October 19, 2019

Engineer says fear of ice falling from new bridge unwarranted

VERONA ISLAND – Maine winters not withstanding, the new bridge being built across the Penobscot River will not be susceptible to the kinds of problem that plagued the Leonard Zakim Bunker Hill Bridge in Boston last weekend, according to a state transportation engineer.

According to newspaper reports, snow built up on the bridge’s cable stays and formed large chunks of ice that around noontime Sunday began to drop onto the deck surface. One chunk hit a passing vehicle, damaging the windshield. No one was injured, but, according to reports in The Boston Globe, state police closed all but two of the bridge’s 10 travel lanes for about two hours, backing up traffic on the major routes into the city.

Transportation officials blamed the incident on “a fluke of nature,” rather than the bridge’s design, but, on Tuesday, the Maine Department of Transportation’s engineer in charge of the bridge project said a major design difference between the two bridges makes it less likely motorists will encounter similar hazards.

The Zakim Bridge is similar in design to the bridge now under construction between the towns of Verona Island and Prospect. It is a cable-stayed bridge that uses cables encased in a protective plastic cylinder – the cable stays – to support the weight of the bridge deck.

“The difference in the Zakim Bridge is that the stays that go to the main span go to the outside edge of the bridge,” Project Manager Tom Doe said. “So the stays have to cross over the traffic as they go from the pylon in the middle to the edge of the bridge.”

With that design, he said, any ice that builds up and drops from the stays can – as it did on Sunday – drop onto a car.

“That was one of the things we were concerned about when people were talking about wanting a double-plane bridge,” said Doe, who had not heard about the Boston incident.

The final Penobscot River bridge design, however, calls for a single-plane design in which the cable stays run from central pylons to anchors located in a median strip along the center of the bridge deck.

“If ice should build up there – as I suppose it probably will – it would drop directly down and onto the median … in the middle of the bridge,” Doe said.

Plans call for the median to be 14 feet wide and to be separated from the two travel lanes by concrete barriers and a 2-foot shoulder.

Although some icing on the bridge is likely, Doe said he did not anticipate large buildup of ice or snow on the bridge or the cables. The cable stays on the Prospect-Verona Island bridge are much larger at about 17 inches than those on the Zakim Bridge, Doe said, making it less likely that ice or snow would build up on the surface.

Wind also will be a factor in keeping ice and snow buildup to a minimum. The Zakim Bridge is “quite a bit lower and it’s protected from the wind,” Doe said. At 420 feet, the Penobscot River bridge will be about 150 feet higher than the Boston bridge and will be subject to more winds that run up and down the river. Doe pointed out that there has been little icing or snow buildup on the structure or cables on the existing Waldo-Hancock Bridge.

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