Structural behavior of large anchor bolts used for sign-support structures

Abstract

Anchor bolts are commonly used to tie sign-support structures to their concrete base through a double-nut moment connection. Over the past decade, loose anchor rod nuts have proven to need frequent attention, incurring significant cost, while endangering the safety of sign-support structures. In particular, connections tightened with the current pre-tensioning specifications are found to have been loosened within only a year (if not sooner) after installation. Considering this long-standing issue, the objective of this study is to investigate the structural behavior of large anchor bolts used at the base of sign-support structures and evaluate the parameters that influence the rod tightening and loosening process most. For this purpose, a set of Skidmore-Wilhelm bolt tension calibrator tests were performed on different sizes of bolt that had been lubricated with various methods. The tests were to evaluate the effects of the lubrication method on the tightening process. In addition, a sign-post structure removed from the field was tested in the laboratory to study the bolt behavior when it was subjected to the lateral load. Finite-element models were then developed and calibrated using the data collected form the laboratory test. A parametric study was conducted on the calibrated model to understand the effects of key contributing factors, such as level of tightening preload, variation of preload level, washer size, and grip length, on the tightening and loosening process. The investigations determined that the current tightening process can generate a stress concentration in the rod in the range of 30 ksi, which was 55% of the rod's yield strength. The clamping force was also noted to experience a significant reduction during the first few cycles of loading (i.e., up to 20 cycles), beyond which further reduction was minimal. The outcome of this study directly helps choose a preload level that ensures adequate tightening, while avoiding permanent damage to the rod. This is critical for preventing or alleviating the issue of nut loosening.