The investigation has started in to the collapse of the road bridge in Baltimore, Maryland USA, on Monday that killed six maintenance workers.

National Transport Safety Board (NTSB) chair Jennifer Homendy confirmed on Wednesday that the bridge, which was used by more than 35,000 vehicles daily, was fracture critical. Homendy said, “What that means is if a member fails, that would likely cause a portion of, or the entire bridge to collapse. There’s no redundancy.” Francis Scott Key Bridge was built in 1976, and she explained that there is less tolerance for fracture critical structures now. “The preferred method for building bridges today is that there is redundancy built in, whether that’s transmitting loads to another member, or some sort of structural redundancy.”

She added, “There are 17,468 fracture critical bridges in the United States out of 615,000 bridges total – and that comes from the Federal Highway Administration.”

Homendy said that fact finding was underway. “Our entire focus on scene is to collect the perishable evidence. That’s documenting the scene, it’s taking photographs, it’s taking any sort of electronics or components. Whatever goes away once the scene is cleaned up.”

She continued, “When it comes to digging through inspections and maintenance records, that can be done when we leave. Right now, it’s focussed on the scene itself.”

Although the NTSB is presently concentrating on the scene, rather than records, Homendy was in a position to say, “This bridge was in satisfactory condition. the last fracture critical inspection was in May 2023. We have not been able to go through that inspection and all the documents, but that will occur after we leave the on-scene portion. But we’ve also requested all fracture critical routine and underwater inspections of the bridge over the last decade. Once we receive that, we will begin to go through all of those documents.”

The collapse was precipitated by a container ship striking one of its piers. The NTSB has requested information on pier protection for four other bridges owned by Maryland Transportation Authority.

As well as investigating the incident, the NTSB is also providing support to the families of the six workers who died in the collapse, and other people affected.