by Paul Bowers
The City Paper received a note from a reader Monday with an interesting question: Could engineers have built an ice-proof Ravenel Bridge?
It's a timely question. After last week's snow and ice storms, which shut down the vital artery between Mt. Pleasant and peninsular Charleston for 43 hours, police reported that chunks of ice were falling from the bridge's superstructure, crushing at least one vehicle's windshield as it crossed underneath.
We called Timothy Mays, a bridge designer who teaches civil engineering at the Citadel, to see what he knew about building bridges and bridge cables to resist ice accumulation.
"Practically speaking, I don't think you're going to put any kind of heating mechanisms in, and I don't think you change the geometry of the bridge so that the ice falls to the side or anything," Mays said. "I don't know if there's any real way to practically, economically do that."
The question of winterizing the bridge has two parts, which we'll address separately: Keeping ice off of driving surfaces and keeping ice off of spans and cables.
Some highway departments have installed systems to keep ice off of bridge decks, but not off of the bridge spans or superstructure. From 1992 through 1997, the Federal Highway Administration provided funding to install heating systems under seven bridges in Nebraska, Oregon, Texas, Virginia, and West Virginia. Three different types of systems were installed: 1. Hydronic systems, which pumped heated fluid in a loop under paved surfaces, 2. Heat pipes, in which vaporized fluid is pumped through pipes, and 3. Electrical systems, in which heat was generated by metal conductors.
Many of the FHA projects were smaller overpasses and walkways, so it's hard to say whether similar measures would be practical on a bridge the size of the Ravenel, but they reportedly were successful in preventing ice from accumulating on bridge surfaces. Costs ranged from $150,000 for a hydronic system under a pedestrian overpass in Lincoln, Neb., to $1.2 million for a hydronic system with geothermal wells under a road overpass in Amarillo, Texas.
Other bridge de-icing projects have had mixed success. In 1999, to combat the frequent formation of black ice on an I-35 bridge over the Mississippi River in Minneapolis, the Minnesota DOT spent $618,450 installing nozzles to automatically spray the surface down with potassium acetate during high-risk weather. The system was effective, but when the bridge collapsed during an evening rush hour in August 2007, killing 13 people, National Transportation Safety Board investigators found that the potassium acetate might have corroded galvanized steel elements on the bridge, partially causing its collapse.
So, could engineers have built an ice-free bridge in balmy Charleston? Technically, yes. But, Mays says, "I'm not sure that it's something practical to design for."
The problem of falling ice is not unique to the Ravenel Bridge. One similar instance took place on Boston's Bunker Hill Bridge in 2005, but bridge engineers there called the event a "fluke" of weather. Boston Globe reporter Mac Daniel wrote:
Michael Swanson, chief engineer for the Central Artery/Tunnel project, said officials would "keep an eye" on the bridge and close lanes on the two-year-old span if ice falls again. He rejected some suggested fixes, saying he had never heard of heating a bridge's cable stays to prevent icing. A deicing agent could not be used because of concerns that it would pollute the Charles River or make the roadway slippery, he said.
Elsewhere in the world, Sweden's Uddevalla Bridge — a massive cable-stayed bridge with diamond-shaped elements similar to the Ravenel Bridge's — closes several times a year due to dangerous chunks of falling ice. In 2011, researchers from Dartmouth College began testing new pulse electro-thermal de-icing (PETD) technology on a few cables and one pylon of the Uddevalla Bridge. By applying short pulses of electricity between the ice and the cables, researchers reported "instant de-icing action at very low energy consumption as compared with conventional de-icers." A report from Dartmouth professor Victor Petrenko noted that conventional heating methods are highly inefficient for de-icing bridges and roads due to heat loss to the earth and air. "Petrenko's super-efficient PETD has demonstrated immediate effectiveness and saves up to 99% of electric energy compared to conventional heaters," a press release from the college noted.
So, could engineers have prevented the Cooper River Bridge from dropping potentially deadly icicles from its spans? Probably not at the time the bridge was built in 2005.
Overall, Mays says he thinks bridge designers did a fine job designing the Ravenel Bridge, and local law enforcement agencies were right to shut the bridge down.
"I think it's probably the right thing, what they did, for the once-every-10-year kind of event we had here," Mays says.