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Infrastructure Technology Institute

Live Strain Measurement Aids interstate bridge Repair

The John F. Kennedy Memorial Bridge carries Interstate 65 over the
Ohio River between Louisville, Kentucky and Jeffersonville, Indiana.

The John F. Kennedy Memorial Bridge, opened in 1963, carries Interstate 65 over the Ohio River between Louisville, Kentucky and Jeffersonville, Indiana. Over 120,000 vehicles travel the bridge on an average day. Four bearings on the large through-truss structure are designed to resist substantial upward forces that might otherwise lift the bridge off its supports. In a 2006 inspection, one of the four anchor bolts restraining one of these bearings was found to have fractured. Since the performance of the uplift bearings is critical to the safety and serviceability of the bridge, the Kentucky Transportation Cabinet was eager to gain insight into the behavior of the compromised bearing assembly prior to designing a retrofit.

During the summer and fall of 2007, the Kentucky Transportation Cabinet engaged ITI’s Research Engineering Group (ITI-REG) to help investigate the anchor bolt failure. As part of a multi-party effort, the ITI team worked closely with Cabinet personnel, University of Kentucky researchers, and consultants. ITI engineers deployed strain gauges, displacement transducers, and accelerometers on all four bearings to compare the behavior of bearings with intact bolts to that of the bearing with the fractured bolt. Each site was monitored under live traffic for several hours during both daytime and evening. The data revealed that the strain in the three remaining anchor bolts restraining the compromised bearing was much greater than those in the four anchor bolts on the other bearings.

The compromised bearing assembly was retrofitted with a replacement anchor bolt in late October 2007. Prior to installation, ITI engineers instrumented the replacement bolt with strain gauges to measure its performance. Using a computer-controlled data acquisition system, the ITI team observed and recorded the behavior of the replacement bolt during installation and tightening and for twelve hours overnight following installation. The data showed that the strains in the replacement bolt and the three original bolts on the compromised bearing were more appropriately distributed after the retrofit. Members of the ITI-REG presented their findings to the Kentucky Transportation Cabinet at their headquarters in April 2008.

Once a more permanent retrofit is in place, the ITI-REG will install a system for the continuous remote monitoring of live strains, displacement, and acceleration at the bearing assembly. This system will help characterize the long-term behavior of the anchor bolts and aid in evaluation of the retrofit. Data will be autonomously delivered to researchers and Cabinet engineers for analysis via Internet display technology previously developed at ITI.