Investigating and Mitigating Fatigue Failures of Electric Transmission Line Davit Arms Due to Conductor Dynamics

Electric transmission line conductors experience a large range of dynamic motions due to wind loading. These dynamic activities impress stresses of varying cycles and intensities on the conductor strands, the suspension equipment and structural supports. Some are fairly well known and managed to mitigate problems and failures. The most common among these are aeolian vibration with its influence on conductor fatigue and vortex shedding of conductors creating large amplitude "galloping conductors". There is, however, another cluster of motion dynamics experienced at a different range of amplitudes, frequencies and cycles that is less well known and is not currently routinely included in line design efforts. The fatigue failure of davit arms on structures induced by this lesser-known dynamic activity has not been a frequent or widespread phenomenon, but it has become a reliability and liability concern for several utilities where the convergence of the right combination of conductor properties, sag and tension, line exposure and structural details develop into fatigue failures. This paper addresses the investigation of several fatigue failures, recommends a cause and develops a mitigation program to avoid future failures. In particular, a unique approach to assessing the dynamic influence on stress concentrations and mitigation plans by utilizing Photostress technology is documented. Final, recommendations provide a vibration mitigation strategy and a recommendation for a research effort to isolate what specific converging parameters and their characteristics lead to these failures.