Fatigue reliability assessment and inspection scheduling for High-Mast Illumination Poles

High-Mast Illumination Poles (HMIPs) are tall and slender poles that provide lighting around highway interchanges and large open public spaces and arenas. Recent collapses have raised safety concerns about these sizable illumination poles. Investigations concerned with such failures have sometimes revealed the existence of severe fatigue-induced cracks at the shaft-to-base connection at the bottom of the poles. These cracks are believed to exist due to the galvanization process prior to deployment. Also, an unanticipated increase of fatigue demand from vortex-induced vibration is believed to exacerbate the failure process. Although no fatigue-related collapses have been reported in Texas, the Texas Department of Transportation (TxDOT) initiated a series of studies directed toward assessing the reliability of HMIPs in the state. One area of interest in these studies is to characterize the fatigue resistance of pre-cracked poles, that were designed in accordance with current specifications. A second area of interest is to characterize typical fatigue loading of HMIPs resulting from wind excitation.

Fatigue experiments on the bottom portion of newly made and galvanized HMIPs are conducted to characterize the S-N curve that describes the fatigue resistance of the tested subject. The S-N curve is specified by parameters estimated from experimental stress ranges employed and associated cycles to failure. S-N curves for pre-cracked HMIPs are compared with those specified for uncracked HMIPs. The fatigue demand due to wind excitation is evaluated using five monitored HMIPs throughout Texas. Stress range data collected are analyzed for their equivalent fatigue load (EFL) to describe the fatigue demand under various wind loading environments. The comprehensive information collected from the field provides an assessment of the expected fatigue demand, on each HMIP studied at the various sites, for different wind speeds and directions. These data are utilized with site-specific wind rose information to estimate the fatigue reliability of HMIPs or, effectively, the probability of safe performance over the planned service life. In principle, even for sites not instrumented, HMIPs can be evaluated using site-specific wind data together with the fatigue demand for different wind speeds and directions derived from the five field tests. Results from the reliability analyses are used to propose inspection schedules for in-service HMIPs with a view toward optimizing costs while maintaining safety and acknowledging fatigue damage accumulation.