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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.

Throughout the US; many state departments of transportation (DOTs) are experiencing issues with the loosening of anchor bolt nuts on overhead sign; luminaire; and traffic signal (SLTS) structures. Retightening loose nuts imposes a significant drain on state DOT resources. In addition; loosening of these nuts increases fatigue stresses on the anchor bolts; possibly increasing the risk of failure. Loose anchor bolt nuts were recorded on both old and new structures; some immediately after installation. Even after retightening by Minnesota Department of Transportation (MnDOT) maintenance workers; anchor bolt nuts were found to come loose within two years. In a previous project; new retightening specifications were developed based on laboratory testing; field monitoring; surveys of current practices; and finite element modeling. This project focused on implementation and evaluation of the proposed specifications from the previous project. Structural monitoring also continued on a previously instrumented overhead sign structure. Difficulties were discovered with the proposed procedures during implementation; including structure clearance; instruction clarity; and retightening timing. Overall; though; the proposed procedures were found to be effective in preventing loosening. Revisions to the specifications were suggested along with recommendations for further review to simplify the procedures through a future laboratory study.