Hi,

When a vessel is determined not to be susceptible to vortex shedding, it will not vibrate when the wind blows across its exposed surface. When the box in PVElite is checked to consider this load the program computes the vortex shedding load and applies it over the top 1/3 of the vessel. If it is not susceptible and you force the load on the structure high moments and stresses will be generated.

On the other hand, if the vessel can resonate as a result of this phenomena (the structure is flexible enough) this will not produce the extrememly high loads you noted when you "forced" the vessel to vibrate by saying yes to message that came up on the screen.

Also realize that these calculations are very theoretical and with enough surface roughness like piping, platforms, ladders etc, vibration may not be a problem. If you have a very slender smooth structure like a stack this would warrant some investigation. ASME's Steel Stack Standard STS-2000 may provide additional insight for these sorts of problems.