US safety investigators yesterday singled out undersized steel plates as the chief cause of last August's fatal collapse of a highway bridge in Minneapolis.
But additional weight from construction material stockpiled on the centre span contributed to the collapse that killed 13 people, they said.
Federal investigators told the National Transportation Safety Board that the collapse on 1 August 2007 of the I35W bridge was "unavoidable" once gusset plates in the centre span failed.
Board members criticised Minnesota transportation officials for allowing the storage of 287t of construction materials for lane-widening on the bridge. The materials were stored above the gusset plates that fractured. But board members said it was not possible to determine if the materials alone — or factors such as weather and traffic, combined with the added weight — pushed the plates to a breaking point.
"Had the gusset plates been properly sized, this bridge would still be there," said Bruce Magladry, director of the NTSB's office of highway safety.
Investigators said the "half-inch thick" plates were inadequate to handle traffic and other stress factors and did not meet engineering guidelines when the bridge was built in 1967. The safety board, as far back as January, had identified design flaws in the plates as a critical factor in the collapse.
The board's final ruling is expected later today.