Implementation of New Guidelines in Tightening Large Anchor Bolts of Supports for Signs & Luminaires

Status:  Complete
Report Date:  06/29/2020


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.

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