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The eventual destruction of the 81-year-old Milton-Madison Bridge over the Ohio River, scheduled for next year, has presented a rare opportunity for researchers at Purdue University.
Robert Conner, an associate professor of civil engineering at Purdue and a national expert in the study of steel fatigue, is hoping to use the bridge to compile research that will help transportation officials throughout the United States better inspect and diagnose “fracture critical” truss bridges.
Bridges like the Milton-Madison span are considered fracture-critical because of the two-girder design that supports the bridge deck. In theory, Conner said, experts assume that a fracture along any part of critical steel members of such a bridge likely would result in collapse of the span under normal traffic loads.
Based on this assumption, federal law requires costly hands-on inspections of “fracture critical” bridges every two years, meaning every critical piece of the bridge must be physically inspected at arm’s length. These inspections are costly — from 1994-2008, Kentucky spent $300,000 on them — and must be done regardless of a bridge’s age and condition, Conner said.
Because there are thousands of fracture-critical bridges throughout the United States, Conner said the requirement places a serious financial burden on the states that own these bridges.
Obviously, the goal of the inspection requirement is to prevent potentially deadly bridge failures, Conner said. But, he pointed out that the requirement is based on assumptions and not scientific evidence, and believes that not all fracture-critical bridges need such frequent inspections.
Through his findings from the Milton-Madison bridge and several other research projects his team is involved with, Conner said he hopes to establish diagnostic criteria that could be used nationally to determine which bridges truly require two-year inspection schedules and which bridges could undergo fracture-critical inspections less often.
Conner said he hopes the study will determine if there is, in fact, “redundancy” in the truss design, meaning that other parts of the structure would support the weight of the bridge and traffic – even if just temporarily – should a critical piece of the span fail.
Monitors already in place on span
Sensors have been put in place on the bridge and are scheduled to be activated next week, Conner said. These will send data directly to servers at Purdue’s main campus in West Lafayette, Ind.
The data collected during this phase of the project will provide “a baseline of how the bridge is behaving,” Aaron Stover, project manager for Michael Baker Jr. Inc., the firm designing the new bridge, said during a media tour of the site last month.
But the serious part of the study will be done after traffic using the bridge is rerouted to the temporary ramps. Once the existing approach to the bridge on the Madison, Ind., side is no longer in use, Conner plans to haul in sand that will be piled onto the deck to simulate the weight of traffic. Then, engineers will use explosive charges to force structural failure on key components underneath the approach to see what happens. The sensors will continue to collect data on the amount of stress the failure puts on other key pieces of the bridge.
“What they hope to learn is how much additional load [the bridge] can handle [in the event of a failure], or if it’s just going to fracture and fall,” Stover explained.
Whether the structure remains standing or collapses, the data collected will be crucial, Conner said. Computer analysis of the data will help engineers determine specific risk factors to help diagnose the condition of fracture-critical bridges and better determine inspection schedules.
Temporary ramps won’t open until spring
Originally, Conner had hoped to conduct the study this summer, when officials had scheduled traffic to be rerouted onto temporary access ramps to the existing bridge.
However, heavy rains and high river levels plagued the project throughout the spring, and set back construction of the temporary ramps. That phase of the project just got under way last week.
Officials have not yet determined the exact date when traffic will be rerouted onto the temporary ramps, but Andrea Clifford of Kentucky Transportation Cabinet District 5 said that won’t happen until spring, so that drivers won’t have to use them during the winter.
“We’re definitely at the mercy of their construction schedule,” Conner admitted.