In-service failure of a support structure for traffic signaling

Traffic signals and their supporting structures are an essential complement in highways and local roadways all around the world. Their accurate operation is today a well-recognized issue by the respective transportation authorities, especially when located on major roadways because of exposure to fatigue cracking by wind-induced large-amplitude vibrations and severe environmental conditions or wind gusts resulting from the passing trucks. The present paper describes the research undertaken to analyze the fatigue failure of a bolted joint in an overhead porticoed structure for traffic signaling located close to a highway entrance to a tunnel. The structure had been in use for more than 25 years and the failure was preventively detected, during a maintenance inspection. The research is a forensic engineering analysis of failure involving optical and scanning electron microscopy observations of the in-service fractured bolts together with the experimental characterization of the fatigue behavior of the bolt steel by means of non-standardized fatigue tests. The failure analysis pointed to the existence of environmentally-induced fatigue initiators as the failure origin, the galvanized condition of the bolts and environment aggressivity gave rise to local dissolution and microcracking of the Zn-coating, damaging and weakening specific zones at the steel interface that acted as fatigue initiators.