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 instrumented in field cantilevered overhead sign structure was also continued as part of this study. The performance and behavior of anchor bolts along with changes to the specification were studied in the laboratory. Finally, improved procedures were recommended based on the field and laboratory work.
During implementation, difficulties were discovered with the proposed procedures. Clearance is a critical aspect for many lighting and traffic signal structures; a wrench cannot be effectively placed inside the base to property pre-tension the anchor bolts in many cases. During pre-tensioning, individual steps from the specifications often needed clarification and tended to be difficult to follow. After investigating structures pre-tensioned with the new specifications, none had loose anchor bolt nuts, likely indicating that the new procedures are effective.
A laboratory testing regimen was derived from the field experience and structural monitoring. Two fatigue tests were completed, one replicating field stresses and the other replicating AASHTO typical fatigue stresses. Relaxation of the anchor rods and lubrication properties were studied in the laboratory. Lab testing indicated that the primary mechanism of pretension loss in properly tightened anchor rods is likely due to relaxation, and not fatigue loading. In addition, accurate pretensioning should be performed with force controlled methods like torque or DTI washers, since displacement based turn pretensioning can result in a high degree of error. Lubrication impacts the final pretension when using torque based pretensioning, but is fairly consistent as long as the same class of lubricant is used.
Overall, the proposed procedures were found to be effective, but sometimes difficult to perform and sometimes not feasible for certain SLTS structures. The improved procedures focus on force controlled pretensioning and an immediate relaxation retightening.