Thousands of bridges around the U.S. may be one freak accident or mistake away from collapse, even if the spans are deemed structurally sound.

The crossings are kept standing by engineering design, not supported with brute strength or redundant protections like their more modern counterparts. Bridge regulators call the more risky spans “fracture critical,” meaning that if a single, vital component of the bridge is compromised, it can crumple.

Those vulnerable crossing carry millions of drivers every day. In Boston, a six-lane highway 1A near Logan airport includes a “fracture critical” bridge over Bennington Street.

In northern Chicago, an I-90 pass that goes over Ashland Avenue is in the same category. An I-880 bridge over 5th Avenue in Oakland, Calif., is also on the list.

Also in that category is the Interstate 5 bridge over the Skagit River north of Seattle, which collapsed into the water this week after officials say an oversized truck load clipped the steel truss.

Public officials have focused in recent years on the desperate need for money to repair thousands of bridges deemed structurally deficient, which typically means a major portion of the bridge is in poor condition or worse. But the bridge that collapsed Thursday is not in that deficient category, highlighting another major problem with the nation’s infrastructure: Although it’s rare, some bridges deemed to be fine structurally can still be crippled if they are struck hard enough in the wrong spot.

“It probably is a bit of a fluke in that sense,” said Charles Roeder, a professor of civil and environmental engineering at the University of Washington.

While the I-5 truck’s cargo suffered only minimal damage, it left chaos in its wake, with two vehicles catapulting off the edge of the broken bridge into the river below. Three people involved escaped with non-life threatening injuries.

The most famous failure of a fracture critical bridge was the collapse of the I-35W bridge in Minneapolis during rush hour on Aug. 1, 2007, killing 13 people and injuring more than 100. The National Transportation Safety Board concluded that the cause of the collapse was an error by the bridge’s designers — a gusset plate, a key component of the bridge, was too thin. The plate was only half of the required one-inch thickness.

Because the bridge’s key structures lacked redundancy, where if one piece fails, there is another piece to prevent the bridge from falling, when the gusset plate broke, much of the bridge collapsed.

Mark Rosenker, who was chairman of the NTSB during the I-35W bridge investigation, said the board looked into whether other fracture critical bridges were collapsing. They found a few cases, but not many, he said.