How to model forces on padeyes during lifting operations

[JeffreyTeo]

Hi all, I tried searching, but either I'm hopeless at doing a meaningful search or no one has posted this question before.

Using a generic example:
I have a box-like structure, let's say a sea-going container. In each corner of the roof is a pad eye, or lifting eye. For lifting, wire-ropes are attached to each pad eye using shackles. The four ropes from the corners come together to a central hook a suitable distance above the top of the structure (forming a pyramid shape). The central hook is of course connected to the main rope of the crane.

I have a couple of objects carried inside the container. These can be easily represented as point loads on the floor of the container.

Because of the resultant forces caused by the wire rope, there should be a horizontal component and a vertical component to the forces acting on the pad eye. Thus the 4 corners of the container are simultaneously being lifted vertically and pulled together horizontally to a central point.

The floor of the container, being suspended from the 4 corners, should show some bending.

Realistically, the proper representation should have the pad eyes connected by the "Remote Force" tool, but with the force being a "Fixed Constraint". This does not seem to be possible to do.

So is there a suggested method to properly represent a container with objects in it, being lifted by a crane, using wire-rope shackled to the four corners?

Thanks in advance.

[swalton]

Using IV 2009 FEA:

1. make a free body diagram of the container and calculate the resultant forces in the rigging. Resolve them to x,y,z components using the same coordinate system as the container model.

2. make a surface(s) on the inside of the container to represent the location and size of the equipment in the container. use the split command.

3. make a surface on the pad eye to represent the area that the rigging hook touches the pad eye.

4. go into the fea tool and apply a fixed constraint to one of the equipment surfaces. apply a frictionless constraint to the other

5. apply the loads that you calculated in the free body diagram to each pad eye. Use the vector mode to enter the x,y,z components of the forces.



If your model is big, consider using symmetry to simplify it and cut it in half or quarters.

[JeffreyTeo]

Hi. Thanks. I'm using IV 2010 and doing the FEA in Assembly.

That was a good idea and I had done something similar. However, I don't think it is entirely accurate. The loads are not symmetric, neither in the longitudinal plane nor the transverse plane, nor even in the vertical position (some had a higher CG than the other). The container frame isn't symmetric either.

I did try to calculate the resultant loads and resolve the proportions of each load on each lifting point. It is complicated, long-winded and messy. Also, everytime I made a change to the structure, its mass and CG would change, plus there is the matter of environmental forces (wind & waves) acting on the container and acceleration of differing Gs in each of the 3 planes.. See what I mean by messy?

It is, of course, the best that can be done right now, but it certainly would be more convenient if there were some way to have a remote point as a fixed constraint.

The other problem comes with presenting all that information to the regulatory body or the approval-powers-that-be. It would be so much clearer to be able to just put down all the forces as though they were the real objects, and the "fixed" reference points would be the point from which the lift is made. Then it would be simple to point to the result and say " This is where the loads act, this is where we are lifting, these are the resultant forces and the resulting stress and displacement relative to the fixed points.

I tried a solution where I modeled some "cables", tried to give it freedom of movement in the requisite directions and have the fixed constraint at the apex of where the cables joined. At least the resultant forces were acting in the expected directions, though I don't know if the magnitude was accurate.

I'm thinking of trying to make one pad eye fixed, put a pin with horizontal faces in to the hole of each pad eye. Constrain the horizontal surface of the pin with a frictionless constraint. Run the simulation to get the vertical forces, then apply those forces horizontally (the angle of rise of the wire-rope is not expected to be any shallower than 45deg to the horizontal) to the padeyes and run the simulation again. I wonder if that would be accurate.

[swalton]

Ah, the non-trivial problem.

I see that you made some cables (steel rods) that went from the container to the hook point and then fixed the hook point, but you were not confident that the manginitudes of the resultant forces were correct. That's were your hand calc comes in. Use it to verify that the FEA forces are correct for one load case. As long as they agree, you method should be right.

The rods would not be quite right because they can carry both a tension and compression load, not tension only like cable, but I think you would need ANSYS or ALGOR to get tension only elements for the FEA study. Also, you may have meshing issues with IV FEA if your container has thin walls. IV FEA does not have any shell elements, so your model will have lots of small elements and take a long time to solve.

I think the rods would get you to the point that you could build a prototype and do real world testing to prove your FEA.