Even while they swarmed the Longfellow Bridge to test steel beams and replace rusty supports, state workers failed to inspect concrete that supports the train tracks on the span, prompting the Federal Highway Administration to demand this summer's frustrating Red Line slowdown.
The 10 mile-per-hour speed limit for the Red Line trains - which is a crawl compared with their usual speed of up to 40 miles per hour on the bridge - will continue at least through the end of this month while the test borings on the concrete are analyzed, officials said.
It was the third major safety precaution that the federal government recommended to the Massachusetts Department of Conservation and Recreation, which owns the bridge.
Three weeks ago, the federal government ordered that the state ban all large trucks from the bridge and prohibit all traffic from the left lanes going in both directions.
Work to shore up the Longfellow Bridge began after an interstate highway bridge collapsed in Minnesota last summer, killing 13 people. An inspection shortly after that catastrophe found that the 102-year-old Longfellow Bridge had seriously deteriorated in spots.
Since June this year, as repair work has been underway, the estimated 87,000 weekday commuters who cross the Longfellow on the Red Line have been inconvenienced by the speed restriction - and many have puzzled how the 64-ton trains could cross the bridge while even cars the size of a
Although they have agreed with the Federal Highway Administration's recommendations, state officials consider the concrete tests to be merely precautionary and the federal approach overly conservative.
State engineers say the tracks are not at risk and they have no reason to believe the concrete is a danger. Unlike much of the bridge's structure, visual checks have shown that concrete appears to be in good shape, state officials said.
Five repair crews have been working on the bridge almost every night.
"There's people climbing over that bridge all night, every night," said DCR Commissioner Richard K. Sullivan Jr. "Nobody has seen anything visually or in any of the tests or watching the operations to have any concern here. No one's expecting anything to be contrary to what they're seeing in the field."
Officials of the Federal Highway Administration declined to comment. State officials said the federal officials' concern is that concrete below the Red Line could buckle or crack, putting the tracks off-balance and derailing a train, said Dave Lenhardt, supervisor of parkways and bridges for the DCR.
The 10 mile-per-hour speed limit "is one of the things they recommend so if they did have that scenario, the T could stop without major injury or equipment failure," Lenhardt said.
The MBTA initially slowed trains on the track last month for repair work. When it continued, state officials said the slowdown was intended to reduce pressure on the bridge during repairs. Officials did not disclose that the slowdown was requested by federal authorities until this week, after inquiries by the Globe.
Sullivan called the speed and traffic restrictions on the bridge "an indication of just how careful everyone is being and how serious everyone takes public safety."
"Every day and every night the bridge is in a little better shape than the day before," Sullivan said.
Since the Minnesota collapse last August, Massachusetts officials have worried about the integrity of aging bridges like the Longfellow, which is built in an arch deck truss style similar to the collapsed Minneapolis span.
State officials have often offered assurances that, unlike the Minneapolis span, the multifaceted Longfellow has plenty of redundancies to protect it from collapse: If one steel segment fails, another carries its weight.
But Lucy Garliauskas, the Massachusetts division administrator of the Federal Highway Administration, expressed skepticism in the June 30 letter to the state about those assumptions. As state engineers often note, the turn-of-the-century Longfellow is not built like other bridges and cannot safely be analyzed using modern computer models.
"We don't have the original calculations," said Lenhardt. "We can't come up with enough historical data to prove how it works."
A full overhaul of the Longfellow has been so long postponed that the costs have ballooned from an estimated $100 million to $250 million. A full overhaul could begin by 2011.