MONTPELIER — A historic Vermont bridge is going to be updated with a technique that seems like something only a superhero could accomplish: Engineers will cut the 350-foot bridge in two lengthwise, push the two sides apart, widen them, and put the bridge back together.
As far as anyone involved in the project has been able to determine, it is the first time such a technique has been tried on such a large bridge, and after 15 months of preparation, engineers are getting ready to separate the sides, move one 300,000-pound truss 10 feet away from the other and then reattach the sides. Once the steel and concrete has been added, motorists crossing the bridge will not have to worry about meeting a truck coming the other way.
The so-called Pennsylvania truss bridge, which carries US Route 2 across the Winooski River about 10 miles east of Burlington, was built in 1929, two years after an epic flood washed out a covered bridge there. The bridges are marked by the crisscrossing I-beams that form a latticework that creates a steel canopy above the road surface.
It was marvel of its time, but by the bridge’s 60th birthday in 1989, Vermont officials had recognized that it needed to be modernized. The two travel lanes were a total of 20 feet across, barely wide enough for two cars to pass; there is no shoulder, no room for bicyclists or pedestrians; and the steel and the supports were showing their age.
‘‘I travel that road every day, twice a day,’’ said Carolyn Carlson, Vermont Transportation Structures project manager, who started working on the Richmond bridge in 1989. ‘‘Driving on it before, when it was 20 feet [from] rail to rail, you’d find it very nerve-racking.’’
Still, state and federal officials wanted to keep the historic look of the bridge; three such bridges are left in the state and the Richmond one is the only over on a major roadway. It took decades to decide on the unusual course of action.
The project’s cost, an estimated $16 million, is more than double that of building a new bridge. At first, officials were reluctant to undertake the more expensive option, but as time went on, it became clear it could work.
‘‘When you are dealing with a historic resource, you don’t look at it as extra money,’’ said Scott Newman, the historic preservation officer for the Vermont Agency of Transportation.
Two federal laws require that the significance of historic structures be taken into account when planning transportation projects, as long as the alternative is ‘‘prudent and feasible,’’ Newman said.