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Design of lifting lugs
2

Design of lifting lugs

Design of lifting lugs

(OP)
I'm curious as to how others design lifting lugs.  I am using AISC D.5 and J.7. The lug I am designing appears to be governed by bearing strength using AISC Eq J7-1  Rn = 1.8FyApb.  But I also found reference to ASME-BTH-1-2008 Eq 3-54 where the bearing capacity is P=0.63Fy(Dpin)t which is a big difference, 1.8Fy vs 0.63Fy.  I don't have the actual ASME document.

Also a co-worker attended an AISC seminar on Practical Connection Designby Larry Muir and it was mentioned that AISC D.5 should not be used for lifting lugs.

RE: Design of lifting lugs

1.8Fy is the nominal stress.  With omega=2, for ASD, you get an allowable stress of 0.9Fy.  This is still more than 0.63Fy, but not as dramatic an increase.  You are going to have an impact factor on the service load; perhaps ASME doesn't use this, and reduces the stress instead?

I'm not familiar with the ASME document, but AISC does have a paper (free for members) on the design of lifting beams, which discusses these checks.

http://www.aisc.org/store/p-818-design-and-construction-of-lifting-beams.aspx

RE: Design of lifting lugs

I'm not surprised you found big differences in the standards. Everyone seems to have their own opinion on how lifting lugs should be designed. What ever you do someone will surely disagree.

Chris
www.value-design-consulting.co.uk

RE: Design of lifting lugs

I usually check block shear (J4-5) and bearing at hole (J3-6b).

RE: Design of lifting lugs

Resources for lift beam design
ASME BTH-1-2008 (replaces the 2005 version) - Design of Below the Hook Lifting Devices.  Similar to AISC 9th Ed. ASD with greater safety factors.  (~$60 from ASME)  Design requirements used to be contained in ASME B30.20 which now covers safety requirements. I believe OSHA will reference ASME B30.20.

Two Articles by David Duerr in ASCE Practice Periodicial on Structial Design and Construction May 2008
  p.53 - ASME BTH-1 Pinned Connection Design Provisions
  p.59 - Design of Stiffened Plate Lifting Beams

Design/Evaluation of Overhead Lifting Lugs by Clement Rajendra,PE - This is an example calculation in Mathcad format (usuable without Mathcad).  This was part of a continuing education course.  Do an online search of the title. This follows the David Ricker Article (not BTH-1)

Design and Construction of Lifting Beams by David T. Ricker - AISC Engineering Journal 4th Qtr. 1991 (+ Errata 2004 4th Qtr.) Uses AISC 9th ED and shows how to modify for greater safety factors.  Good Article.  I have not had time to compare this with BTH-1 calc by calc.

Another note - If I remember correctly, The ASME books reference AWS D14.1 which uses a lower allowable tensile value.  The Ricker article also references the Blodgett book.   

RE: Design of lifting lugs

(OP)
The major difference between AISC and ASME is bearing strength.

AISC  1.8FyApb/omega = 32.4k
ASME  0.63Fy(Dpin)t/SF = 7.56k  w/SF=3.0

I have the Ricker article.

Thanks to all

RE: Design of lifting lugs

FYI, I believe OSHA requires lifting devices to resist at least 5* their rated load.

RE: Design of lifting lugs

You are looking at several different cookbooks and debating whether we should use a pinch of salt or a pinch and a half, however much that is and whatever kind of salt you are using.  And, we don't have the vagues idea what the hell we are cookin or how it works.  I don't have your edition of the AISC cookbook so I can't comment on your exact recipe (formulas).  I've worked around some of the ASME codes too, but am not absolutely sure what the one you mentioned says about the subject since I don't have a copy of it either.  I can see generally what the formulas represent, but can't comment explicitly.  And, I can't see your attachment either.  I'm not even sure what omega = 2 is in nutte's world, or if he really didn't mean phi; is omega something akin to the inverse of phi?  He was heading in the right direction with his conclusion though, in saying maybe the allowable design stresses aren't that different.  An acceptable bearing stress, when looked at from the Theory of Elasticity standpoint, is considerably higher than the bending or tension stress that we would allow, it just works differently.

It's not hard to imagine that AISC would say that their code doesn't really address lifting lugs, although they do function pretty much like eyebars if you understand the real intention of the design criteria for eyebars.  They certainly don't function like we assume a bolt bears in its hole.  They don't function much like our normal tension members in the areas critical to their design.  The AISC code is a fine guide to understanding steel design in general, and in most any field, if we really understand the basis of what they are doing, and where their equations came from.  But, more and more that basic understanding is lost in the manipulation of the many factors included in the recipe.  No need to understand, just follow the recipe.

The design of lifting lugs involves a Hertz stress problem as relates to the bearing btwn. the pin and the lifting lug pl.  Let's say the lift is vert., then the pin is in bearing at 12:00 noon.  And, they involve a combined stress problem with the max. stress being tension in the regions btwn. 2&4:00 and 8&10:00.  They involve transmitting the loads properly to object being lifted, and this doesn't have to mean a full pen. weld, but does req'r. some special attention to quality weld details.  They req'r. some special attention as to shape, pl. grain orientation, manufr'g. methods, etc.  You should also have an understanding of the mating lifting equip. to be used.  Those are some of the design considerations.  That leads to a vague understanding of how the lifting lug works and is loaded.  Then we can argue about load factors and resistance factors, etc.; and for the most part that will depend on the design code that your product must meet.  Oil fields and rigs, pressure vessels, OEM equip., etc. they all have their own cookbook, and measure a pinch of salt slightly differently, and all the while the lifting lug doesn't know the difference.  It just keeps acting the same old way.
 

RE: Design of lifting lugs

The design of below-the-hook lifting devices are standardized in the United States by ASME B30.20 and further detailed in Below-the-Hook Lifting BTH-1-2008 Design of Below-the-Hook Lifting Devices.  I found the BTH-1-2008 publication, commentary sections very usefull.  I did not find the David T. Ricker article usefull.

I think you are only taking one part of the ASME code section.  BTH-1-2008, 3-3.3.4 Bearing Stress has two equations for bearing stress, they are:

Bearing stress limitation to control deformation and wear, not a strength limit was give by:
Fp= 1.25*Fy/Nd (3-53)

The allowable bearing stress for connections that will
rotate under load for a large number of cycles was give by:
Fp= 0.63*Fy/Nd (3-54)

Eq. (3-54)] is 50% of the eq. (3-53) allowable bearing
stress for large number of cycles.

RE: Design of lifting lugs

Below-the-Hook is the way to go. The commentary is a great document.

dhengr, I agree on the cookbook discussion, however, I will vouch for this particular ASME Code. It is well written and taken in context with 1989 ASD makes good sense. It takes care of the impact factors by assigning design categories. The logic is straightforward and well defined. The only additional check I sometimes perform is to make sure lug does not yield if pulled in the perpendicular to the face of the lug.

RE: Design of lifting lugs

It depends on the application.  But note that the ASME BTH assumes you're going to use the item over and over, perhaps for thousands of cycles.  Whereas, a lot of lifting lugs are used to initially set the item, and then seldom if ever again.  I think that's one reason why BTH is so conservative.

Note that the equations for bearing strength generally assume yielding at the pin hole.

RE: Design of lifting lugs

I have designed one time use fixtures, that are now 20 years old that our riggers still use.

RE: Design of lifting lugs

ANSI C57-12 also has criteria for lifting fixtures - i.e. a FS of 5 of the ultimate strength of the steel material.

RE: Design of lifting lugs

I saw my name mentioned so I thought I would chime in.

Though my current AISC seminar does not directly address lifting lugs, it has come up, and I am sure I have cautioned against using Chapter D "as is". The pin-connected members discussion in Chapter D of the AISC Specification is not intended to address lifting lugs. One reason for this is that the pin connections in Chapter D assume a tight fitting pin with less than 1/32" clearance. It would be unusual for a lifting lug to have 1/32" clearance.

This is not to say that Chapter D cannot be modified for use with lifting lugs. The first thing that would have to change is the factor of safety (omega in the 13th Edition Manual). There can be a lot of uncertainty surrounding the loads and impact factors for lifting lugs. The factors of safety in the AISC Specification are not set to account for this level of uncertainty. Ricker in his paper suggests a factor of safety of 5. This corresponds to the OSHA requirements as I read them. ASME BTH-1 seems to use a lower factor of safety.

As stated, it would be unusual for a lifting lug to have 1/32" clearance, so adjustments must be made. Typically I have made these adjustments based on the paper, "Pin Clearance Effect on Pinned Connection Strength" by Duerr and Pincus. Duerr is a member of the committee that writes ASME BTH-1.

Concerning the bearing check: The ASME BTH-1 bearing check is not a strength limit state but rather is used to limit deformation. The AISC bearing checks serve the same purpose. Again, the checks in J7 assume a close fit, so this is not an appropriate check. A better option might be the checks in J3.10. As was stated by some of the other contributors, the ASME checks depend on the number of cycles. Often lifting lugs are subject to few, or even one, cycle so the comparison between AISC and ASME requirements should be based on the 1.25 coefficient in ASME not the 0.63. Another difference that must be accounted for is that ASME uses Fy and AISC uses Fu. Since this check is based on empirical data, it is not surprising these kinds of differences exist. If we adjust the 1.25 in ASME BTH-1 from Fy to Fu assuming A36 steel and assume a factor of safety of 3, the more conservative value used, we get (1.25/3)(36/58)= 0.295. For the AISC requirement, assuming a bearing check where significant deformation is tolerable, we get 1.5/5=0.3. If significant deformation at the hole cannot be allowed, we get 1.2/5=0.24. From this we can see that the two requirements are not all that different. Ricker recommends 0.9Fy, which when used with the suggested safety factor of 5 would be much more conservative than either ASME or AISC.

The Commentary to ASME BTH-1 states that there all 4 strength limit states that must be considered: net tension perpendicular to the load, single plane shear rupture parallel to the load, double plane shear rupture parallel to the load, and out-of-plane buckling (dishing) of the plate.

Both the AISC and ASME requirements address all of these limit states, though at times in different ways.

The net tension check is essentially the same. Using the AISC D5 check with a factor of safety (omega) of 5, as suggested by Ricker and OSHA, the AISC check will always be more conservative than the ASME check. It should be noted that Duerr and Pincus sound that the clearance effect was less pronounced when the strength was governed by net tension, so it makes sense that the two standards take essentially the same approach.

Buckling in both standards is prevented by limiting the effective width of the plate. This is done somewhat differently, but the intent is the same. Since the approaches involve different variables, it cannot be determined whether one approach is always more conservative.

The most significant difference is in the checking of shear. The AISC equation assumes a tight fitting pin, and since Duerr and Pincus found a significant clearance effect on this limit state, the AISC values must be adjusted to account for this effect. The two approaches also differ in that AISC limits the dimension "a" relative to "b". I believe the intent is to prevent the single plane shear failure. Again since the two approaches use different variables, it is not easy to compare the two. However, the intent of each is the same.

Some comments should also be made relative to the Ricker paper, which is often used as a reference by the structural steel community. The Ricker paper does contain the same "a" to "b" relationship that is noted in AISC Chapter D. The Ricker paper does limit the "b" dimension (he calls this "a"), so does address the dishing problem. His limit is not as severe as that in AISC D5, it is the same as one of the ASME limits, but does not seem to correspond to the other ASME limit, so it might deserve a closer look. He also makes some recommendations relative to a lower bound thickness relative to the hole size, which he states are intended to prevent dishing. He also explicitly addresses the shear limits states, though again differently than either AISC or ASME. He does not seem to address the net tension limit state.

I agree that the basics have largely been overlooked in this thread, but the "cookbooks", though they take somewhat different approaches, are all trying to address the same problem and all are based on the same sources. Johnston's work conducted back in 1939 is common to AISC, ASME, and Ricker. Tolbert is cited by both ASME and Ricker. I have suggested modifying AISC D5 using Duerr, who is on the committee that writes ASME BTH-1 and whose work they cite.

Finally, I do not believe OSHA cites a standard in their regulations (though I may have missed something), so the design decisions seem to be left to the discretion of the engineer, which is probably a good thing. I have however seen OSHA reference the ASME standards in some answers to questions they have received.

Larry Muir
www.LarryMuir.com
http://larrymuir.building.officelive.com
 

RE: Design of lifting lugs

Lift rigging assembly and rigging is covered under: Occupational Safety and Health Administration (OSHA) 29 CFR standard 1926.753(e)(2) and 1926.251
 

RE: Design of lifting lugs

As far as I know, you need to satisfy the 0.63Fy for the code bearing calculation. 1.8Fy (I use 1.6Fy) is the shakedown stress and is used to limit Herstzian contact stresses in the pin connection. If you need to satisfy both of them, the pin design and the gap between pin and hole diameters governed by the Hertzian Stresses.

In the pin calculations, you had better satisfy both to make sure the pin connection will be functional/not under plastic deformation under the design load.

For bolting or pin application, in case the plastic distortions at the connected members are acceptable, you can only use 0.63Fy bearing stresses.

Hope it helps.

Ibrahim Demir

 

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