Bridge Engineering Center

About the research

Many state Departments of Transportation (DOT) across the US, including MnDOT, are experiencing problems associated with loose anchor bolts used in support structures (e.g., overhead signs, high-mast light tower (HMLT), and tall traffic signals). Specifically, MnDOT inspection crews have found loose nuts at most anchor bolt locations, even at some newly installed signs. Many of these nuts became loose in less than two years, even after being tightened by the maintenance crew following current recommended procedures. This situation has placed great strain on the resources from the districts’ maintenance group and also causes concerns related to inspection frequency and public safety. This project investigated causes of the loose anchor bolts and proposes solutions based on site surveying, field monitoring, laboratory study, and numerical analysis. The research team found that the tightening process proposed in AASHTO’s specification is a sufficient alternative for MnDOT, though it requires modification in three key areas: defining snug-tight, accounting for grip length, and recommending verification procedures. The research team quantified snug-tight values, and defined the relationship between torque, tension, and nut rotation through empirical constants. Recommendations are made for a new specification for MnDOT structures.