Falling ice that smashed windshields on the Arthur Ravenel Jr. Bridge on Jan. 31 reflects a wider problem reported on similar bridges in the U.S. and Canada.
Preventive solutions are being tested.
The cable-stayed design of the Ravenel Bridge has become increasingly popular since the 1970s, for aesthetic, cost and engineering reasons. But it has also been associated with incidents like what happened here when large chunks of ice hit cars.
In British Columbia, "ice bombs" fell on the new Port Mann Bridge in December. In response, officials there devised a system to brush and scrape away snow and ice on dozens of cables.
In Toledo, Ohio, experts created an automatic early-warning system to advise of hazardous conditions on the ice-plagued, cable-stayed Veterans Glass City Skyway.
Maine is evaluating whether heat could be used to thaw cables in the wake of ice sheets hitting cars last month on the Penobscot Narrows Bridge.
Conditions that lead to falling ice on bridges do not occur very often, said Paul Ziehl, a professor in the Department of Civil and Environmental Engineering at the University of South Carolina.
"That is more true for Charleston and locations south of Charleston," he said.
Chunks of melting ice dropped from the Ravenel Bridge cables and diamond-shaped towers in the wake of an unusually fierce winter storm that hit a few days earlier. At least nine people called 911 to report damaged vehicles, and the bridge closed for six hours beginning in mid-morning.
"The culprit appears to be the diamond-shaped towers that have the cables extending over the roadway," said Tim Mays, an associate professor of civil engineering at The Citadel.
He pointed to a different design for the cable-stayed Talmadge Memorial Bridge leading into Savannah. On that span, support cables parallel the bridge deck rather than rising over it.
"Another shape may have prevented the problem, but in my opinion, at this point, it was a freak thing that would rarely happen, and closing the bridge is the best response," Mays said in an e-mail.
State Department of Transportation officials said they will explore use of large heaters to melt ice on bridge roadways. Possible measures to manage overhead ice build-up have not been publicly identified.
Six people have contacted the DOT's Charleston office to request information about filing claims for car damage from falling ice.
"All were sent forms, but no claims have been filed as of yet," DOT spokesman Pete Poore said Thursday.
The DOT is not liable for a weather-related loss resulting from snow or ice conditions unless it is "affirmatively caused" by negligence on the part of an employee or the department, according to the state Tort Claims Act.
"S.C. DOT is considering all options for the Ravenel Bridge to address the roadway deck itself as well as the overhead hanging ice that formed as these two issues were at the core of the bridge closures," the department said in a statement.
The falling ice was preceded by two days during which the Ravenel Bridge was closed because of icy travel lanes. The Don Holt Bridge on Interstate 526 also was closed because of ice on the road. It is a different truss-type bridge that does not have overhead cables.
Bridge cable icing-preventive measures identified by researchers include anti-icing chemicals, a cable coating to reduce ice accretion and changing the shape of the sheath over cables to inhibit ice formation. In addition, high velocity water, air and steam could be used to force ice off cables.
A professor at Dartmouth University is exploring the use of short pulses of electricity to prevent bridge icing. The method proved successful when applied to a cable-stayed bridge in Sweden where falling ice is a regular problem, according to the School of Engineering.
Although ice falling from cable-stayed bridges onto travel lanes is an issue, the design is considered a fast and economical way to build medium-to-long spans.
"The result is a cost-effective bridge that is undeniably beautiful," says the website for the PBS science show "Nova."
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