Infrastructure Technology Institute
ITI’s Research Engineering Group (REG) has partnered with the Kentucky Transportation Cabinet (KYTC) and the University of Kentucky (UK) since 2007 to address questions about the end bearing anchor bolts on the John F. Kennedy Memorial Bridge, a large through-truss structure which carries Interstate 65 over the Ohio River between Louisville, Kentucky, and Jeffersonville, Indiana. The bridge has two large bearings at each end which, due to the design of the continuous span, must resist substantial uplift forces. In a 2006 inspection, one of the four anchor bolts restraining one of these uplift bearings was found to have fractured, while measureable section loss was noted on the other three.
The results of a 2007 short-term strain gage study by ITI engineers (described in a previous ITI newsletter article, “Live Strain Measurement Aids Interstate Bridge Repairs”) contributed to a retrofit design by KYTC in which the remainder of the fractured bolt embedded in the concrete pier was drilled and tapped to receive a replacement anchor bolt. Four large “keepers” anchored directly into the concrete pier would provide additional redundancy. ITI was asked to monitor the performance of the replacement bolt and the entire bearing assembly during and after retrofit. ITI engineers installed a variety of instruments on the assembly, including strain gages to measure axial, torsional, and bending strains on the replacement bolt and displacement and acceleration transducers to monitor the overall movement of the bearing assembly. Load cells were also installed on the keeper anchors to indicate if the keepers became loaded due to movement of the bearing assembly.
ITI returned to the bridge in late 2008 to install a continuous remote monitoring system at the troubled bearing. This system logs and transmits readings from strain gages on the anchor bolts, load cells on the keepers, and displacement sensors on the bearing assembly itself, along with ambient temperature and humidity, back to ITI via a cellular data connection. This information is automatically processed and posted daily on the secure project web site.
Within two months of installation, the continuous remote monitoring system recorded an event involving significant redistribution of loads on the bearing assembly, including the unloading of the replacement anchor bolt, indicating possible failure of the new bolt. Authorities were made aware of this anomaly promptly. ITI and KYTC engineers confirmed fracture of the new anchor bolt during a subsequent site visit. During that visit, ITI also installed two Internet-accessible cameras to provide visual confirmation of any future indications.
Through the use of continuous remote structural health monitoring, ITI was able to detect very quickly the fracture of a structural member in a bridge over 300 miles away without the benefit of an on-site inspection. When the next retrofit of the bearing is performed, KYTC will continue to use this successful monitoring technique. Demonstration of the benefits of monitoring technology to retrofit design and performance validation continues to be a major thrust of the ITI REG’s ongoing research.