The Baltimore bridge that collapsed Tuesday was “fracture critical,” the National Transportation Safety Board, or NTSB, said in a briefing Wednesday.

To be “fracture critical,” a bridge has to lack built-in redundancies, meaning if a single vital piece of the structure were to fail, either a part or the entire bridge could crumple.

Jennifer Homendy, the chair of the NTSB, said, “the preferred method for building bridges today is that there is redundancy built in.”

This includes additional protective structures or the ability for a bridge to share the load with “another member” in the event of a failure, she said.

Fracture-critical bridges are a minority in America — but the Francis Scott Key Bridge isn’t the only one.

Of about 615,000 bridges in the United States, 17,468 are fracture critical, Homendy said, citing data from the Federal Highway Administration.

She added that the Key Bridge was in “satisfactory condition” and last underwent a fracture-critical inspection in May 2023.

Construction on the bridge started in 1972 and it opened to the public in 1977. The bridge was comprised of three spans totaling 9,090 feet in length, Homendy said, adding that an average of 30,767 vehicles crossed it daily.

The collapse on Tuesday raised questions about the safety of America’s infrastructure, including whether the bridge’s age meant its protections were outdated.

It’s unclear if possible protections such as a group of pilings called “dolphins” or buffering “fenders” would have made a difference. Some experts told BI that nothing could have withstood the Dali cargo ship’s massive blow.

On Wednesday, the NTSB reported the ship radioed for help from nearby tugboats for help minutes before it crashed, but it was too late, data the NTSB retrieved from the ship’s voyage data recorder shows.

Two construction workers were rescued after the crash, while six were presumed dead. Authorities said Wednesday that two bodies were recovered from a submerged pickup truck.