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Abaqus Standard: Overconstraints & Negative Eigenvalues

hi, i am using Abaqus standard to model a reinforced beam-column connection subjected to a tip load at the end of the beam. 

i first applied a gravity load followed by a tip load at the end of the beam. halfway through the application of the tip load, abaqus standard
stop and give 2 errors.

1) overstraint at certain nodes of my longitudinal reinforcement

2) negative eigen values

i did research on this and some people said it might be due to insufficient number of boundary conditions (in the right places) to prevent rigid body motion under the action of the applied force. i rechecked and re-assign my boundary conditions but the same error still occurs.

i hope someone can offer my advice on this. appreciate and thank you.

rgds,
joseph

Comments

Hi Joseph,

I suppose you are receiving these warnings through your ABAQUS FE Analysis. About the overconstraint warning, you've probably constrained some unnecessary degrees of freedom of certain nodes (these node numbers are printed in the message part of analysis). For instance, in a 2D analysis, you don't have to constrain rotations about Z axis.

 

Regarding the negative eigenvalues, it might have caused by local instability during analysis which would be recovered if an Implicit analysis is conducted or it could have been caused due to sever instability conditions which does not allow the model to reach equilibiruim. For the latter case, it's better to check the deformed shape of the model seeking any ill-conditions within your model. otherwise, should your model converge without any unexpected deformation, you don't need to be concerned abou this warning as it is due to local instabilities. 

others might have better experiences with these type of warnings, I would be so grateful if they share that with us.

Cheers~Yousef

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Hi Yousef,

Thank you for sharing with
me. I managed to resolve the overconstraints issue. My model is made up
of different parts of concrete surface-tied together. For the reinforcement, I
did it as an embedded element within the concrete. Abaqus Standard does
not allow

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embedded nodes to be tied by surface-based tie. So I redid my model and
resolve this issue.

I am still stuck with
negative eigenvalues. for the sever instability conditions that you have
suggested; I have checked the deformed shape and it looks fine. The tip displacement
of the end of the connecting beam is only very small, less than 1mm. As for the
other reason that you have mentioned (local instablity), i am not too sure how to
resolve that.

i did more runs yesterday
to try to resolve this issue. There are a couple of interesting stuff that I
have found. I did 3 models with the following conditions.

1) Coarse mesh - 0.1m
element: NLgeom OFF; 10 increment of 0.1 sec each; Abaqus Standard completes
the analysis.
2) Fine mesh - 0.025m element: NLgeom OFF; 10 increment of 0.1 sec each; Abaqus
Standard stops running after awhile with NEgative Eigenvalues and Numerical
Singularity warnings.
3) Coarse mesh - 0.1m element: NLgeom ON; 10 increment of 0.1sec each; Abaqus
Standard stops running after awhile with NEgative Eigenvalues and Numerical
Singularity warnings.

Discussion:
i) Comparing model 1 &
2, I am not sure why Abaqus Standard stops running when I use a finer mesh.
ii) My model have material non-linearity and so I should use NLgeom ON swtich
on non-linearity). But the error starts once I switch this option on.

I really appreciate you
writing in. Abaqus Standard is really proving to be a challenge. I will
continue to try to resolve the above issues. Hopefully, someone can share their
experience with us.

Thank you and cheers!
joseph
 

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