mm ice accretion rendered the system ineffective.
Then there is the thermal solution.
On Hakucho Bridge, Japan, a study of the effects between different heating methods was made, including use of metal foils, air and water, but large amounts of energy were required in each case. In Toledo, the Skyway’s stays were hollow, so heating was possible, Nims says.
“We did some thermal analysis: the issue was pushing hot air up all of the stays, having 20 heaters on the bridge. Then, when the air gets warm, it picks up moisture, so then you have to dehumidify it, which of course is a big issue, so we never even got down to the question of cost.”
In Sweden, a high pressure hot air system has been employed on the Udevalla Bridge. The air is pushed through small holes in high density pollyelthelyne tubing. Again, large quantities of energy are required.
We’re dying for another icing event, to test all our models.
Finally, passive systems were considered, using chemical coatings. Nims says research conducted with the US Army’s Cold Regions Laboratory – one of the foremost expert bodies in this area – fielded good data, but no solution.
“People turn up to the US Cold Regions Lab and expect that they’re going to get a solution. And it’s like the Kubler-Ross model for five stages of mourning: first there’s denial… But you have to manoeuvre to admitting that there is no solution, and it then becomes a question of how to manage it in the best way,” says Nims.
The Toledo team has instead developed an “ice hazard mitigation strategy”, using bespoke sensors on stays to measure ice thickness and weather. A conventional icing sensor system, commonly used in airports, has also been adapted for the stays, but is less than ideal, Nims says. Not least because it can cost more than £20,000.
Data from sensors is converted to a model, which has been proven to predict ice accretion and ice shedding on other bridges.
Suspension ice 3
Some of the models’ mathematics were developed from previous work done on powerline icing.
“Powerlines, which have icing issues, they can rotate along the longtitudinal axis, and get cylindrical casings of ice. Our stays don’t rotate, so we revised those models, adapted them and hindcast (testing against past weather patterns) them,” Nims says.
Nims says the mitigation strategy can be further developed – all that is needed is another major icing event during which more tests can be carried out; while there have been six icing events since 2007, when the bridge opened, there hasn’t been another since 2011.
“And we’re dying for another icing event, to test all our models.”