ESDU Engineer

Vortex-shedding Response of Structures in the Wind

The vortex-shedding response of structures in the wind is increasingly an important design consideration, especially as architects become more adventurous whilst under pressure to reduce costs. We increasingly see taller, lighter and more flexible structures. Historically, the response of circular-sectioned structures has been the focus of concern but increasingly checking for other shapes is necessary.

ESDU has provided calculation methods for circular sections since 1985 and for non-circular sections since 1990. Since the early 1990s, national Codes of Practice have introduced methods for consideration of vortex-shedding response, the current Eurocode 1991-1-4:2005 being one example.

ESDU 96031, for response of circular-sectioned structures, implements a theoretical model with empirically-derived parameters described in ESDU 96030. One of the worked examples provided is that of a rebuilt stack that had failed due to fatigue damage that might be attributed to vortex shedding. The before and after cases provide good simple test cases for the ESDU model and the Eurocode.

The results of the calculation of the ratio of expected maximum deflection to stack diameter are summarised in the table below.

It can be seen that both methods predict responses for the original design that are large enough to be of concern. The ESDU method allows prediction of the response of the rebuilt stack with strakes but the Eurocode only allows consideration of the effect of increased mass and damping. However, both methods predict responses that are likely to be considered acceptable.

The ESDU program also provides calculation at off-critical conditions as shown in the sketch below and the response calculated includes the contribution of across-wind turbulence buffeting which may be important at higher wind speeds.

The Eurocode is complex to implement and includes a need to iterate in a hand calculation whereas the ESDU method is implemented in a spreadsheet without the need to involve the user in the calculation. ESDU also provides the response in the form of various loading parameters as well as the displacement.

The case discussed above is of a very simple structure. Use of the Eurocode for more complex cases will often involve the user in making simplifications and judgements. The ESDU method has been extrapolated in a rational manner to cover cases of stepped, tapered, straked or otherwise suppressed structures and structures which may have a varying mass distribution. All of the cases are implemented with direct input of properties to the spreadsheet.

Queries regarding this article should be directed to the Wind Engineering Group via esdu@ihsesdu.com or the‘feedback’ form at www.ihsesdu.com.