Infrastructure Technology Institute
In the right circumstances, structural health monitoring of large bridges can be a useful management tool, with the potential to improve safety, extend the useful life of a bridge, and reduce the costs of maintaining the bridge over its decades. Unfortunately, standard sensing technology is not always well-suited for the unique and often unpredictable monitoring challenges found on many bridges.
Encountering such a situation while designing a structural health monitoring system for the John F. Kennedy Memorial Bridge in Louisville, Kentucky, the ITI Research Engineering Group (REG) adapted off-the-shelf sensing technology to develop a new sensor capable of measuring the performance of threaded rod assemblies used in a bridge retrofit.
Due to its asymmetrical cantilever design, the two ends of the JFK Bridge must be held down by large bearing assemblies anchored into concrete piers. This type of design, unusual for large through-truss bridges, required an equally unusual repair when inspectors discovered that one of the bearings was suffering from corrosion damage caused by runoff of deicing chemicals and precipitation. To ensure the downward restraint of the end of the JFK Bridge, the Kentucky Transportation Cabinet (the bridge owner) installed a retrofit that effectively clamped the end of the bridge more securely to the concrete pier below. The clamping strength of the retrofit is obtained from sixteen threaded anchor roads embedded into the top of the concrete pier. Nuts on the end of the rods were tightened against the bearing retrofit to prevent the bearing from pulling away from the top of the pier.
While designing a system to monitor the performance of this retrofit, the REG soon realized that it would be quite useful to know exactly how much these threaded rods were “pulling” down on the bridge. If this pulling lessens over time, it can be inferred that the threaded rods are being wrenched out of their anchor holes and must be repaired. To use off-the-shelf sensing technology to measure tension in these rods, a portion of each rod would have to be flattened to attach the sensor, damaging the rod and compromising the repair. The REG had to invent a new sensor to measure the tension on these rods without damaging the rods themselves.
The solution took the form of an external clamping mechanism composed of off-the-shelf strain gages, several large nuts and a simple bar of steel. First, the two nuts are split in half so that they can be re-assembled around the threaded rod with six small screws. Then, a steel bar is welded between the two nuts. Finally, strain gages are affixed to the bar. When the bracket is placed on a threaded rod, the strain in the bar is approximately equal to the strain in the rod, allowing calculation of the tensile forces in the rod.
The REG installed four of these sensors on the JFK Bridge in February, after the rods were placed in the concrete pier but before the clamping force was applied. The sensors reported the tension developed in the rods as the retrofit was tightened down and continue to provide insight as to the continued performance of the retrofit. The REG is in the process of analyzing the data from the bridge and performing additional controlled laboratory testing to further calibrate the sensors. Once this testing is complete, the new sensors may be used to evaluate the performance of the other retrofits that employ threaded rods to develop tensile forces.