Environmentally assisted fatigue failure of a structural bolted-joint connection

•Failure analysis of the galvanized threaded bolts from a supporting structure for traffic signaling;•The fatigue resistance of the bolt steel complies with the Eurocode 3 requisite after 30 years of service;•The damaged Zn-coating propitiated bolt failure from environmentally induced fatigue initiators. The paper addresses the analysis of the failure detected during a routine service inspection in an overhead porticoed structure for traffic signaling installed across the major north–south Spanish highway. The failure, which occurred after more than 30 years of service, consisted of the partial or full loosening of the bolts from the middle-span bolted joint and warned of a structural collapse. The failure analysis is based on the structural design data provided by the Spanish supervising authority for road and transportation and on exhaustive research regarding the structural integrity condition of the bolts removed from the dismantled joint. The experiments performed with the bolt material involve mechanical properties, fracture behavior with respect to the failure criterion of Eurocode 3 and fatigue resistance determination. These are accompanied by fractographic study and microstructural analysis of the bolts broken in-service. The analysis indicates that failure originated from environmentally induced fatigue initiators due to the environment aggressiveness and the galvanized condition of the bolts. The combination of these two factors caused local damage and weakening at the steel interface, after dissolution and microcracking of the Zn-coating protection.