Published 4 October 2013
Of the 607,380 bridges listed in the recent U.S. National Bridge Inventory, 65,605 bridges are classified as “structurally deficient” and 20,808 as “fracture critical,” with 7,795 of those bridges designated as both structurally deficient and fracture critical. Experts say this indicates significant disrepair and a risk of collapse. These 7,795 structurally deficient, fracture critical bridges carry more than twenty-nine million drivers a day.
The AP has reported that of 607,380 bridges listed in the recent U.S.National Bridge Inventory, 65,605 bridges were classified as “structurally deficient” and 20,808 as “fracture critical,” with 7,795 of those bridges designated both structurally deficient and fracture critical. Experts say this indicates significant disrepair and a risk of collapse.
Engineers and officials, however, say bridges are safe despite their classifications as structurally deficient or fracture critical.
A fracture-critical bridge does not have redundant protections and is at risk of collapse if a vital component fails. A structurally deficient bridge is in need of rehabilitation or replacement because at least one major component of the span has advanced deterioration, or other problems that lead inspectors to deem the bridge as unfit.
One example of a bridge which is designated as both structurally deficient and fracture- critical is the Frederick Douglass Memorial Bridge in Washington, D.C. Bridges across the United States which are considered both structurally deficient and fracture critical together carry more than twenty-nine million drivers a day. Many of the bridges were built more than sixty years ago, including the Brooklyn Bridge in New York and the Main Avenue Bridge in Cleveland.
Many fracture critical bridges were built between the 1950s and 1970s during the construction of the interstate highway system. These bridges have now exceeded their life expectancy but are still operational, often carrying more cars and trucks than originally planned.
Thousands of inspectors across the country monitor the safety of bridges, and “If a bridge is found to be unsafe, immediate action is taken” says Victor Mendez, head of the Federal Highway Administration. Cities and states have little funding to replace aging and vulnerable bridges so they resort to juggling repairs in an effort to stay ahead of deterioration.
The Frederick Douglass Memorial Bridge was designed to last fifty years, so now it operates thirteen years past its life expectancy. Local transportation officials have inserted catcher beams underneath the bridge’s main horizontal beams to keep the bridge from falling into the river in case a main component fails.
Engineers note that all it takes for a fracture-critical bridge to collapse is a single unanticipated event that damages a critical portion of the structure. “It’s kind of like trying to predict where an earthquake is going to hit or where a tornado is going to touch down,” Kelley Rehm, bridges program manager for the American Association of State Highway and Transportation Officials (AASHTO), told the AP.
Peter Vanderzee, CEO of Lifespan Technologies of Alpharetta, Georgia, an engineering firm which uses special sensors to monitor bridges for stress, said steel fatigue is a problem in the older bridges.
“Bridges aren’t built to last forever,” he told the AP. He compared steel bridges to a paper clip that’s opened and bent back and forth until it breaks.
“That’s a fatigue failure,” he said. “In a bridge system, it may take millions of cycles before it breaks. But many of these bridges have seen millions of cycles of loading and unloading.”Many of the bridges included in the APanalysis have sufficiency ratings. A bridge with a score less than 50 on a 100-point scale may be eligible for federal funds to help restore some components. The latest federal inventory shows that more than 400 bridges which are fracture-critical and structurally deficient have a score of less than 10. The Frederick Douglass Memorial Bridge has a sufficiency rating of 60. The bridge has been rehabilitated since its previous sufficiency rating of 49. Ronaldo Nicholson, the chief bridge engineer for the Washington, D.C. area notes that a new retractable bridge which allows ships to travel the Anacostia River would cost $450 million, yet the cost could be as low as $300 million for a non-retractable bridge. Officials are extending the bridge’s life for another five years while a replacement bridge is to be built. Inspections of the bridge have been shifted to every six months instead of the regular 2-year intervals for most bridges.
In the end, keeping bridges healthy and safe, and preventing them from reaching the point of being designated as structurally deficient or fracture- critical, is a question of money.Congress showed some interest in fixing bridges after the 2007 bridge collapse in Minneapolis, but efforts to allocated billions of federal dollars to repair and replace deficient and obsolete bridges have so far failed.
“Do we have the funding to replace 18,000 fracture critical bridges right now?” AASHTO’s Rehm asked. “No. Would we like to? Of course.”
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