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Dating to the 1930s, the structurally deficient Chelsea Street Bridge needed to be replaced for more than the sake of motorists crossing the span between Chelsea and East Boston. The narrow waterway between its piers, less than 100 feet wide, allowed only older, single-hull fuel tankers to ply Chelsea Creek and reach nearby terminals.

The completion of the $125 million replacement bridge, ending a year of detours for motorists, allows for 175 feet of horizontal clearance in the channel, making room for the double-hulled tankers outfitted with outer layers of steel to guard against spills.

“From an environmental standpoint, that’s probably the biggest benefit,’’ said Frank DePaola, administrator of the state Department of Transportation’s Highway Division, which oversaw the project. “The fact that we can now allow modern tankers to come in is good for us all.’’

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A decade ago, the old bridge was targeted by state energy regulators as a threat to the environment and the region’s economy. In a report, regulators warned that the narrow passage posed a high risk for accidents and presented “one of the most serious vulnerabilities’’ to the state’s petroleum supply.

Three years ago, the same bridge and its impending reconstruction provided a backdrop for Governor Deval Patrick to announce his Massachusetts Recovery Plan, creating construction jobs with a blend of federal, state, and private investment.

Monday, officials gathered again to celebrate the completion of the bridge, the largest of its kind in the state.

Federal funds covered about 80 percent of the Chelsea Street Bridge work, DePaola said in a phone interview after the ceremony. The work was coordinated with Chelsea and Boston to manage traffic detours and also timed to coincide with a federal dredging project to improve the creek for tankers, he said.

The old span was a traditional drawbridge, known as a bascule bridge. The new span - 450 feet across and lined for two lanes of traffic and a sidewalk in each direction - operates like an elevator and rises to a height of 175 feet above the water at low tide, DePaola said.

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Chelsea Street was closed to traffic during the final year of work to allow for demolition of the old bridge and installation of the new span, which was constructed on a neighboring lot and moved into place using winches, he said.

Additional nighttime traffic closures will be needed until mid-June to allow for some finishing work, DePaola said.


Eric Moskowitz can be reached at emoskowitz@globe.com.