FROM

The next big bridge replacement won’t be nearly as quick as the Comm. Ave. project

The Commonwealth Avenue project was massively disruptive but was completed quickly. –Aram Boghosian for The Boston Globe

One bridge is more than a century old, and 1,100 feet long. The other is about half that age and a few hundred feet shorter. Both are on city streets that carry tens of thousands of cars a day in and out of Boston, one over a highway and rail tracks, the other over a river.

One took less than six weeks to rebuild. The other will take five years.

The Commonwealth Avenue bridge project was that rare thing in construction: a big job, done fast. State highway officials used a technique called accelerated bridge construction, in which many of the main components are built off-site and then dropped into place by crews working in 24/7 sprints. Though finishing work will continue in the coming months, the hard part is already over.