(Purdue School of Civil Engineering/Ryan Sherman)
Federal law requires that the fracture-critical bridges be inspected every two years. Workers must perform a detailed "hands-on" inspection of such spans.
Connor suggested piling a load of sand onto one of the approach spans to simulate the weight of trucks and then cutting a critical member of the bridge with an explosive charge. He took his idea to engineering consultancy Michael Baker and general contractor Walsh Construction, as well as officials from the Federal Highway Administration and Indiana and Kentucky state transportation departments. The proposal was approved and federal funding provided.
Connor and his team of research engineers and a student have fitted the 30m approach span with 50 sensors and will damage a portion of the span and record the findings. The researchers will take high-speed video in addition to recording sensor data. The test is tentatively set for mid-August.
"We expect to be able to show whether, if one piece fails, the whole bridge won't fail because there are secondary load paths that kick in," said Connor, who has been involved in numerous steel bridge failure investigations, the most recent being the 2007 I-35W Mississippi River bridge collapse in Minneapolis.
Connor believes it is the first study of its kind on a truss bridge that is in place. As part of the fracture-critical work, Purdue researchers also will test steel members from the bridge.
Asset management
Bridge demolition set to reveal clues about 'fracture-critical' spans
A civil engineer at Purdue University in the USA is taking advantage of the demolition of a bridge spanning the Ohio River to learn more about collapse in efforts to reduce the annual cost of inspecting large spans.
"We are looking at 'after-fracture redundancy,' or whether a bridge does remain standing after a key element fails," associate professor of civil engineering Robert Connor. "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 a 30m approach span leading to the Milton-Madison Bridge in southern Indiana. The 82-year-old bridge is being demolished in stages and a replacement bridge is scheduled for completion in 2012.
