Errors in Modal analysis (load stiffened) with Rigid Elements and remote load

[punisher]

We've been fighting a recurring issue where the solver errors out.

simplified test

Modal analysis with load stiffening

Using previously successful modal analysis results

4 rigid elements (single part) sharing common node

Remote force applied to common node

The error reads 

error: rigid element number: 2 
             node already assigned:165 

We get a bunch of these errors.

When the rigid definition is replaced by beam elements, all is well and solves. (this is what we've done in the past)

I am trying to help another fellow who has to work with rigid elements.

Any ideas?

[AstroJohnPE · Accepted solution]

If you are able to run the analysis using linear static stress and modal (without load stiffening), then it sounds to me like a software bug in the modal with load stiffening. Here are two options:

1. Beam elements, assuming that the rigid elements are connecting all 6 degrees of freedom (or whichever degrees of freedom the elements connected to the rigid elements have). As far as the results are concerned, a "rigid" beam and a rigid element will give the same result. Be sure to make the beams massless! From the results, you can check the rotations about X, Y, and Z. If they are equal on each end of each beam element, then the beam was rigid enough.
2. You would need to research whether modal analysis with load stiffening has the capability to use Multi-Point Constraints (MPC). MPCs will connect nodes together using mathematical equations, which I believe is somewhat similar to how the rigid elements behave.

[punisher]

John,

Thanks for the reply.

The PA team did in fact confirm that it was a 2014 issue (2013 as well), and that they would pursue the fix when the development cycle came around to it.

Unfortunately the gentleman will have to go back to the client or go to another solution because rigid are part of the contract requirements.

We've used beams successfully, but I want to follow up more specifically with your notes on them. Thank you very much for all the help here.