City closes Northern Avenue Bridge in South Boston

The Old Northern Avenue Bridge, built in 1908, has teetered on the edge of demolition a number of times.JOSH REYNOLDS FOR THE GLOBE

The bridge has been around for more than a century, beloved by preservationists for its unusual design and used by pedestrians and bicyclists trekking between downtown Boston and the South Boston Innovation District.

But now the city has closed the Old Northern Avenue Bridge, citing “deterioration beyond repair” to the floor beams. The city offered no clear plans for the future of the 640-foot steel and concrete span across the Fort Point channel, raising fears that it may not be returned to service.

“This bridge really captures the character of the South Boston innovation district and it’s a part of our history,” said Vivien Li, the president of the Boston Harbor Association. “It would be a shame if we lost that.”

The closing marked the second time in recent months that the city has shut down a bridge maintained by its Department of Public Works for safety reasons. In October, city officials abruptly shut down the Long Island Bridge, used to shuttle homeless people and others to social service programs on the island in Boston Harbor.

In closing the Old Northern Bridge on Wednesday, city officials said they were acting in an “abundance of caution,” according to Bonnie McGilpin, a spokeswoman for the mayor’s office.

This week, city consultants reported they found 13 floor beams on the bridge were no longer safe for pedestrians, according to a Dec. 18 e-mail from the company to the city engineer, Para Jayasinghe.

“Due to the severity of the deterioration, it is not possible to perform immediate repairs to restore these members to the required capacity,” wrote Paul Norton, an inspector with TranSystems, a firm that has long worked with the city on evaluating its bridges.

In the e-mail recommending the closure, consultants from TranSystems Corp. offered few details about its recommendation for the future of the structure. They promised to meet with the city to review the details of its recommendations.

The city is currently “evaluating its options” and working with the Boston Preservation Alliance to balance preservation interests with the need for public safety, McGilpin said.

Pedestrians and bicyclists are advised to use one of three other bridges over the Fort Point Channel — the Evelyn Moakley Bridge, the Congress Street Bridge, and the Summer Street Bridge — while the Old Northern Avenue Bridge is closed.

For years, the Old Northern Avenue Bridge teetered on the brink of demolition as city officials pointed out drawbacks in continuing to maintain it. But preservationists say the bridge, constructed in 1908, is one of the state’s few “swing” bridges, spans that rotate horizontally when water traffic needs to pass.

The Boston Preservation Alliance has worked with the city to keep the bridge up in its original form, according to Greg Galer, the executive director of the nonprofit. At one point, city officials wanted to lift the bridge, so that it would no longer have to operate as a swing bridge.

“We understand it’s a challenge,” he said. “It’s a very expensive project, but we believe that this bridge is an integral part of the city’s character.”

It once carried vehicles, but changed to pedestrian- and bicycle-only traffic in 1997.

In 1998, the director of the Boston Redevelopment Authority said saving the bridge was too expensive. Around the same time, J. Joseph Moakley, then a US representative, supported demolishing the bridge to clear the view to the nearby bridge named for his late wife, Evelyn.

Former mayor Thomas M. Menino had once opposed keeping the bridge open, even vetoing a unanimous Boston Landmark Commission vote to give it landmark status.

City officials later changed their stand, with Menino proposing to rebuild the span in 2008. But money never materialized for a massive reconstruction.

Bridges like the Northern Avenue span are inspected about every five years, according to a city official.

TranSystems last inspected the bridge in July to determine how much weight it could carry. City officials then asked for another TranSystems inspection, which was conducted in November, according to the city.

Supporters have kept their hopes alive that the bridge would be rebuilt. In 2012, the Boston Harbor Association was awarded a $50,000 grant to put garden planters on the bridge, which are now in place.

And earlier this year, the Boston Redevelopment Authority said it hoped the bridge would again be able to host cars in a plan to revitalize a strip between the North End and the Fort Point Channel.

Lauren Grymek, the executive director of the nonprofit Seaport Transportation Management Association, said the bridge should be kept open to bikes, pedestrians, and eventually perhaps include rapid transit.

“It has a ton of potential, and I would hope there’s a way to balance preserving it from a historical perspective, but maximizing that potential,” she said.

Li, of the Boston Harbor Association, also said the pedestrian bridge is an essential way to keep revitalizing the neighborhood and she hopes plans to rebuild it come through.

“I think it’s a really important gateway between the South Boston innovation district and the downtown area,” she said. “It’s absolutely key.”