Measured Anchor Rod Tightening of High-Mast Light Poles in Alaska

Abstract

The Alaska Department of Transportation and Public Facilities owns and maintains 124 high-mast lighting poles (HMLPs) in south-central Alaska. Some of the anchor nuts that secured these poles, 100 ft and taller, to their foundations had been found to loosen without apparent cause. In this study, the axial force was measured in the 1½-in. (38-mm) diameter anchor rods of one HMLP during the tightening of new rods. These rods had a relatively low grip-length to diameter ratio of approximately 3. The force was measured by strain gauges mounted in a small hole along the anchor rod axis and a computer-controlled data acquisition system. The FHWA tightening procedure was followed. This included a snug-tight condition, followed by 60° of turn-of-the-nut method. The snug-tight condition was created by the full force of a workman pulling on a 24-in. (610-mm) wrench. A specified verification torque was applied to the nuts after 1 week to ensure adequate tension in the anchor rods. It was found that after the tightening procedure, the axial force in several of the 12 rods exceeded their yield capacity. In each rod that yielded, the axial force caused by the snug-tight condition was higher than the anticipated values. It was concluded that several variables contributed to the yielding. These included snug-tight axial loads that were higher than expected, effects of low grip-length to diameter ratios, and an unnecessarily large verification torque.