Fatigue Strength of Signal Mast Arm Connections

Mark T. Koenigs, Tamer A. Botros, Dylan Freytag, and Karl H. Frank


Changes in the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals and an increasing rate of fatigue related problems have raised awareness of fatigue concerns in traffic signal mast arms. Prior research has indicated that the most commonly used connection details exhibit poor fatigue performance. This study was initiated to confirm the previous research results, as well as to investigate a larger variety of connection details and a weld treatment method. During this study, 55 full-size mast arm connection detail specimens were tested for fatigue resistance. The present specifications were found to overestimate the fatigue life of connections with stiffeners. The results indicate that the Ultrasonic Impact Treatment weld treatment can significantly improve the fatigue life of a fillet-welded socket connection detail. Several other connection details exhibited improved fatigue lives; however, the improvements were not as significant as the UIT treated specimens. An extensive finite element analysis generated stress concentration factors for a variety of connection geometries. These finite element analyses extended the range of geometries beyond those included in the experimental study.

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