The purpose of this bulletin is to provide site owners and contractors with recommended best practices for identifying and inspecting flagpoles and shrouded monopoles that may be potentially affected by Vortex Shedding fatigue and for mitigating the potential damages caused by repetitive Vortex Shedding vibrations, up to and including structural fatigue causing failure. Vortex Shedding tends to happen when wind is blowing steadily at lower wind speeds in the range of 18 km/hr to 60 km/hr. Generally, the height and the diameter of the pole determine the wind speed at which these vibrations will occur. The wind speed at which these vibrations occur is known as the critical wind speed. (For more information about the effects of Vortex Shedding Vibrations, please see STAC Alert SA-0003 here.)
This bulletin also provides recommendations relating to inspection frequency of flagpoles and shrouded monopoles, as well as mitigation strategies for new towers. These recommendations are expected to serve as stop-gap measures until the release of the CSA S37-18 standard, which will provide further guidance on the design requirements.
Please note that a related bulletin was previous released providing information about a similar issue relating to shrouded tripoles. For more information, please see STAC Bulletin SB-0001 (February 16, 2018).