Development of the Guidance on the Tightening of Large Anchor Rods of Support Structures for Sign and Luminaires

Across the United States, various state Departments of Transportation (DOTs) are finding that the base anchor-rod nuts on critical support structures for overhead signs are coming loose. Retightening loose nuts imposes a significant drain on state DOT resources. In addition, the loosening of these nuts increases the failure risk of tall and overhanging structures. In many of the inspection engineer's observed cases, anchor-rod nuts were loose immediately after installation. Even after tightening, it was found that anchor-rod nuts had consistently come loose just 2 years after retightening. The objective of this study was to evaluate the resistance to the loosening of anchor-rod tightening procedures proposed by Chen et al. and to develop improvements to the procedures based on stakeholder feedback. To achieve this objective, field deployment of the previously developed procedures was conducted on multiple sites. After field deployment, improvements to the Chen et al. procedure were investigated on a full-size laboratory specimen to ensure revisions did not affect connection performance. Overall, it was found that the pretension levels proposed by Chen et al. were effective, and the revised procedures only use torque, or force-controlled tightening, as opposed to displacement-controlled turn-of-nut.

Automated summary

Transportation departments across the United States are facing the issue of loose nuts on critical support structures for overhead signs. This poses a drain on resources and increases the risk of failure. The objective of this study was to evaluate and improve anchor-rod tightening procedures to prevent loosening.