Lifting Lugs with ASME BTH-1-2008 2

[MachinaMan]

Hi Everyone,

Baseed on thread507-273125, I am working on designing a standardized set of lifting lugs using ASME BTH-1-2008.
For now, I am working on a single case and detailing the calculations.

The lug is as per figure C3-3. I have also attached a sketch.

I need to vary the angle of the applied load in the plane of the lug so it will be usable from 0º to 90º from the position shown in figure C3-3.

When analysing the welds, I can determine the tensile and shear limits of the weld (0º and 90º) however, when the angle is anything but 0º, a moment is created at the interface between the lug and existing structure requiring a flexion analysis in the strong axis as per 3-2.3.

The problem I have is that it's not clear to me if I am dealing with a compact or non-compact section as per table 3-1.
I also need to determine this for a flexion analysis in the weak axis. Any help would be appreciated.

 

[JAE]

Would you use AISC's Table B4.1 for the plate (Case 6 is for flexure in legs of single angles - which gives the compact limit for b/t as 0.54 x sqrt(E/Fy).

b could be the distance from the root of the lug out to the center of the hole. t is the thickness of the lug.
Back-calculate a t required to ensure a compact behavior and you go from there.

 

[MachinaMan]

Hi JAE,

Thanks for the reply.
I don't have AISC's table B4.1 however it seems like ASME BTH-1 table 3-1 has a similar entry.

"Legs of single angle struts; legs of double angle struts with sparators; unstiffened elements, i.e., supported along one edge"

Which has the following limits for width/thickness ratio (b/t):
- Compact = 0.38*sqrt(E/F_yf)
- Noncompact = 0.45*sqrt(E/F_yf)

Would you agree that b would be the total width minus the hole diameter, i.e., the weakest section?
b is defined as: "width of a compression element"

 

[JAE]

Usually the "b" is the length of the free, unstiffened edge which undergoes compression.

In your case, I would see the b distance as the straight vertical edge to the base (center of hole to base weld line).

 

[WillisV]

I have never seen a lifting lug that would not be classified as compact by a long shot. Their thickness is generally driven by bearing checks on the pin hole that make this plate much much stockier than typical structural elements.

 

[oldrunner]

We just recently designed some lifting lugs where the dimensions were pretty much set by the client. In the end, the bearing on the lifting lug did control and we ended up using A514 (Fy = 100 ksi) steel. Also because of the configuration of the total fixture and what it was connected to (a transformer), we provided a NASTRAN analysis of the metal plates and a separate NASTRAN analysis of the welds. Most of the supporting pieces were A572 (Fy = 50 ksi) and all of the transformer tank was A36. The fixture was designed to IEEE C57.12.10 where the FS is 5 based upon the ultimate strength. (Fu)

 

[NS4U]

Compact/non-compact criteria is used to determine if local buckling will occure prior to the complete cross-section of the plate yielding.

For a square plate, like your lug, local buckling is not possible. Therfore it is inherently compact. I think JAE's recommendation to check AISC B4.1 is conservative and good guidance (I would probably do somethign similar) but I don't think it is 100% necessary.

 

[NS4U]

You teally should design the plate according to AISC Section F-11. (flexural capacity of square plates and rounds). That is the governing provision for square plates. You'll see that there is no criteria for checking whether the section is compact or non-compact. Instead, you're checking for a glocal buckling (Lateral torsion buckling) mode of the plate.

I'm not sure LTB of a lug could ever happen (unless you had a really funky looking lug), I would probably just use the full plastic capacity of the pad eye. But F-11 is the section you want to check to be thorough.

 

[dhengr]

MachinaMan:
Could you show your sketch as a pdf file, I can’t see it in the format to showed it. Alternatively, I will survive without see it. Almost invariably, pin bearing, shear and yielding in the area of the pin, and good quality welding detailing controls the design of lifting lugs.

 

[JAE]

I think some of you above need a thanks from me - getting my head out of the sand.

I think what I posted above is correct but WillisV and others posts are right on - buckling isn't really an issue with lugs like this unless the plate is thin and the lug is really long and extended. Even then, I wonder about how much compression would even get into the lug with an angled hanging "tensile" load.

 

[MachinaMan]

Hi All,

Thanks for your help.

WillisV, Agreed, this makes sense to me despite being a mechie.

NS4U, The 2008 version of ASME BTH-1 I am using is commented and most calculations refer to AISC. I beleive that the ASME code mostly describes certain cases covered by AISC.

dhengr, I will post the .pdf version. The base of my lugs will be wider, however, to be conservative I am designing with a constant width plate for now.

JAE, Still good advice, since I am doing the calcs in Mathcad to semi-automate the design, I am still going to check the section for it's compactness, however there's also a simplified clause for rectangular sections I will check out.

Thanks again, of course further comment is always welcome.

 

[dik]

Your force diagram assumes that the pin dia and the pin hole dia are the same... often the pin is 1/4" or so smaller.

Dik

 

[MachinaMan]

Hi Dik,

That's correct.
I select the closest available Crosby G-209 shackle for the load.
The next available drill size will be used to make the hole slightly larger than the pin.
AMSE BTH-1 accounts for the difference in diameter between the pin and the hole.

Regards,