Washington - Three months before proposing in his State of the Union speech to speed up construction projects, President Barack 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 U.S.'s 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 five 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 I-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 association.

The average bridge is functional for about 50 years, depending on weather and other conditions, he said.

"These bridges have an amazing safety record to this point, but they are getting old and have to be watched," he said in a telephone interview.

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 U.S. 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. "If the crack is not arrested, it can run the length of the steel and jeopardize the integrity of the structure," he said.

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.

Fracture-critical bridge maintenance "consumes a large portion" of states' inspection budgets, the report said. A fracture-critical inspection can cost "well into the six digits," compared with $15,000 or less for more routine work, said Todd Niemann, a bridge inspection engineer for the Minnesota Department of Transportation.

It cost about $100,000 to hire a consultant to inspect the Milton-Madison bridge, which carries US 421 across the Ohio River between Milton, Ky., and Madison, Ind., according to data compiled by Michael Baker Jr. Inc., an engineering services company. A more general bridge inspection by Kentucky Department of Transportation employees cost about $10,000, according to the data. Michael Baker Jr., a unit of Moon Township, Pennsylvania-based Michael Baker Corp., is a consultant working on the 81-year-old bridge's replacement.

The Sherman Minton Bridge, another span connecting Kentucky and Indiana over the Ohio River, was closed Sept. 9 after cracks as wide as a 12-ounce can were found. The discovery prompted the Federal Highway Administration on Sept. 12 to "strongly" recommend that states double-check fracture-critical bridges using the same type of steel.

"When you notice something on these bridges, they have to be shut down right away because they don't give a warning before collapsing," the University of Minnesota's Fisher said. "It's not like they sag or start to shake first."

The bridge, completed in 1961, carried about 80,000 vehicles a day, according to the U.S. Department of Transportation.

The New York State Thruway Authority spent $389 million from 2007 through 2011 to maintain and improve the Tappan Zee Bridge, Betsy Feldstein, a spokeswoman for the agency, said.

The Federal Highway Administration estimates it will cost $5.2 billion to $16 billion to replace the bridge, depending on whether it has bus lanes and tracks for rail.

"The Tappan Zee is being held together with glue and duct tape," said Barry LePatner, founder of LePatner & Associates, a construction-law firm in New York, and author of "Too Big to Fall" about the state of the nation's bridges.

"It is inexcusable why each of these bridges cannot be structurally given redundancy to ensure that they will be safe," LePatner said.

Repairing all U.S. bridges would cost about $140 billion, three times what the federal government receives in taxes annually for road, mass transit and bridge projects, according to the engineering society.

Funding for bridge repairs comes in part from U.S. surface- transportation legislation, the last of which provided $20 billion between 2005 and 2009. That bill has been extended at about the same funding levels since 2009. The current extension expires March 31.

The Highway Trust Fund, which receives fuel taxes for use on highway, bridge and mass transit projects, is spending more money than it takes in and will probably be insolvent by the first quarter of 2013, according to the Congressional Budget Office.

The shortage of repair money means the U.S. should inspect fracture-critical bridges regularly and allocate inspection money to the structures in the worst state of repair, said Robert Connor, associate professor of civil engineering at Purdue University in West Lafayette, Ind.

"I don't think the sky is falling today," Connor said. "But if we don't train the future workforce in at least how to maintain these structures, we're going to have some pretty bad things eventually happen."