Thousands of bridges around the United States may be one freak accident or mistake away from collapse, even if the spans are deemed structurally sound.
The crossings are kept standing by engineering design, not supported with brute strength or redundant protections like their more modern counterparts. The consequence of that risk was evident Thursday on Interstate 5 over the Skagit River, where a large section of the bridge collapsed into the water after officials say an oversized truck load clipped the steel truss.
While the truck's cargo suffered only minimal damage, it left chaos in its wake, with two vehicles catapulting off the edge of the broken bridge into the river below. Three people involved escaped with non-life threatening injuries.
Public officials have focused in recent years on the desperate need for money to repair thousands of bridges deemed structurally deficient, which typically means a major portion of the bridge is in poor condition or worse. But the bridge that collapsed was not in that deficient category, highlighting another major problem with the nation's infrastructure: Although it is rare, some bridges deemed to be fine structurally can still be crippled if they are struck hard enough in the wrong spot.
"It probably is a bit of a fluke in that sense," said Charles Roeder, a professor of civil and environmental engineering at the University of Washington.
Major roadways used by thousands of U.S. drivers cross over "fracture critical" bridges — a term meaning that if a single, vital component of the bridge is compromised, it is at risk of collapse.
Many are not truss-style bridges like the one now in the river. In Boston, a six-lane highway 1A near Logan airport includes a "fracture critical" bridge of a different style over Bennington Street. In northern Chicago, an I-90 pass that goes over Ashland Avenue is in the same category. An I-880 bridge over 5th Avenue in Oakland is also on the list.
The most famous failure of a fracture critical bridge was the collapse of the I-35W bridge in Minneapolis during rush hour on Aug. 1, 2007, killing 13 people and injuring more than 100 others. The National Transportation Safety Board concluded that the cause of the collapse was an error by the bridge's designers — a gusset plate, a key component of the bridge, was too thin. It should have been an inch thick but instead was a half inch thick. Because the bridge's key structures lacked redundancy, where if one piece fails, there is another piece to prevent the bridge from falling, when the gusset plate broke, much of the bridge collapsed.
Mark Rosenker, who was chairman of the NTSB during the I-35W bridge investigation, said the board looked into whether other fracture critical bridges were collapsing. They found a few cases, but not many, he said.
"Today, they're still building fracture critical bridges with the belief that they're not going break," Rosenker said.
Fracture critical bridges, like the one in Washington, are the result of Congress trying to cut corners to save money rather than a lack of engineering know-how, said Barry B. LePatner, a New York real estate attorney and author of "Too Big to Fall: America's Failing Infrastructure and the Way Forward."
The crossings are kept standing by engineering design, not supported with brute strength or redundant protections like their more modern counterparts. The consequence of that risk was evident Thursday on Interstate 5 over the Skagit River, where a large section of the bridge collapsed into the water after officials say an oversized truck load clipped the steel truss.
While the truck's cargo suffered only minimal damage, it left chaos in its wake, with two vehicles catapulting off the edge of the broken bridge into the river below. Three people involved escaped with non-life threatening injuries.
Public officials have focused in recent years on the desperate need for money to repair thousands of bridges deemed structurally deficient, which typically means a major portion of the bridge is in poor condition or worse. But the bridge that collapsed was not in that deficient category, highlighting another major problem with the nation's infrastructure: Although it is rare, some bridges deemed to be fine structurally can still be crippled if they are struck hard enough in the wrong spot.
"It probably is a bit of a fluke in that sense," said Charles Roeder, a professor of civil and environmental engineering at the University of Washington.
Major roadways used by thousands of U.S. drivers cross over "fracture critical" bridges — a term meaning that if a single, vital component of the bridge is compromised, it is at risk of collapse.
Many are not truss-style bridges like the one now in the river. In Boston, a six-lane highway 1A near Logan airport includes a "fracture critical" bridge of a different style over Bennington Street. In northern Chicago, an I-90 pass that goes over Ashland Avenue is in the same category. An I-880 bridge over 5th Avenue in Oakland is also on the list.
The most famous failure of a fracture critical bridge was the collapse of the I-35W bridge in Minneapolis during rush hour on Aug. 1, 2007, killing 13 people and injuring more than 100 others. The National Transportation Safety Board concluded that the cause of the collapse was an error by the bridge's designers — a gusset plate, a key component of the bridge, was too thin. It should have been an inch thick but instead was a half inch thick. Because the bridge's key structures lacked redundancy, where if one piece fails, there is another piece to prevent the bridge from falling, when the gusset plate broke, much of the bridge collapsed.
Mark Rosenker, who was chairman of the NTSB during the I-35W bridge investigation, said the board looked into whether other fracture critical bridges were collapsing. They found a few cases, but not many, he said.
"Today, they're still building fracture critical bridges with the belief that they're not going break," Rosenker said.
Fracture critical bridges, like the one in Washington, are the result of Congress trying to cut corners to save money rather than a lack of engineering know-how, said Barry B. LePatner, a New York real estate attorney and author of "Too Big to Fall: America's Failing Infrastructure and the Way Forward."