Stable Beam Model

[hungrydinosaur]

Hi Friends,

I am designing a offshore container for my company. The beam and column model is modeled with lifting slings as "tension only" members. This makes the model unstable. So as per DNV 2.7-3 recommendation I have added horizontal springs at the bottom to stabilize the model. But as the model was still unstable, I added vertical springs at the bottom with a very low stiffness of 1kgf/m, which helped in stabilizing the model. Can someone please advise what effects will the vertical springs have on the results (as the vertical springs will "take the load out" from the structure)? Please see attached model. Thanks in advance.

Cheers!

Hungry Dinosaur

 

[BigInch]

Why model such a simple system? A hand calculation is quite easy, as long as the CG of the load is at the center of the horizontal frame and the center of both frames are under the hook. The CG of the package will always remain somewhere directly under a single lifting point. If that is true, all slings will have the same tension load as well, as long as their angles to the hook are all equal.

Are the slings the only "tension only" members or have you modeled the columns and X bracing that way too?

It would appear that a pure lifting load would only create tension stresses in the slings and columns, so you could model them as tension-compression members, with the known result that the tension-compression members would always be in tension in a pure lifting condition. The upper horizontal frame members would always be in compression under a pure lifting condition. That load can be calculated by hand from the horizontal components of the sling loads. The X bracing has essentially no stress under a pure lifting load.

X bracing and columns would only have load if there were horizontal accelerations from lateral swing. These loads should be minimized during a lift, and if they were significant, some vertical X bracing between the horizontal frames would probably be necessary.

Independent events are seldomly independent.