Fatigue Assessment of High Mast Illumination Poles Using Field Measurements

Failures of high mast illumination poles (HMIPs) in recent years have raised concerns on the long-term fatigue performance of the poles by various transportation officials around the US. The thesis documents a study sponsored by the Texas Department of Transportation focused on the fatigue behavior of in-service HMIP systems. This study is an extension of previous investigations on the fatigue behavior of the poles that have demonstrated that many poles have poor performance and fail in fatigue before the AASHTO category E' limit. Galvanized specimens were also tested and some of them showed evidence of initial cracking, impacting the fatigue performance such that the galvanized poles behaved worse than the uncoated specimens.

Ultrasonic Testing (UT) has shown several poles around the state of Texas contain cracks in the welds between the shaft and base plate. To further investigate the performance of the poles in-service, a field study was initiated to measure the wind speed and direction, as well as the corresponding stresses in the pole shaft. This thesis presents results from the field investigation.

A data acquisition system was developed to gather wind data and induced stresses. The system was powered by a solar panel and can be remotely accessed via a wireless modem. Data collected throughout the year details the intensity and number of stress cycles experienced by the poles, and could be correlated with the measured wind velocity. Using the field data, more accurate estimates of expected fatigue life for the poles were made. The study provides TxDOT with valuable data on the performance of in-service poles so that the most critical fatigue cases can be identified and proper decisions can be made on the appropriate inspection or repair schedule.