Bridge's falling ice called fluke of nature

Big Dig officials and the engineer who designed the Leonard P. Zakim Bunker Hill Bridge yesterday dismissed concerns about falling ice from the span's cable stays, calling Sunday's alarming cascade a ''fluke" caused by the weather.

But Big Dig officials could not guarantee that it will never happen again.

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.

''Cables don't normally accumulate ice like these did, and the reason for that is because there's a high angle to the cables and there's a spherical shape to them," Swanson said at a news conference. ''So what we really saw [Sunday] was kind of a fluke of nature. It's not something that happens very often."

He said the ice buildup was not caused by the one-of-a-kind design of the covers on the Zakim's 210-foot-high steel cables. The white plastic covering that shields the cables from rain, snow, and sun is ringed by a continuous spiral of plastic intended to break up rivulets of water that could throw the cables out of balance and cause the bridge to sway or vibrate. It is the first bridge in the world to use this feature, according to an award the bridge received from the American Society of Civil Engineers.

''We don't believe they had anything to do with the snow buildup on Saturday, as evidenced by the fact that there was snow buildup on top of the sheathing in locations where the beads were located on the underside of the sheathing," Big Dig spokesman Doug Hanchett wrote in reply to questions about the cable-stay's design.

A spokeswoman at HNTB, the engineering firm that helped design the bridge, referred all questions about the Zakim to Big Dig officials. Big Dig officials, in turn, referred reporters to the US Army Corps of Engineers and Cape Cod Canal manager Fran Donovan, who said both the Bourne and Sagamore bridges often deal with falling ice from their steel spans.

''I don't think you're talking about anything that has to do with steel versus plastic," Donovan said. ''It's just the right weather and temperature conditions."

Christian Menn, the engineer who designed the bridge, said from his home in Switzerland that while ice falling from the cable stays is unusual, ''if you have certain weather conditions, it can happen."

''And it happens everywhere, even in the big bridges in New York such as the George Washington and the Verrazano-Narrows Bridge," he said.

The Sunday ice fall on the Zakim did not injure anyone, but did damage one driver's windshield as she drove northbound in the bridge's far right lane around 12:30 p.m. After pulling off at Exit 28 in Sullivan Square, the woman contacted nearby State Police, who later closed all but two lanes on the bridge until about 2:30 p.m., backing up traffic on Interstate 93, the Tobin Bridge, and Storrow Drive. It was the first major problem with the Zakim, considered the crown jewel of the $14.6 billion Big Dig project, which has recently been plagued by a series of underground leaks as well as chunks of spray-on fireproofing falling from the tunnel ceiling.

''It didn't happen at all last winter, and we almost got through this winter without it happening," Swanson said.

''We're very much, you know, hostage to whatever Mother Nature wants to throw at us," he added. ''But it was an unusual circumstance and one that we wouldn't expect to get very often, certainly not again this year."

Mac Daniel can be reached at mdaniel@globe.com.