A bridge that collapsed in Washington state and sent two cars plunging into the Skagit River was knocked down by a truck that crashed into at least one girder.
The truck, after the accident, rumbled across the bridge safely before a portion of the structure gave way, sending a car and pick-up into the frigid river on Thursday evening, along with a mass of concrete and steel. Three people were rescued.
While no one was killed, the collapse of the steel truss bridge built in 1955 prompted renewed calls from lawmakers in Washington, DC and elsewhere for greater investment in the nation's ageing infrastructure.
But Washington state officials said preliminary indications were that the bridge, which was inspected twice last year, was not structurally deficient and fell because of the impact from the truck striking its support beams.
"We had a collision between a very heavy vehicle travelling at probably not a small amount of speed crashing into not just one but probably multiple girders, and it failed," Governor Jay Inslee told an afternoon press conference in Mount Vernon.
Officials say the bridge, 90km north of Seattle, was not among the spans listed by the state as "structurally deficient", which in some cases relates to bridges that cannot carry their intended traffic loads.
But the privately run National Bridge Inventory Database listed the bridge as "functionally obsolete", widely defined by public officials as not built to current standards and demands.
"Based on our inspecting, the bridge is not structurally defective," said Lynn Peterson, state secretary of transportation. "We do take hits on almost every one of our bridges. This is just bad luck where and how it was hit."
She said there were a number of bridges along Interstate 5 in Washington state rated lower than the bridge that collapsed.
A new bridge that suffered a similar blow would have likely reacted the same way, Washington State Department of Transportation spokeswoman Jan Katzenberger said.
US National Transportation Safety Board investigators were on the scene of the collapse, which occurred on the four-lane Interstate 5, the principal highway between Seattle and Vancouver, Canada.
The NTSB said its members would inspect the 339-metre Warren Truss bridge, which links Mount Vernon and Burlington, including its substructure, deck and superstructure. Underwater inspections would also be conducted, the agency said.
David Goldberg, a spokesman for the non-profit policy group Transportation for America, said that the bridge's status as "functionally obsolete" did not necessarily mean that it needed to be replaced but that it might not accommodate modern needs.
"A lot of these older bridges are carrying much more traffic, and heavier traffic, than was envisioned when they were built," Goldberg said.
"There is no redundancy built in for the key structural elements. So if something happens - being struck by a vehicle or watercraft, a slight tremor, or vibrations from the traffic - it could set off a chain reaction that brings it down," he said.
The truck driver, identified as 41-year-old William Scott, made it across the bridge and pulled his vehicle to the side of the road after the accident. He has not been arrested and was co-operating with authorities.
Truckers must obtain a permit from the state Department of Transportation to carry an oversize load. The driver also receives a proposed route from the state, Washington State Patrol spokesman Sergeant Kirk Rudeen said.
STATE OF EMERGENCY
An official with Canada-based Mullen Trucking, which employed the truck driver, said state transportation officials had given clearance to take the oversize load across the bridge.
"Hopefully we will get some answers," said Ed Scherbinski, vice president of operations for Mullen Trucking, adding that the company was sending its own investigative team to the scene.
Scherbinski declined to immediately provide the height of the oversize load the trucker was hauling. The truck had been bound for Vancouver, Washington, he said.
Inslee declared a state of emergency in Skagit County and two neighbouring counties, citing the disruption of normal traffic for weeks or months, and said it would cost an estimated US$15 million (NZ$18m) to fully repair the bridge.
Inslee said officials hope to either secure or build a temporary replacement for the 160-foot span of the bridge that collapsed. It would likely take the form of a prefabricated truss bridge known as a Bailey bridge, he said.
"We are searching the entire country right now" for the right piece of infrastructure, he said.
US Transportation Secretary Ray LaHood said he would make US$1 million in federal emergency funds immediately available to Washington state to help repair the bridge.
In August 2007, a bridge fell into the Mississippi River in Minnesota, resulting in the deaths of 13 people and raising concerns about faulty infrastructure in the United States.
A 2013 report card from the American Society of Civil Engineers gave US bridges a grade of C+ and ranked Washington state as having the 11th-highest projected cost to perform what the organisation deemed necessary repair or replacement of deficient highway bridges. The cost was pegged at US$2.1 billion.
Washington state lawmakers are debating a proposed US$8.4 billion transportation funding package championed by Inslee and fellow Democrats. A key point of contention has been whether to pay for a new Interstate 5 bridge over the Columbia River connecting Vancouver, Washington, and Portland, Oregon.