Not So Good Vibrations—Design Considerations for Slenderness Limits

As engineers, we work to refine our designs to ensure the most efficient use of material to provide an optimized design and best service to our clients. In doing so, we’re able to design some very slender members that will still meet the project design criteria. However, when left unchecked and so narrowly focused on material usage and efficiency, in some cases, these slender designs may be susceptible to the detrimental effects of vortex shedding and wind-induced vibration. While wind-induced vibration is largely a location-based phenomenon, it may be possible for owners, line designers, and fabricators to work together to reduce the probability of problematic vibration. Cross braces and cross arms for H and A Frame structures are two examples where overly efficient designs can lead to some unintended consequences with structures installed in susceptible locations. This paper and presentation will provide a real-world example of one such design. Based on analysis, investigation, and research, this paper and presentation will also propose slenderness limits and equations that any engineer can use to determine the approximate critical wind velocity for vibration to occur and help them assess the risk for their cross brace or cross arm design to the effects of vortex shedding and wind-induced vibration.