Out of plane load on flat plate lifting lug

[DerChad]

I have four lifting lugs on a skid. The lugs are unstiffened vertical flat plates (with cheek plates) that go through the flange of the skid member and weld to the top flange, bottom flange, and web of the skid member (lug is aligned with the web of the skid member). The skid is being lifted with a spreader bar. In order to avoid a conflict with steel above, I have to have an outboard angle of about 8 degrees but I've not planned to pre-bend the lugs to match this angle.

In all the literature, there are guidelines for accounting for the out of plane load on the lugs. I understand this requirement if the lug is very stiff and might cause excessive torsion to the member to which it is attached, particularly if the lug is attached at a point where there is little resistance to torsion. I have addressed the torsion issue by locating my lugs where there are fully welded lateral members in the skid. It is a very stiff point - much stiffer than the weak axis capacity of my lifting lugs (strong axis bending in a heavy wide flange beats weak axis in a 1" flat plate). Can I not allow the flat plate to deflect in the direction of the tension load, much as you would consider a fan plate rod?

 

[thaidavid40]

@DerChad,
If your plate lugs are aligned with cross-bracing steel members, then I don't think that torsion of the primary skid member is a concern - assuming your connections are stiff enough to take that load. That means the connections will need a substantial amount of cross-member strong-axis capacity. Are the cross pieces the same depth as the skid beams, and can you weld them rigidly to the skid beams? If so, then I think you have the torsion component covered.
Dave

Thaidavid

 

[dhengr]

DerChad:
We can’t see it from here, and without being able to see the actual details, dimensions, proportions, weld arrangement and loads, it is pretty tough to make a meaningful engineering judgement about the pro’s and con’s of what you want to do. Is this skid already built, so that you would have to bend the plates, on the skid? If not, I would just bend the lug plates, or lean them in 8̊ to conform with the sling angle. If the lug is straight (vert.) and as long as you have a fairly tight shackle pin fit in the lug hole, I would think that you primarily have a two component load on the lug plate. One is the vert. load and the other is a lateral load at the shackle pin, and they relate to the sling loading as a function of the 8̊ sling angle. What are the combined stresses in the plate due to these loads, and are the weld details and quality able to accommodate them? The shackle pin will not exactly be in uniform double shear under these conditions. It will have a higher shearing load at one of the faying surfaces than at the other and will cause some nonuniform bearing yielding in the pin holes.

 

[DerChad]

ThaiDavid: Thanks for the response. The lateral member is full depth and fully welded. The torsion is definitely covered.

dhengr: I appreciate that it's tough to develop a mental picture without actually having a sketch. I appreciate you taking the time to address my question. Based upon your comments, I think you're visualizing the situation correctly. The pin is 6% smaller than the hole (DNV requirement) and the plate thickness + cheek plates is greater than 80% of the jaw width of the connection hardware (Crosby advice). The pin is pretty snug so we should be ok for bearing.

I considered bending the plate to match the design angle. Internal discussions at the company and my own thoughts kept me away from that solution because the actual angle will not match the expected angle. Even if I bend it, I'm still going to have out of plane load. See below.

Since I posted this, I gave in and applied the lateral load to the plate. The design configuration works for the combined stress of the strong axis, weak axis and tension loading. But my question still stands. Why are we worried about an unstiffened weak axis plate bending toward the direction of the pull? It's like an opposite p-delta effect, reducing the moment as the plate deflects. Yet codes are pretty specific about accounting for it.

 

[NS4U]

I can't picture your rigging arrangement and lug position based on your description, but here are some general thoughts.

Do the welds work? How much does the plate deflect? If the plate deflects do the rigging angles change and does the load distribution change? What level of impact are you considering? Is it realistic or could it be higher?? If it could be higher then your out of plane load will be higher. In plane loading is more ductile and stiffer so I'm less worried about that.

I always avoid out of plane loading on lugs because it is an ugly load path. In cases I have come across the out of plane moment is usually produced from a small moment arm, therefore if you have small differences in the field your actual moment could double or triple.

It's too easy for bad stuff to happen during lifts. I always keep the load path simple and in plane. I save the cute designs for architects, not for lifts that could kill someone or destroy the lifted objected.