Ice from high-rises a menace in US cities

New York City dwellers battling one of the most brutal winters on record have been dealing with something far more dangerous than snow falling from the sky: ice tumbling from skyscrapers.

Streets around New York's new 1 World Trade Center, the nation's tallest building, were recently closed when sheets of ice were seen shearing from the face of the 540-metre-tall structure, turning them into potentially deadly, 160km/h projectiles.

And footpaths around high-rises in cities big and small have been cordoned off because of falling icicles and rock-hard chunks of frozen snow, a situation that experts warn could get worse during the next few days as a thaw sets in over much of the country.

"The snow starts to melt and the liquid drips off and makes bigger and bigger icicles, or chunks of ice that break off skyscrapers," said Joey Picca, a meteorologist at the National Weather Service in New York, which has had 123 centimetres of snow this winter and several cycles of freeze and thaw.

"Be very, very aware of your surroundings," he said. "If you see ice hanging from a building, find another route. Don't walk under hanging ice."

Some architects say newer, energy-efficient high-rises might actually make the problem worse.

"They keep more heat inside, which means the outside is getting colder and that allows more snow and ice to form," said engineer Roman Stangl, the founder of consulting firm Northern Microclimate in Cambridge, Ontario.

Stangl helps developers opt for shapes, slope angles and even colours - darker colours absorb more melting sunrays - to diminish ice formation.

High-tech materials can be also be used, such as at Tokyo's Skytree observation tower, where heaters are embedded in the glass to melt the ice.

Exactly how many pedestrians are hit by falling ice is not clear, but dozens of serious injuries are reported annually.

It's a perennial problem in St Petersburg, Russia, where dozens are reported to be injured or killed every year.

Seven people were hurt in 2011 near Dallas, Texas, when huge sheets of ice slid off the roof of the Cowboys Stadium.

Fifteen people were injured in 2010 by a shower of ice from the 37-storey Sony Building on New York's Madison Avenue.

Outside Chicago's 100-storey John Hancock Center in January, people scrambled with backpacks and purses over their heads to avoid falling ice.