Evaluating the Behavior of Anchor Rod Foundations for High-Mast Light Poles Using Nonlinear Finite-Element Analysis

Abstract

This study examines the behavior of high-mast lighting pole (HMLP) foundations through the use of nonlinear finite-element modeling. HMLPs utilize nuts and threaded rods to clamp the light pole structure’s base plate to the foundation. Inspections by the Alaska Department of Transportation and Public Facilities (AKDOT&PF) have revealed widespread loosening of the nuts used on HMLPs over time. The threaded rods used are F1554 Grade 55, which are heat treated from mild steel rods, Fy=248 MPa (36 ksi) to gain additional yield strength. This study briefly highlights the difference in postyield behavior in mild steel and high-strength steel, and proposes a probable mechanism behind the clamp-load loss experienced by Alaska’s HMLPs. Because the HMLPs are much taller than traditional lighting poles, they experience larger external wind loading. This wind load is potentially stressing threaded rods into the postyield range. A previous study shows that the current tightening procedure is likely not undertightening nuts, which could be a major culprit behind loosening. As a result of this and previous research, it is believed that the nuts aren’t loosening in the traditional sense—losing clamp load by rotating on the threads. Instead, the threaded rods undergo significant postyield strain during external loading. This plastic deformation is large enough in magnitude to cause the bolted joint interface to lose clamp load when the external force is removed. One factor that exacerbates this effect in some pole configurations is prying action, which is not accounted for in the design. The clamp loss mechanism is examined by applying wind loads to three pretensioned HMLP configurations using finite-element analysis software (ABAQUS). The analysis uses 3-D solid elements, contact surfaces that allow for separation, bonding surfaces where contact surfaces are not required, boundary conditions that approximate reality, displacement controlled pretension, and external wind loading represented by coupled moments.