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Frame for lifting

Frame for lifting

Frame for lifting

(OP)
I am trying to design a frame for lifting a boat out of the water in Risa 3d. The frame is in the horizontal and is simply a rectangle frame at each corner is a plate which the lifting chains attach to (i.e a single lift point ) and straps go below the boat's hull. How would I put boundary conditions in Risa for this?

RE: Frame for lifting

You could use a single point of fixity at the top (where the four chains come together).
Then model the four chains, the rectangular frame, and perhaps the straps. The straps under the boat might just be point loads on your frame.

Then put in small spring restraints in the two horizontal directions at a pair of diagonally opposite corners of your rectangular frame.
The springs will "satisfy" RISA's matrix solution and avoid an instability error.

Check the springs after the analysis to make sure there is little or no reactions there..should be very small numbers.

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RE: Frame for lifting

(OP)
Thanks Jae
that is what was coming up - an error about the model boundary conditions not being adequate.
I will try your suggestion of putting a low spring in the diagonal corners and I will revert if I run into difficulties. Thanks again

RE: Frame for lifting

Mike,

The way I handle these cases depends on if I know the loads in my rigging.

If you do not know the rigging loads, such as assuming a CoG perfectly in the center of your frame, then you would have to do something along the lines JAE has suggested. You will need to put those springs in (usually very light springs) in order to keep the model from spinning and thus giving you errors. You will want to check your spring reactions and deflection diagram to make sure nothing wacky is going on.

If you know your loads to each of your padeyes, I would just model the frame, and put "dummy members" that mimic your padeyes (lifting lug or whatever you want to call them). The dummy members are rigid members, with height equal to the distance from center of framing-member to center of padeye hole. Then apply your horizontal and vertical components of your rigging loads to these dummy members (to mimic the moment induced from the out of plane force on your padeye). I hope this description makes sense, as I am currently too lazy to screen shot an example.

RE: Frame for lifting

(OP)
Thank you andriver.
I will look up 'rigid members' which I never used before and approach it from this method also to compare the results.

You mention 'out of plane' loads of the pad-eyes : in my model, with the COG in the exact(unrealistic) centre, I have got no out of plane loads. I would probably expect these come in to play when the COG is not as such?

RE: Frame for lifting

Mike,

Apologies for the late response, I have been on a nice long vacation.

In your example, you will have out of plane forces from the rigging, because you have four points going to one hook. You will have a vertical force, and horizontal component forces in two directions.

In my attached figure as an example,the y component will create an out-of-plane force on your lifting eye which you need to be accounted for. The x component in my figure will create an in plane force with the lifting eye, this in plane force will also create a moment on your frame which I account for by creating a rigid member, the length of my rigid member is the distance from center of frame to lifting eye hole. Technically the out of plane force also creates a moment on your frame, but this is usually small in comparison to your in plane force.

Now, this may not be applicable if your lifting eyes are orientated so that they are in line with your rigging. However, you will still create a moment on your frame from the rigging force acting at the lifting eye hole, again where your moment arm is from the lifting eye center hole and the center of your lifting frame member.

hope this helps

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