Help on ASME-STS1 in regards to steel stack design. 1

[hoots711]

Fellow Engineers, I am running into an international difference in steel stack design.
Per the ASME, STS-1, Section 4.4, Eq 4-6, allowable stress cases are only valid when equation 4.6 is satisfied.

4-6 states that t/D <= 10Fy/E

basically, it limits your thickness based on diameter as Fy and E are going to be constants in my design.

The issue is in design of something like a 1.2M Diameter steel stack that is 40M high (free-standing). Per the above formula/condition, i cant use plate thicker than 15-16mm (after corrosion @ FY=34ksi & E=29Mill) and with that parameter i cannot design the stack to pass the allowable stress cases.

Our foreign counterparts design the above stack with most thickness around or over 20mm over the lower half of the stack.

Before i can debate which method is acceptable i have to understand why on Earth the ASME would be limiting thickness based on D in steel stacks.

The only thing i can possibly think of is that the rolling of thicker plate to smaller diameters could compromise the steel??? If this was the case i would think there would be thorough documentation on the subject. This also apears to not be the case as FY is on the right side of the formula (stronger steel would allow larger thicknesses... not likely a rolling issue)

Im striking out finding anything explaining equation 4-6. Can anyone offer any advice or interpretation?

I guess the basic question is: What do you do when equation 4-6 is not satisfied?

Am i mis-reading something here? I have brough this issue to 3 or 4 of our senior US structural engineers and they have all be puzzled by this (mostly by the fact that they havnt noticed this stipulation in previous designs..)

Why on earth would ASME limit MAXIMUM plate thickness by diameter??
Thanks and Best regards.

 

[JLNJ]

That code has been suspect since the 1st day the 1st edition was published.

Call the guy who does the STACKDES program and ask him. He's probably the most knowledgable guy with regard to stack design.

 

[dhengr]

Hoots:
By limiting t to 16mm for your dia.; I wonder if they aren’t saying, ‘any thicker than that and you are no longer working within what we consider the thin shell regime, certainly stiffer than any of our testing, so these allowable bucking stresses may not actually apply.’ I would not ignore ASME’s stress formulas, and it sounds like you haven’t since you said you needed about 22mm to meet them. So, maybe they are saying that given your D/t, you are working below the lowest D/t values considered for these testing programs and for these empirical formulas. Above t = 16mm, you are moving out of the thin shell regime; into a no-mans land, a long way from the 12 or 16" round col. though. Does that ASME STS1 code have a commentary you should be looking at and telling us about. Or, as paddington suggested any discussions of the standards in journals, prior to adoption.

The ASME limitation is because you are moving out of their known range of, thin shell, thin plate, elastic bucking regime for these cylinders, as you increase t too much. Their formulas are for thin shell, elastic buckling, and the thicker pl. will push you into another buckling regime, if buckling happens at all. The tougher problem may be making this stack check as a canti. beam/col., where kl/r = 2l/r, for Euler type buckling. It seems to me that you are working at such a low D/t level that you just won’t get their, thin shell, kind of buckling, if you get any buckling at all. We should maybe looking at another form of buckling for these kinds of cylindrical structures, and that is that they can go into a flatter, or oval, shape. Maybe we should be looking at the fact that the actual max. stress which might cause buckling only occurs over about a 90° arc of pl. at any given time. I just don’t see that thickening the pl. and working near (or to be within) their stress ranges leads to a buckling problem.

What do the power transmission people do with their towers? They are break forming their poles in 8, 10 or 12 sides, certainly as tall as your stack, and with much higher imposed loads and moments. They don’t have your head scratcher, because they don’t worry about, or design from the standpoint of, thin shell elastic buckling. They undoubtedly do pay some attention to pl. buckling, the way we normally look at it, but they have the advantage in that they think in terms of stiffened edges on a 10 or 12" wide pl.

I wrote the above before I saw the ASME & Gaylord mat’l. you sent. I think you’re on the right track now. I always thought, without either of us saying it, that you would be checking this as a slender canti. beam/col., with kl/r = 2l/r, and an interaction equation, and that that might control the design. But, I didn’t see your ASME approach until late yesterday. I’m not so sure that I would be so bold as to say you have a compact section (.66Fy) with your dimensions though; but you are below their D/t range for thin shell theory and testing. AISC doesn’t comment on non-compact sections in their table B5.1, because pipes are a nice stable shape and most dias. we deal with are probably compact. But, see their footnote ‘c’ and appendix B5, pg. 5-100 and there-abouts. In your 4th para. under (1.), ‘allowable yield stress’ seems like bad wording, and what is table C-36?

It just took TERIO, with few words, to turn on the light bulb. In effect, the above is about what I meant when I said you are moving out of the thin shell regime, but into something of a no-mans land, since this sure isn’t a 16" round pipe col. either. Thanks TERIO!

In most buckling problems, you are always looking for a min. thickness, a width/thickness or dia./t criteria for a given stress, to prevent buckling. So, it shouldn’t be hard to wrap your head around this, but it’s a min. t for a given width or dia. at a particular stress. ASME is saying if you go above this t/D these stress equations are suspect as you are moving out of the thin shell regime. And, we have been arguing this back and forth through this whole thread. And then, just to confuse things every other formula or graph uses t/D instead of D/t or R/t which tends to do funny things with your thinking and the inequality symbol.

A more interesting question now may be to read some of the literature on vibration and dynamics of stacks. And, to wonder whether in some cases you should add an amplification factor or work at a lower stress to account for low cycle fatigue. For both of our, all of our edification, I would like to know what the ASME committee says on this. With a little more digging and work, you and I could teach a seminar or semester course in steel stack design.

 

[hoots711]

Great post Dhengr, I agree... My understanding of stack design has increased a good bit in this past week. We so often get caught up in using templates and excel sheets that we forget we need to actually understand the math behind them.

Yes, my wording was poor in regards to table C-36. (Page 3-16) Allowable Stess for compression members of 36KSI specified yield stress steel.

I mentioned this as a "common sense check" in that my allowable at Fy=34 using equations E2-1 & E2-2 from the AISC is close to the numbers provided by table c-36 for 36KSI steel with a Kl/r of around 172. Gaylord's Eqs return similar results

In hindsight... It seems this was a not seeing the forest bc of the trees issue.

I (and other coworkers) stared at the line about "all other steel members shall comply with the requirements of the AISC..." but we all were thinking of "members" as stiffening bars, or part of our breach design, or spoiler (strafes) designs or pieces in our compression rings... Some still question this line, and feel it should be addressed by the asme.

The biggest curve ball, to me, was no mention or relation to slenderness in equation 4-6. The pure D to t ratio is what was throwing me off. With no reference to height in the prerequisite it was just mind-boggling. (and threw many off us off in the wrong direction)

Now, it appears, (to me) that equation 4-6 is determining if you can use AISC design methods shown in chapter E (Columns and other compression members) or if you need to use the ASME equations. (ill need to use both in this case as our plate with range from 22mm at the base to 6mm at the top)

I do plan on asking ASME about this issue. One of our Engineers asked the following in regards to legality

Can a stack with with a t/D not meeting equation 4-6 be designed per the ASME code? (basically, my same original question... What do you do when Eq 4-6 is not satisfied)

If the answer to the above is yes, what design method is acceptable.

Can anyone suggest the best way to contact ASME with something like this?
Thanks

 

[TERIO]

Here is the link to the STS committe page. I would contact the person listed and request the interpretation.

http://cstools.asme.org/csconnect/CommitteePages.cfm?Committee=C81000000

 

[hoots711]

Thank you Terio. I sent an email and will update the thread with any response.

 

[hoots711]

(Below is the ASME's opinion on the issue)

Dear Mr. XXXX

Please see below in bold letters the personal opinion of a committee member. Please note that since this is the personal opinion of a committee member, it is not endorsed by ASME.

If you require a formal written interpretation, you must follow the guidelines noted in page vi (correspondence with the committee) of the Standard. I could take about 2-3 months to get an official response.

Sincerely,

XXXX XXXX XXXXX
Project Engineer
ASME
Three Park Avenue, MS 23E4
New York, NY 10016
Tel. (212) 591-8018
Fax (212) 591-8501

(This is my original email with the ASME response in brackets)

Mr. XXXX



My colleagues and I have been requested to estimate and design free standing steel stacks that do not satisfy equation 4-6 (ASME STS-1-2006). (As an example, a 36.7M tall stack by 1.18M Diameter). Once stresses are calculated, the design would require 20-22mm plate at the base. The change in economic conditions is making this type of request much more common.

[Response: I’m not sure why the thickness is so high (20-22 mm). This is a very slender stack and I can imagine that wind induced vibration (vortex shedding) may be controlling the design. Usually you would add some vibration damping device or helical strakes to address the vortex shedding, otherwise if you increase the thickness to handle the vortex shedding loads then you will end up with a very thick stack.]



Section 4.4 does not clearly address what to do when equation 4-6 is not satisfied. The 2nd paragraph (below) could be interpreted as “when Eq 4-6 is not satisfied, design per the AISC”. Is this interpretation correct?

[Response: That is my interpretation as well, and this is what I have done in practice. Typically this condition occurs when we have a relative thick shell, and so my conclusion was that it behaves more as a “Beam” and we can ignore local buckling concerns of a thin wall shell.
]


“All other steel members shall comply with the requirements of the American Institute of Steel Construction (AISC) specification for the design, fabrication, and erection of structural steel for buildings”



After reviewing Troitsky, Cheng, Gaylord and the AISC, it is apparent that when eq 4-6 is not satisfied, you are within the limits of a compact section (D/t < 3300/Fy). The sections of the stack that do not satisfy eq. 4-6 could then have their allowable stresses calculated by using equations E2-1 & E2-2 of Chapter E of the AISC. Again, does this thought and design process, when eq 4-6 is not satisfied, coincide with the intended design of the ASME?



Can a stack containing thicknesses that do not comply with eq. 4-6 still be “Designed per the ASME STS-1”?

[Response: In my opinion, since the standard indicates that if eqn 4-6 is not met then AISC should be followed, that you would meet the intent of ASME STS-1. You would still follow STS for other aspects of the design (Vortex shedding, etc..). ]



Thank you for your time Mr. X.



Any clarification or direction you could provide would be appreciated.



Best Regards,