Lifting lug design per ASME BTH-1 and what it doesn't address

[Finn_McCool]

Hello,

I have been developing a design procedure for lifting lugs based on ASME BTH-1, but it seems that there's a potential failure mode that BHT-1 doesn't address. It doesn't seem to address Hertzian contact stresses that arise in the lug, which produce a maximum shear stress in the lug some small distance below the point of contact between the lifting pin and the lifting lug. Lifting lugs that I have evaluated per ASME BTH-1 suggest that the lugs I've designed are sound, yet when I compute the Hertzian contact stresses in these lugs they are definitely NOT fine. Am I missing something in ASME BTH-1; does it actually address this failure mode in some way that isn't apparent? Initially my thought was that ASME BTH-1's criteria for "single plane fracture strength" was actually referring to failure by Hertzian contact stresses, though I'm fairly certain what this is actually referring to it hoop-tension failure in the lug. Any insight would be greatly appreciated.

Thank you,
Finn McCool

 

[JStephen]

Avoid triple posting- but see my answer in the Structural Engineering General Discussion post.

 

[Finn_McCool]

JStephen,

I apologize - thank you for bringing that to my attention. I thought I was posting in three separate boards and thus not triple posting. I was wrong; will avoid in the future.

Honestly, I have done my due diligence and spent a good amount of time searching the board for an answer to this question -- could you provide a link to the specific thread you're referring to?

Thank you,
Finn McCool

 

[SSCon]

Was responding to another thread but it got deleted while I was typing it...

This is interesting, and is unfortunately an issue you're likely to encounter throughout your career.

The ASME standard is based on test data and the critical paper by Williams is based on theory, test data always wins.

In fact the BTH standard is very well founded, I'd sugest you have a look at: ASME BTH-1 Pinned Connection Design Provisions, David Duerr, (2008).

I only had a glance at the critical paper but it appears to criticize the ASME criteria on the basis of first principles hand calculations and linear FEA. I don't think it even attempted to address the test data which is referenced in BTH. Frankly, the paper is silly and sounds like something a graduate or academic would write.

There are numerous offshore codes with equations for lifting lug design (I wouldn't bother digging into them if I was you). They don't typically mention Hertzian contact stress but will have a bearing stress criteria which which is more conservative than BTH. The equations won't be sourced though.

 

[IRstuff]

Your duplicate threads were deleted; unfortunately, whatever answers there were are no longer extant.

TTFN (ta ta for now)
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[Finn_McCool]

@SScon: Thank you for the reference -- I will absolutely look at that. I agree that empirical data should win in a match with theory; I was unaware that what you reference existed, and I'm looking forward to investigating it.

Why is it you say "this is interesting, and is unfortunately an issue you're likely to encounter throughout your career"? Unfortunately I'm at a point in may career (fairly new) where I lack the experience to be able to make design decisions based largely on experience. Thus my orientation is to attempt to square the things I'm told with what theory suggests. I frequently find that they don't line up, which is frustrating.

I apologize for the triple-posting; I thought they would be considered separate posts since I posted them in different boards. I won't do that again.

Thank you,
Finn McCool

 

[SSCon]

I was referring to the Williams paper criticising BTH-1 on the basis of elastic FEA. The interpretation of FEA results is difficult and it is common for people to think that localised areas of stress above yield indicate failure and consequently create overly conservative designs. This is briefly addressed in section A-4.4 of BTH-1.

 

[Finn_McCool]

@SScon: I see A-4.4:

Analysis of a lifting device with discontinuities using
linear finite element analysis will typically show peak
stresses that indicate failure, where failure is defined as
the point at which the applied load reaches the loss of
function (or limit state) of the part or device under
consideration. This is particularly true when evaluating
static strength. While the use of such methods is not
prohibited, modeling of the device and interpretation
of the results demand suitable expertise to ensure the
requirements of this Standard are met without creating
unnecessarily conservative limits for static strength and
fatigue life.


This statement seems confusing. It refers to the "limit state" of the member in question. The "limit state" is prescribed by the standard in accordance with it's restrictions on allowable stresses, so how could one fault the use of the FEM in determining what these stresses are? Provided one knows what they are doing with a FEM package, this paragraph doesn't seem to discredit using it to determine how close internal stresses are to the limit states prescribed by the standard. I'm assuming Dr. Williams probably knows what he's doing with it, so why should we not accept his results?

Thank you,
Finn McCool

 

[Finn_McCool]

@SScon: I think I see what you may be saying, though correct me if I'm wrong: what your saying is that the standard allows a certain amount of yielding in the lug, and Dr. WIlliams' criticism is erroneous because he is defining ANY yielding as failure.

Thank you,
Finn McCool

 

[r13]

The Hertzian contact stresses addresses the very high contact stress between two spherical elements. The extremely high stress is localized (a sharp point), it will cause localized yield with finite distribution width and depth, and the subsequent deformation at the contact point. However, the local yielding is non-progress, as other stresses do, that will leads to global failure of the elements. I have respond to another thread of yours with excerption from an article, but that thread may have been deleted for site rule violation.

 

[SSCon]

That's roughly correct. If an FEM has a discontinuity it will show a peak stress and the more you refine the mesh the higher the peak will be, the actual value of the peak stress is largely meaningless. Any stress above yield in a linear model is fundamentally incorrect as a non-linear material model would be needed to accurately represent material behaviour.

In the real world most any heavily loaded structure will exhibit some local yielding, this local yielding isn't generally a problem but with FEA it can look like one.

If you consider a bolt under simple tension. If you wanted to know how strong the bolt was you would only need the tensile stress area of the bolt and ultimate strength of the material. Using; Force = Stress * Area you would determine the strength of the bolt and apply a design margin to determine the working load.

Simple, but what if someone created a linear FE model which included the threads? They'd see huge stresses at the thread roots (about five times the average stress through the cross section). How would you justify your previously determined working load?

The answer is that peak stresses - like those at the root of a thread - are typically only relevant for fatigue analysis. For static strength calculations you're concerned with the overall strength through a failure cross section, not localised "hot spots".

So, when you're doing elastic FEA with solid elements you need some sort of method to interpret the results, the ASME BPVC has a method to do that called stress linearization.

We might be biting off a little too much here but the answer to the original question about whether or not BTH-1 considers all relevant failure modes is; yes it does.

 

[Finn_McCool]

@r13: That thread was deleted because I'm an idiot and I triple-posted this thread. I saw your post in the other (now deleted) thread. What do you mean by "the local yielding is non-progress"?

Thank you,
Finn McCool

 

[r13]

The Hertzian contact stresses does not cause the entire cross section to yield. The yielding is instantaneous upon contact and limited to a small local area. I suggest to review how this method was developed, in combination with elastic/plastic theories.

 

[Finn_McCool]

@r13 & @SSCon: you've both given me some great feedback and things to consider. I appreciate you both taking the time to educate someone like myself new in the profession. I'll be investigating some of the items you mentioned today and might solicit further responses if your willing to give them.

Thank you,
Finn McCool