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Steel Stack Design 1

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SVaughn

SVaughn

Structural
Jul 11, 2012
12
Hi all,

I am designing a laterally braced, unlined steel stack and need a little help. The stack is to be a constant diameter of 4.37' with a height of 87.67'. It is to be made of 1/4" plate and will not be lined. Please see attached screen shot for layout of bracing. The top two braces are for lateral support only whereas the bottom brace location can be used for both lateral support and vertical support if need be. I only have two pieces of reference material and they are The Structural Engineering Handbook and the ASME STS-1 - 2006. Where I am struggling, is with the multiple brace points and how that effects the design as far as crosswind loads and vortex shedding is concerned. Do I determine an equivalent static force perpendicular to the wind direction accounting for the vortex shedding of each segment or eliminate this all together? Would this be the same for ovaling? Does anybody know of or have any other reference material that may help me? As always, any and all help/guidance is greatly appreciated.

Thanks,
Shaylon
 
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Another useful reference in "Tubular Steel Structures - Theory and Design" by M.S. Troitsky. It is published by the James F. Lincoln Arc Welding Foundation. You can buy it off their website for a pretty cheap price (something like $30). It is a little dated, but it has a whole section dedicated to stack design.
 
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As far as Vortex shedding goes you should seek to eliminate it all together (not try to accommodate it). I had to go into this one time and the reference I relied upon was the ‘Shock and Vibration Handbook’ [4th edition, by C.M. Harris; I think another edition has come out since then]. It had some good formulas for those types of bodies (i.e. what frequencies vortex shedding occurs at). Your objective is to make sure the natural frequency of your structure (at any of the various modes) will be will not be the same as the vortex shedding frequency you calculate (with some additional safety factor; remember, this type of analysis isn’t exactly an exact science).

With the tube braced like it is, I wouldn’t think there would be an issue…..but with it being such a small diameter, checking it is probably a good idea.


 
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Pardon me: above I meant to say: As far as Vortex shedding goes you should seek to eliminate it all together (not try to accommodate it). I had to go into this one time and the reference I relied upon was the ‘Shock and Vibration Handbook’ [4th edition, by C.M. Harris; I think another edition has come out since then]. It had some good formulas for those types of bodies (i.e. what frequencies vortex shedding occurs at). Your objective is to make sure the natural frequency of your structure (at any of the various modes) will not be the same as the vortex shedding frequency you calculate (with some additional safety factor; remember, this type of analysis isn’t exactly an exact science).

With the tube braced like it is, I wouldn’t think there would be an issue.....but with it being such a small diameter, checking it is probably a good idea.

 
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For something this tall, it's pretty common to use anti vortex strakes (spiral fins that break up the vortices). It increases your wind area, but it's better than having vortices.
 
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For something this tall, it's pretty common to use anti vortex strakes (spiral fins that break up the vortices). It increases your wind area, but it's better than having vortices.
Click to expand...

Quite true. I've seen similar outer skins on [tall, cylindrical] process equipment.
 
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as was mentioned, calculate the natural freg of your setup(1st mode), then calculate the critical vel and if below 60mph, address the oscillation problem..from looking at your sketch it does not look like you will have that problem..however, ovaling stiffeners will probably be required spaced @ about 1 1/2 dia....
 
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WARose has a good reference. As far as a safety factor, I generally try to avoid the vortex frequency by +/- 20%. This comes from "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications" published by ASCE.

namanges
 
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Thanks to everyone here for all of your help. I have managed to work through this and have come up with a pretty solid design. However, I am not working on the base plate and anchor base ring design and was wondering if anyone out there has a worked example that they may be willing to share with me.

Thanks,
Shaylon
 
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Other references

BRITISH STANDARD BS EN 1993-1-6:2007
Eurocode 3 — Design of steel structures
Part 1-6: Strength and Stability of Shell Structures

BRITISH STANDARD BS EN 1993-3-2:2006
Eurocode 3 — Design of steel structures
Part 3-2: Towers, masts and chimneys
Chimneys
 
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Hi all,

My previous post stating that I have come up with a solid design has turned out to be false. Some assumptions I was told to make were not correct. Anyhow, I am having trouble interpreting the ASME STS-1-2006 standard as far as VORTEX SHEDDING goes and whether or not helical strakes should be used. If you can remember, I have a stack that is braced at three points. Please see attached screenshot. I have determined that stiffeners are required to combat ovalling at a spacing of 3D which is twice as far as what Gaylord suggests in the Structural Engineering Handbook. Can anybody shed some light on how to proceed with this design?

Thanks,
Shaylon
 
 http://files.engineering.com/getfile.aspx?folder=78115ddd-2d6c-45e8-b1c6-8f551a58954b&file=Vortex_Shedding.JPG
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