Issue with stack structural analysis 1

[atrizzy]

I'm analyzing an existing steel stack for power plant effluent to determine adequacy for wind loading and am running into a bit of an issue. Please see the attached sketch.

The stack was designed with 4 vertical stiffeners at 90 degrees to each other running vertically for the top 40' of the stack. There are intermediate horizontal angle stiffeners, but none exists at the lower termination of the vertical stiffeners. The stiffeners just stop, abruptly, about 40' below the top of the stack.

I'm trying to determine if there is a risk of premature local buckling in the primary shell due to bending in the stack at the point where the stiffener terminates.

I'm having a tough time wrapping my mind around this. On one hand it seems if there is stress in the stiffener, it has no path but to transition to the main body but through a localized point. On the other hand, how could the entire stiffener be under stress if it terminates and there's nowhere for it to go?

I'm thinking of providing a beefy horizontal ring stiffener at this location to mitigate any potential for this issue, so that I can be sure that the flexural capacity of the entire tube section can be utilized.

Any thoughts and insight welcome. Thanks in advance.

 

[Klitor]

Whats the total height of stack?

 

[atrizzy]

@Kiltor - the stack is about 200' tall.

 

[WARose]

It probably is going to deliver some localized force......which a analysis could tell you. But usually stacks are stiff enough (without the stiffeners) where I wouldn't think it would be that high.....as long as the stiffeners are small.

By the way, why the stiffeners up high (rather than below)?

 

[SlideRuleEra]

I'm analyzing an existing steel stack for power plant effluent...

If the stack if for combustion flue gas, be sure to verify steel thickness, flue gas is very corrosive.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[atrizzy]

@WARose - I'm not sure why the stiffeners are that high, which is part of what I'm trying to understand. NDT thickness tests show a thicker tube wall in the lower segments, but I'm unable to determine if this is due to corrosion or not. They terminate at a location where, as far as I can tell, the tube thickness remains constant above and below. And the stiffeners are large... 1" x 12" compared to a (corroded) 0.15" (or less in some cases) wall thickness. I'm thinking due to the corrosion in some cases the stiffeners may be taking all/most of the flexure in a tension/compression couple.

@SlideRuleEra - agreed, we have good NDT data for thickness, much of the uppermost portions are extremely corroded, which is why I'm involved.

 

[WARose]

@WARose - I'm not sure why the stiffeners are that high, which is part of what I'm trying to understand. NDT thickness tests show a thicker tube wall in the lower segments, but I'm unable to determine if this is due to corrosion or not. They terminate at a location where, as far as I can tell, the tube thickness remains constant above and below. And the stiffeners are large... 1" x 12" compared to a (corroded) 0.15" (or less in some cases) wall thickness. I'm thinking due to the corrosion in some cases the stiffeners may be taking all/most of the flexure in a tension/compression couple.

With that wall thickness I can see what you are getting at. (My guess was about 3/8" thick.)

They could have stiffened it up for the wind (i.e. a vortex shedding issue).....or really a whole bunch of reasons. It's hard to say.

If you don't mind me asking: why are you messing with it at all? Are you modifying it?

 

[Compositepro]

It does not make sense that stiffeners are in that location. It is more likely that those are strakes to reduce vortex shedding. If they are solid then they would also add mass which would de-tune the stacks resonant frequency. Normally these strakes are a spiral around the stack, but other configurations work.

https://en.wikipedia.org/wiki/Vortex_shedding.

https://www.google.com/search?q=tal...#imgdii=4PcCGkGFTjnsUM:&imgrc=yKrpk-3nYiCv_M:

 

[SAIL3]

yes, I would agree that they are strakes to counter vortex shedding and thus wind-induced vibration which derives the majority of it's forcing function from the upper portion of the stack...there are many different variations on the strakes..some are continuous spirals while others are in a spiral pattern using short straight strakes...I have not seen a pattern of 4 long continuous strakes before..not saying they will not work....which brings up the question...how much of the bending stress in the stack migrates into these vertical stiffeners....on the plus side...top portion of stack should not have much moment in it compared to the lower portion....the bending stress is longitudinal and I would assume gradually migrates into the stiffener....I would calculate the wind moment in the stack at the location where the stiffeners terminate....assume the x-section of the stack includes the stiffeners and then calculate stress in stiffeners
...based on that stress I would make an engineering judgement....having said all that, I would not hesitate in putting a horiz stiffener in that location mainly because of the 0.15" stack wall thickeness...

 

[atrizzy]

At first I thought they were strakes, but I was only familiar with the spiral types. Regardless of if they're strakes or not, they're acting as big fat stiffeners.

@WARose - I've been tasked with assessing the capacity and potentially providing a repair scheme for this thing due to very heavy corrosion occurring at the uppermost portions. In checking the overall capacity of the thing I noticed that this interface could be a potential weak point due to the strakes/stiffeners/whatever they are terminating at that point.

I suppose a FEM analysis of the sections is in order. I'm leaning more and more towards providing a big fat horizontal stiffener there to engage more of the entire section below the stiffener.

@SAIL3 - You raise an interesting question. I believe that the level of corrosion in the upper tube body is directing much of the bending effect directly into the strakes/stiffeners/whatever they are.

This thing is a bit of a lipstick-on-a-pig scenario, but I feel I should go through the analysis prior to telling the owner that they should rebuild the top 100' of the thing.

Much appreciate the responses, all. Keep em coming!

 

[JStephen]

I would go with the strake idea, also.
If the shell without the stiffeners is stable, then I would assume that adding the stiffeners didn't make it unstable.
If you are showing problems with stress concentration there, putting a repad at that point or putting a tapered transition at that point might be easier than the circumferential stiffener. You could accomplish the taper by trimming the strakes, rather than having to weld anything to it.
Stress concentrations may be more likely to cause a crack at that point due to fatigue than the localized buckling.

 

[JLNJ]

With such a thin stack, would ovalling be an issue?

If so, vertical stiffeners don't help much for ovalling (other than changing the stack profile and maybe spreading out some local effects), but terminating them at a ring would help.

 

[SAIL3]

JLNJ makes a good point..check ovaling....

 

[Klitor]

I was also leaning to strakes, hence the total height of stack question...ive seen similar unusual attempts at welding vertical strakes at about upper 6th of the stack.
Since you said there are horizontal ring stiffeners, maybe designer thought about ovaling also.
Conveniently upper 6th is also the area most susceptible to corosion so maybe owner is lucky they are there in the first place
I would def do a detailed analysys before proceding.

 

[atrizzy]

Re ovaling, there are horizontal stiffeners, just not at the bottom termination of the strakes.

On a related note, could I request that someone with a 2016 version of ASME STS-1 confirm something for me (I'm reviewing the 2011 version at the moment... I know, I know.)

There's an equation in section 4.4 that reads:

'The following forumlas for determining allowable stresses are applicable for circular stacks and liners provided that eq 4-6 is satisfied:

eq 4-6, t/D <= 10Fy/E

I don't see why they would be setting a maximum limit on the thickness for these equations to apply. Does anyone know why this is? Surely it won't hurt things if I exceed the thickness to depth ratio provided here...

 

[dhengr]

Atrizzy:
Take a look at the old Struct. thread #507-271837, from about 11MAY10, by a person named Hoots711.

 

[hokie66]

dhengr,
You can link that just by typing it in, without any spaces or the #.

thread507-271837

 

[atrizzy]

@dhengr - that was very eye opening and makes perfect sense.

Much obliged.

 

[dhengr]

Hokie:
I did know there was a way to do that, but I wasn’t having any luck finding it, so after a couple tries I gave up. Thanks for the info.

 

[KootK]

My thoughts:

- I'm not so concerned about the local buckling. While the faux stiffeners themselves may terminate abruptly, that may not be nearly so true of the flexurally induced axial stresses within them. I suspect FEM results would confirm that. Rather, I believe that axial stress would transition out of the stiffeners more gradually. Anecdotally, in more conventional steel members, there are many examples where partial depth stiffeners are used with neither incident nor special provision for concentrated force transfer where the stiffeners terminate.

- To the extent that the local force transfer may be a problem, I don't feel that horizontal stiffeners would effectively address that problem anyhow. The stiffness of that spongy, cantilevered backstop of sorts would pale in comparison to the stiffness of the natural force transfer which is the axial force being stripped out of the stiffeners via longitudinal weld shear. The horizontal stiffeners might add some additional buckling stability to the region but that's about it as far as I can tell.

- How quickly the axial forces in the stiffeners is siphoned of into the main member via longitudinal weld shear would seem to the be issue du jour. If it happens at a very slow rate, I think that we'd all agree that there is no issue. It's only if it happens at a very fast rate that there's a potential local buckling issue to worry about. In this regard, the addition of the horizontal stiffeners might actually worsen matters. By providing a backstop for abrupt force transfer, you invite it to a degree. Not that I'd give this any serious consideration in practical design mind you. Just a theory thing.

- Axial load transfer to long stiffeners is somewhat of a self solving problem with regard to local stability. If the rate of force transfer to the stiffeners is too abrupt, the high local compression in the adjacent plate should soften that area and tend to spread the force transfer out over a longer length. I have no idea how to formally quantify that phenomenon but do take some comfort from it. I've often imagined similar buckling issues when detailing things like partial height bearing stiffeners.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.