The Michigan Department of Transportation (MDOT) recently awarded a two-year, $278,000 grant to Christopher Eamon, Ph.D., associate professor of civil and environmental engineering.
MDOT inspection reports have noted diagonal cracks near the supports of prestressed concrete girders on a few bridges in the state. Officials solicited proposals to analyze the significance of the cracks and develop a procedure to mitigate the issue, as well as to prevent such cracking from occurring in future bridges. Eamon and his fellow researchers will review inspection data on a sample of bridges provided by MDOT.
Diagonal cracks may indicate that allowable concrete shear stresses have been exceeded, Eamon said, and may or may not be significant. He noted that reinforcing steel and draped prestressing cables are placed inside bridge girders to carry a portion of the shear force as well.
"Clearly, you don't want the concrete cracking," he said. "That allows water to penetrate and deterioration to occur. It also indicates that previous design methods might not be properly accounting for the actual strength of or expected loads on the structure."
His team's job is to determine if the structures in question have significant shear cracks, and, if so, to determine the cause and make recommendations on how to repair them, as well as to make recommendations on design procedures. MDOT designs structures according to American Association of State and Highway Transportation Officials specifications.
Researchers will visually inspect the bridges and document their findings with measurements and pictures. Then they will obtain the bridge construction documents and analyze the as-designed shear capacities using the latest modeling techniques. The team also will estimate loads over the bridges based on available MDOT records and MDOT legal truck loading criteria.
"Hopefully some pattern will emerge," Eamon said. "We'll look to see if the diagonal cracking is associated with a particular structure type, construction method or design procedure, all of which change over time as new procedures are implemented."
While early indications are that some structures may have some shear deficiencies, he said there's no cause for alarm from motorists.
"These structures for the most part are highly redundant," Eamon said, adding that bridge collapses are "extremely rare events."
He believes researchers are more likely to find cracking concerns that may set the stage for long-term maintenance problems.
"Ultimately capacity is an issue that we will investigate, but at this point we're not certain this is a structural capacity problem," Eamon said. "There are plenty of issues on any type of structure that have to be dealt with, and at this point we haven't concluded that these cracks are more troublesome than a variety of other undesirable issues that exist."
The purpose of his research is to maintain MDOT's commitment to the safety of the motoring public using the highway infrastructure. Department officials will review the team's findings in September of next year before planning any repairs or design procedure revisions, Eamon said.
Provided by Wayne State University
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