Stiffening Ring on Vessel for Wind Loading Bending Moment Design?

[mechiemech]

Hi all,

I have some experience on general plant maintenance but I'm not a designer, so please forgive me if this is a really silly question. For tall vessels where the ruling design case is wind load bending moments, is it acceptable to weld on stiffening rings or lugs external to the vessel to reduce the required shell thickness? I know that stiffening rings are used for vacuum cases, but just wondering if the same principle can be applied for wind load cases.

Thank you

 

[prex]

Nope, bending due to wind results in axial compression in the wall or skirt, and a circumferential ring won't relieve that condition.

prex
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[racookpe1978]

You can add a set of axial ribs to the outside, but then you have a (slight) increase in wind cross-section. The vertical 9axial) ribs also increase the CD (drag coefficient) since it is no longer a simple round CD = 1.0.
Re-calculate the drag with the new cross-section area and the new CD, then use the slightly larger rib that will be required.

Stitch welding probably adequate - the outside welds will not be pressure tight.

 

[mechiemech]

Hi, thanks for your replies. Yes, I understand that stiffening rings all the way round would only stiffen it for vacuum or external pressure cases. What I meant was to apply similar idea of external stiffening by welding beams vertically on to the shell, maybe 4 or 6 beams around the circumference? I hope you can picture what I'm trying to describe.

Thank you.

 

[JStephen]

On large-diameter tanks, it is not uncommon to use a top girder and intermediate wind girders for wind stability. However, this is a different kind of design condition from that encountered in tall vessels.

Theoretically, you can add vertical stiffeners to a cylindrical shell for increased axial load capacity. For new construction, it is generally going to be cheaper to increase the metal thickness than to build up a composite section. In analyzing the stiffened structure, I don't think there is any standardized way to account for the increased stability, which is another drawback.

Note that the fluted-pedestal water towers use a similar concept to increase the strength of the pedestal (the pedestal is not pressurized in those tanks).