Methods to Ensure Appropriate Installation and Prevent Loosening of Anchor Nuts on Ancillary Highway Structures

Report No: 20-R30

Published in 2020

About the report:

Loose anchor nuts on foundations of highway ancillary structures have been implicated in numerous structural failures in the Transportation industry for several decades. Loose anchor nuts can increase the stresses in the anchor rods, which can lead to potential collapse of the ancillary structure under wind loads. The exact cause of anchor nut loosening is unknown but is generally believed to be influenced by three variables: improper tightening, wind-induced vibrations, and thread fabrication tolerance. This study included both large-scale and small-scale vibration testing of ancillary structures to investigate levels of contribution from each of the three variables. The purpose of the vibration testing was to establish a relationship between the number of vibratory cycles and anchor nut loosening. This study also reviewed the current tightening procedures for double-nut moment connections on ancillary structures, and evaluated the effect of initial snug-tight condition and thread fabrication tolerance on anchor nut loosening.

The study found that the current Virginia Department of Transportation tightening procedures for double-nut moment connections contain some discrepancies, which can lead to under- or over-tightening of anchor rods. Recommendations were provided for properly tightening grade 36 and 55 anchor rods. The study also showed that the current manufacturer recommended tightening torque for single nut connections on T-bases are inadequate to prevent loosening. Recommendations were made for specifying deep sockets, long extensions, and proper lubrication to facilitate tightening these connections. Snug tight testing showed that the snug tight condition is highly variable. Recommendations were provided for using an appropriate wrench length depending on the diameter of the anchor rod being tightened. The recommendations provided in this study will aid in preventing anchor nut loosening, which will make for safer ancillary structures requiring less maintenance.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


  • Japsimran Singh, Matthew H. Hebdon, Ph.D., P.E.

Last updated: November 9, 2023

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