Improved pretensioning procedures for anchor bolt connections in sign, luminaire, and traffic signal structures

MnDOT, along with numerous other state departments of transportation (DOTs), are finding that anchor bolt nuts are coming loose at a concerning rate for overhead signs, luminaire, and traffic signal (SLTS) structures. Anchor bolts are critical to the structural stability of a structure, since, for MnDOT, they are the only connection to the foundation. Re-tightening loose nuts imposes a significant drain on state DOT resources. More importantly, the 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. In addition, even after retightening by MnDOT maintenance personnel, anchor bolt nuts were found to come loose within two years. The pre-tension force developed in anchor bolts during nut tightening is critical to keeping them sufficiently fastened to the structure and foundation. Anchor bolts loosening after installation and maintenance suggested a deficiency in MnDOT’s previous anchor bolt pre-tensioning procedures. To alleviate the anchor bolt pre-tensioning limitations, new specifications were developed in a previous study titled Re-Tightening the Large Anchor Bolts of Support Structures for Signs and Luminaires. The new specifications were developed through laboratory testing, field monitoring, surveys of current practices, and finite element modeling. This project focuses on the implementation and improvement of the previously proposed specifications. For a specification to be effective, constructability is critical; if the procedures cannot feasibly be performed in the field, they will likely not be utilized to the fullest extent. Previously proposed specifications were attempted on a variety of MnDOT SLTS structures. Both new installation and maintenance practices were investigated. Monitoring on a previously