WASHINGTON - Three months before proposing in his State of the Union speech to speed up construction projects, President Obama designated replacement of New York’s Tappan Zee Bridge for “fast-track’’ approvals, saying he wanted to cut red tape and create jobs.
He didn’t mention another reason: The failure of one main part could send the structure, which carries about 140,000 vehicles a day, tumbling into the Hudson River.
The Tappan Zee is one of the United States’ 18,000 so-called fracture-critical bridges, of which about 8,000 are classified “structurally deficient,’’ according to Federal Highway Administration records. The bridges require inspections that may cost cash-strapped state and local governments 5 to 15 times as much as routine checks.
“Fracture-critical bridges work fine if maintenance is perfect and everything goes as designed,’’ said Thomas Fisher, dean of the College of Design at the University of Minnesota. “But if you start to change anything, they become very fragile. Their fracture-critical nature means they don’t give any warning at the point of collapse. It is sudden and catastrophic.’’
The Minneapolis Interstate 35W bridge, a fracture-critical design, collapsed without warning in August 2007, killing 13 people and injuring 145 others.
Many fracture-critical bridges were built in the 1960s and 1970s to finish the interstate highway system quickly and inexpensively, said Andrew Herrmann, president of the American Society of Civil Engineers, a Reston, Va.-based industry group.
The average bridge is functional for about 50 years, depending on weather and other conditions, Herrmann said.
“These bridges have an amazing safety record to this point, but they are getting old and have to be watched,’’ he said.
Bridges rated structurally deficient require “significant maintenance and repair to remain in service and eventual rehabilitation or replacement to address the deficiencies,’’ according to the American Association of State Highway and Transportation Officials.
Every US highway bridge must be inspected at least once every two years, he said. Fracture-critical bridges may need to be inspected more often. How frequently depends on their condition. While many bridges can be checked visually, engineers need to use boats, cranes, cherry pickers, and other equipment to inspect fracture-critical bridges up close, he said.
Inspectors look for cracks as small as one-eighth of an inch, Michael Johnson, chief of special investigations for the California Department of Transportation, said in a telephone interview.
Underwater inspections using divers are required every five years, he said.
“These hands-on inspections have revealed numerous fatigue and corrosion problems that otherwise might have escaped notice,’’ according a report by Transportation Research Board, part of the National Academy of Sciences that provides inspection information to engineers.