Galvanizing Crack Formation at Base Plate to Shaft Welds of High Mast Illumination Poles

High mast illumination poles (HMIPs) are tall cantilevered structures used to efficiently illuminate large portions of highways and interchanges. Great interest in the performance of HMIPS has arisen from the discovery of extensive premature cracking at the toes of base plate to pole shaft welds of poles currently in service. These cracks, in some cases, have become so severe that HMIPs have actually collapsed, and therefore present a great threat to public safety.

Previous research at the University of Texas at Austin sought to solve the design problems posed by these pole failures by conducting both full-scale and analytical tests on optimized designs of HMIPs for fatigue loads. These studies indicated that using full penetration welds to connect 3" thick base plates to relatively thin shaft walls minimized warping of the base plate during fatigue loading, and maximized fatigue performance.

Toward the end of these studies when researchers sought to test an uncoated optimized HMIP back-to-back against a galvanized HMIP of the same design and material, researchers discovered the galvanized specimen had cracked during the galvanizing process. This finding prompted an in-depth study to determine the cause of these cracks, and to determine if practices could be implemented to prevent crack formation.

Initially, bend radius, chemistry, and shaft to base plate thickness studies were conducted to find how these parameters affect HMIPs during galvanizing. These parameters were found to play a minor role in the cracking of HMIPs relative to the thermal effects induced during the galvanizing process. Full-scale and analytical tests verified the impact of thermal straining within HMIPs during galvanizing. Instrumenting HMIPs and smaller HMIP stub sections with thermocouples and strain gages provided temperature and initial strain gradients resulting from exposure to the molten zinc bath. This data, as well as observations of cracks in the tested HMIP sections, aided the development of a finite element parametric study comparing HMIPs of the same 150' length and 80 mph design but varying shaft thicknesses. This research concludes that reducing the pole shaft diameter to thickness ratio reduces the likelihood of galvanizing crack formation.