The design flaw that brought down a highway bridge in Minneapolis went undetected for 40 years. Federal safety officials say that, even today, government design reviews might not detect similar problems.
The National Transportation Safety Board made official Friday what has been clear for months — that the failure of undersized, steel gusset plates was the probable reason Interstate 35W bridge buckled and then fell into the Mississippi River.
The board found that engineers who designed the bridge in the 1960s either failed to calculate or improperly calculated the thickness needed for the plates that were to hold the bridge together.
They also blamed state and federal highway officials for not catching the design flaw at the time the bridge was approved for construction.
On Aug. 1, 2007, under stress from 287 tons of construction material and rush-hour traffic, the bridge’s center span shuddered, then collapsed, dragging other spans into the river. Thirteen people were killed and 145 were injured.
One of the chief lessons of the tragedy, board members said Friday, is that state transportation officials may not probe deeply enough into the design details of their bridges.
“As the stewards of the public dollar, it’s the obligation of government to trust but verify,” board member Debbie Hersman said.
Program to detect errors recommended
The board recommended the federal government establish a program to detect errors in bridge designs to prevent a repeat of the tragedy. The Federal Highway Administration has guidelines for how states should review bridge designs, but no formal standards.
In a statement, Transportation Secretary Mary Peters said she was directing the Federal Highway Administration to work with the states to improve quality controls during bridge designs.
Major bridge collapses are rare. Over the last four decades, the NTSB has investigated 24 bridge collapses, only five of which resulted from something other than a ship or a truck running into the bridge.
Design flaws, however, may have occurred more often than thought before the Minneapolis bridge collapse. After the tragedy, safety investigators asked transportation officials in 14 states if they had found instances in which bridges had design flaws. Ten states responded they had found at least one.
That’s only 10 examples out of about 80,000 bridges built during the time frame state officials were asked about. Not all the flaws found were “safety critical,” said Mark Bagnard, chief investigator for the safety board. “Thankfully, it’s not all that common.”
'A bigger problem'
Board member Kitty Higgins called the bridge collapse “a wake-up call” for the government and those who design and build their bridges.
“We found through our conversations that there are a number of examples (of bridges with design flaws) that states fortunately caught on their own,” Higgins said. “It says to me that this is a bigger problem.”
Some board members were reluctant to blame Minnesota and federal transportation officials for not catching the design flaw in the bridge, saying they followed the accepted practices at the time. The bridge opened in 1967.
Hersman, however, said that was no excuse.
“Just because standards or checklists don’t exist doesn’t mean we don’t hold people accountable when that happens,” Hersman said. “The people who are accountable are the people who designed the bridge and the people who accepted that design.”
Although there may be problems in detecting design errors, some states, including Minnesota, now have a second engineering firm do independent reviews on major bridges.
Dan Walsh, an official in the NTSB’s highway safety office, said that states now have additional safeguards, such as requiring that firms designing complex bridges have engineers on staff with experience in such design projects.
The bridge, which opened in 1967, was called “fracture critical.” That meant a failure of any number of structural elements would bring down the entire bridge.
Lawsuits filed over collapse
Investigators said bridges of this type generally aren’t built today, not because the overall design was faulty, but because technology has progressed and there are new types of bridges that require less maintenance.
With the NTSB inquiry completed, independent engineers are getting access to bridge parts to make their own analyses for lawsuits filed over the collapse.
In Minneapolis, attorney Jim Schwebel filed the first lawsuits on behalf of four victims on Thursday. He sued URS Corp., the San Francisco consulting firm hired by Minnesota to evaluate the bridge in the years before it failed, and Progressive Contractors Inc., the St. Michael, Minn. company resurfacing the span when it fell.
The state set up a $38 million victims compensation fund. To collect, victims must sign away their right to sue the state. Nothing bars them from going after private entities in court.
Chris Messerly, a Minneapolis attorney for the largest group of victims, said his clients will probably wait until the compensation process concludes early next year before suing.