Evaluating Vortex Shedding on Existing Stacks: Why "No Failure" Isn’t Proof of Safety
Engineers evaluating an existing stack that has been in service for years or even decades without failure may be tempted to conclude that vortex shedding is not a concern. While that may ultimately be true, it's important to understand why the stack has performed well—and whether the observed performance can be relied upon moving forward.
Vortex Shedding: The Basics
Vortex shedding occurs when alternating low-pressure vortices form on the downwind side of a cylindrical structure like a stack, inducing periodic forces that can excite the stack's natural frequency. If the wind speed aligns with the structure's critical wind speed, resonant oscillations can develop, potentially leading to fatigue damage or even failure.
The “No Failure” Assumption Is Incomplete
Just because a stack hasn't failed doesn't mean it has never experienced vortex shedding—or that it won’t in the future. For vortex shedding to be ruled out as a concern, two things must be true:
The critical wind speed that would excite the stack’s natural frequency must have occurred during the service life.
Someone must have been present and paying close attention to visually observe the stack during that condition to confirm no noticeable oscillations.
Without this, there's no guarantee that the stack hasn't simply avoided the resonant wind speed thus far.
If the critical wind speed is relatively low and it seems highly likely that it would have been experienced and the stack is in a location where it would have been highly visible and vibration would have been noticed, then perhaps you can draw the conclusion that vortex shedding is not critical on that stack. If the stack is not in a highly visible location or perhaps the critical wind speed is very high and most likely nobody would have been observing the stack when that event occurred, then vortex shedding shouldn't be ignored. Between these two extremes, there is a lot of grey area that will be left to the Engineering to make a judgement as to the risk of ignoring vortex shedding.
The Role of Structural Damping
Another possibility is that the actual damping in the structure is higher than the conservative values typically assumed in engineering analysis. Field conditions such as friction at supports, corrosion buildup, or connected ductwork can provide additional damping, suppressing the amplitude of oscillations that might otherwise occur during resonance. If the actual damping is higher than assumed, it could explain why the stack has not exhibited problems despite being theoretically susceptible.
What Engineers Should Do
Don’t dismiss vortex shedding just because the stack is old. Review the stack’s natural frequency and calculate the associated critical wind speed.
Compare historical wind data to determine whether the critical wind speed has likely occurred.
Consider field testing or vibration monitoring during windy conditions to assess behavior directly.
Evaluate damping assumptions. If inspection or historical data suggest the structure is behaving better than expected, damping values may need to be updated.
Conclusion
A history of no visible failure is not sufficient justification to dismiss vortex shedding concerns. Proper evaluation requires confirming that the right wind conditions have occurred and understanding the role of damping in the stack's performance. By revisiting the assumptions and comparing them to real-world conditions, engineers can make more informed decisions about the long-term safety of existing stacks.