Vortex Shedding and Wind-Induced Vibration in Slender Structures
Wind pressure on a building facade is usually treated as a static or quasi-static load, but wind flowing past a slender, roughly cylindrical structure − a chimney, a steel stack, a slim tower, a suspension bridge deck, or even a mast antenna − does something a flat facade does not: it separates from the surface and sheds discrete vortices alternately from each side, generating an oscillating lateral force perpendicular to the wind direction, entirely separate from the drag force acting along the wind. This across-wind force is the mechanism behind some of the more dramatic wind-induced structural failures on record, and it behaves nothing like the pressure-coefficient calculations used for ordinary wind loads on buildings.
The Shedding Mechanism
As wind passes a bluff (non-streamlined) cylindrical section, the flow cannot follow the surface around the back side and separates, rolling up into vortices that peel off alternately from the top and bottom (or left and right) of the section in a regular, periodic pattern known as a von Kármán vortex street. Each time a vortex sheds from one side, it creates a brief pressure imbalance pushing the structure toward the opposite side, and the next vortex from the other side pushes back, so the net effect is an oscillating transverse force at a frequency governed by the wind speed, the structure's characteristic width, and a dimensionless Strouhal number that depends mainly on the cross-section's shape (roughly 0.2 for a circular cylinder over a wide range of flow conditions).
The shedding frequency fs is estimated as fs = St · V / D, where St is the Strouhal number, V is wind speed, and D is the across-wind dimension of the section. For a given structure, this frequency scales directly with wind speed, so there exists a specific wind speed − the critical wind speed − at which the shedding frequency coincides with the structure's own natural frequency of vibration.
When shedding frequency and structural natural frequency coincide, the response does not simply peak the way a forced-vibration resonance curve would suggest and then recede as wind speed continues to rise. Instead, a phenomenon called lock-in can occur: the structure's own motion begins to influence and synchronize the shedding process itself, holding the shedding frequency locked to the structure's natural frequency over a range of wind speeds around the critical value, rather than the shedding frequency continuing to track wind speed as it would for a stationary object. This is why vortex-induced vibration can persist and even grow over a band of wind speeds rather than occurring only at one precise value.
Why Slender Circular Sections Are Most Vulnerable
Vortex shedding is a concern mainly for structures that are both bluff in cross-section (so flow separates cleanly and sheds regular vortices, unlike a sharp-edged section that fixes separation at the corners and sheds less periodically) and lightly damped with low mass relative to the air displaced around them, which is precisely the combination found in steel chimneys, unlined concrete stacks, guyed masts, and long-span bridge decks. Structural damping in welded steel stacks is often extremely low − sometimes under 1 percent of critical − which allows vibration amplitude to build to many multiples of what a similarly loaded but well-damped structure would reach at resonance, because low damping means very little energy is dissipated per cycle relative to what the sustained lock-in excitation puts in.
Amplitude at lock-in also depends on a mass-damping parameter (sometimes called the Scruton number) that combines structural mass, damping ratio, and air density; low Scruton number structures − light, lightly damped, in denser near-ground air − are the ones most prone to large-amplitude vortex-induced response, which is part of why the classic vortex-shedding failure cases in the literature tend to involve slender steel chimneys and stacks rather than heavy masonry towers.
Mitigation
The most common fix is a helical strake − a spiral fin welded around the upper portion of a chimney or stack − which disrupts the coherence of vortex shedding along the structure's height, preventing the vortices from forming a single, correlated periodic pattern over the full length and instead breaking the shedding into an incoherent, less organized wake that no longer excites a clean resonant response. Strakes are a common retrofit on existing stacks discovered to vibrate excessively after construction, since they can be added without changing the structure's stiffness or mass significantly.
Where strakes are impractical, adding damping directly − through a tuned mass or tuned liquid damper matched to the vulnerable mode, the same class of device covered in tuned mass damper design for building sway − increases the effective damping ratio enough to keep lock-in amplitude within acceptable limits without altering the structure's aerodynamic shape at all. For new designs, simply detuning the structure − adjusting stiffness or mass so its natural frequency sits well outside the range of shedding frequencies expected at the site's typical and extreme wind speeds − avoids the problem altogether, though this is not always possible once other design constraints (height, diameter, wall thickness for internal process reasons) are fixed.