Charles Sullivan says the Longfellow Bridge was already in bad shape when he got his job as director of the Cambridge Historical Commission in 1974.

"You could always look up and see the rusting girders, and see that things were falling off it," Sullivan says.

Things got so bad that in 2007 the MBTA had to limit the speed of Red Line trains crossing the bridge to 10 miles per hour. And the state regularly closed the bridge on the Fourth of July to keep people from breaking through the rusty railings and tumbling into the Charles River.

It wasn't until 2008 that the Longfellow Bridge and other decaying bridges in Massachusetts finally got some love, in the form of a $3 billion bridge repair fund.

The Longfellow was "at the top of the list" for rehabilitation because of its condition, according to Sullivan. "It would have been a regional disaster if they had let it go any longer."

But fixing the historic bridge is turning out to be an even bigger challenge than engineers expected. A massive overhaul that began in 2013 was originally expected to wrap up this year. Contractors now expect the work to continue until mid-2018, thanks to two big complications.

The first problem was rust.

The big steel arches connecting the 12 masonry piers and abutments were in good shape. So were the bridge’s 58-foot “salt and pepper” stone towers, although the 529 granite blocks in each tower, some weighing as much as 3 tons, needed a good cleaning — a process that has now been completed.

But the 2,000 steel “spandrel columns” that rise from the arches to support the deck were beyond hope of repair. Many of them were so rusted through they had to be replaced.

A view of the old, rusted spandrel columns rising from the main arches to support the road deck on the bridge's downstream side. (Wade Roush for WBUR)

The second problem was history.

The bridge is an important landmark to Bostonians, Sullivan points out. “We’ve got the State House on Beacon Hill. We’ve got the Bunker Hill Monument. We’ve got the Longfellow Bridge. So it’s right up there as an iconic object.”

More to the point, Sullivan says, the bridge and the entire Charles River Basin were put on the National Register of Historic Places in 1976. Federal preservation regulations required that every visible part of the bridge — down to the smallest piece — had to be restored using the same technology and material used when the bridge was built.

And that included thousands and thousands of rivets.

Riveting is an ancient technology for fastening metal, dating back 5,000 years. But its heyday was from the 1800s until World War II.

“In the old days, when they would build a high-rise steel frame, the men would have a furnace on the upper floors of the building,” Sullivan explains. “They would heat the rivets until they were red hot. Somebody would toss that rivet. Somebody else would catch it in a bucket, and, with tongs, would push it into a hole.”

One worker would hold one end of each rivet, while another attacked the other end with a pneumatic hammer until it had deformed into a half-ball.

“I remember, as a child, that was sort of a background noise on a construction project — somebody way up high going bang, bang, bang, bang, bang with a rivet gun,” Sullivan says. “And at some point that disappeared.”

Workers pour concrete on the walking and biking path along the Longfellow Bridge. (Joe Difazio for WBUR)

Hot riveting died out in the '50s, replaced by faster, cheaper methods like welding and high-strength bolts. Engineers on the Longfellow Bridge project knew that in order to follow the letter of the historic preservation law, they’d have to bring back this nearly lost art.

That job fell to Cianbro Fabrication and Coating Corp. The company is making all of the new steel parts for the Longfellow Bridge at its factory in Georgetown — including spandrel columns and other components containing more than 50,000 rivets.

Why so many? That’s partly a matter of steelmaking technology. Today, the spandrel columns would be made from simple I-shaped steel. But the technology to extrude steel in that shape didn’t exist in 1906-'07, when the Longfellow Bridge was being built.

Instead, as Cianbro manager Jack Klimp explains, each of the bridge’s spandrel columns was built using two long C-shaped pieces of steel, “placed back to back, about a foot part, connected in a lattice pattern with bars that crisscross.” And at every single connection, there’s a rivet.

To handle all that riveting, Cianbro turned to a modern hydraulic press that requires only two workers to do the job that used to take four.

“The press has got many, many tons of force and the rivet just deforms, not like it’s butter, but like it’s clay,” Klimp says. “So there’s no banging, booming, there’s none of that. It’s a much more civilized process.”