Bridge destruction to reveal clues about 'fracture-critical' spans

A civil engineer at Purdue University is taking advantage of the demolition of a bridge spanning the Ohio River to learn more about how bridges collapse in efforts to reduce the annual cost of inspecting large spans.

"There is a whole family of bridges called fracture-critical," said Robert J. Connor, an associate professor of civil engineering. "This means that if an important tension member breaks, it's thought the bridge will fall down."

However, modern analysis techniques could be used to learn whether such bridges really are fracture-critical, or whether other structural elements would share the load if a major piece failed.

"We are looking at 'after-fracture redundancy,' or whether a bridge does remain standing after a key element fails," Connor said. "There is a lot of interest now in this issue because bridges classified as fracture-critical are very expensive to inspect and are subjected to more rigorous inspection requirements that are somewhat arbitrary. However, as is well known, bridge owners have limited resources. But if we could show they have redundancy, that a bridge won't collapse, more rational inspection strategies could be developed, allowing resources to be placed on the bridges that really should be inspected."

To test his hypothesis, Connor will purposely damage an approach span leading to the Milton-Madison Bridge in southern Indiana. The 82-year-old bridge, on U.S. Route 421 connecting Milton, Ky., and Madison, Ind., is being demolished in stages. A replacement bridge is scheduled for completion in 2012.

Federal law requires that the fracture-critical bridges be inspected every two years. Workers must perform a detailed "hands-on" inspection of such spans.