Welded Circular Lug Subjected to Moment Load

[MikeC88]

Hello All,

I am looking into a welded lug for an application that will be subjected to an end load. The lug itself is circular in cross section and welded to a flat plate.

After reviewing my copy of Shigley, and looking through the forum, and coming across a number of interesting threads, in particular

https://www.eng-tips.com/viewthread.cfm?qid=377125,

I am confident that I have assessed the shear in the weld.

I am now onto assessing the attachment stress, both to the flat plate and lug itself, and would like to confirm that my understanding is correct:

For the base plate, I believe I need to utilise [Normal Stress] = [M]x[r]/

Where
M = Moment
r = radius of circular section (25mm in this case)#
I = Second moment of area of a hollow circle (Weld cross section) = 0.25 x Pi x (Ro^4 - Ri^4)

For the weld attachment to the lifting lug I have to admit I am unsure how best to assess, my initial thought is to simplify this to a weld under shear based on half the circumference - So any guidance for this would be appreciated.

I have uploaded my scribbles to date to aid with understanding the problem, any feedback would be appreciated.

Thank you in advance

 

[MikeC88]

Hi All,

So some positive progress my end, I have now worked on a bit further and calculated out the attachment strength in the plate that the lug is welded to, and m satisfied with this part.

The point I am stuck on now is how to calculate the attachment strength of the weld to the circular lug.

My assumption initial thoughts are that I should consider primary shear across half of the weld length, however I believe there should be another element to account for the bending stresses, is anyone able to advise on this point?

 

[12345abc6ttyui67]

I'd take shear over half the weld.

For the moment, I'd treat the weld as a line per the link below and resolve bending moment into the weld.

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html

 

[MikeC88]

Hi Random,

Thanks for the response, I've now completed my calcs and come out with an answer I believe to be on the side of conservative. The approach I have taken has been to:

Use the approach outlined by the roymech, blodgetts, and shigley site for assessing shear in the weld.

Assess the attachment strength of the weld from the rear base plate based on the second moment of area of a hollow circle, and shear based on the total area underneath the weld (again hollow circle). This is similar to a method outlined in Shigley, but modified for a circular weld rather than two vertical rectangular welds.

For the Stress at the weld attachment to the circular lug I have based shear on half the area under the weld, for the stress due to bending I have considered the stress at the OD of the Lug using a second moment of area based on a circular profile. I believe this to be conservative. But have to admit it is the area I am a bit dubious on.

 

[desertfox]

This site might be of use.

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html#Table_Weld

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MJCronin]

Have you checked Roark ? I believe that I recall a case like this ....

For piping and pressure vessel nozzles, a complex methodology exists that can handle complex nozzles and curved surfaces. It sounds like you do not quite have a nozzle situation (no hole in the plate and no pressure loading). A hole in the plate typically requires a reinforcement plate

Additionally, you may not know that stress analysis of your circular lug has been a staple of piping/pipe support and pressure vessel engineering for decades.

You should expand your GOOGLE search of analysis methods to include the term "trunnion" (derived from the old french term of cylindrical support for cannon)

Also search this forum with the terms "trunnion" and " lug stress analysis" ..... also try "lifting lug"

https://www.eng-tips.com/viewthread.cfm?qid=152699

MJCronin
Sr. Process Engineer

 

[MikeC88]

Desertfox, thank you for your response,

MCronin:

Thank you for your comments, I will have a look into trunnions and see where this takes me.

 

[Ron]

I would approach this much simpler than you have.

The moment is in the lug, not the weld. The forces in the weld are a resolved force couple from the lug moment. This results in shear to both halves (upper and lower) of the weld holding the lug. The bottom shear will come from compression and the top shear will come from tension. The compression shear can be neglected.