BCs for flexbodies

Hello,

Is there a way to include SPCs in your model when creating and MNF file for a flexbody? I am asking because when I tried creating an mnf file for a table-like structure, the 1st mode was completely off than when running a simple sol103. The points which were supposed to be constrained were moving freely in the modes (even though I had also included them in ASETS).

Is there another way that I can ensure the validity of the constraints when creating an mnf file?

Thanks,

Josef

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0 Josef K. over 2 years ago

Hello Bruno,

Reading my own post now I see that I did not state my question properly. Let me clarify.

My model is a table-like structure, with its "feet" bolted on the ground. This should mean that the nodes at the bottom of the feet should be completely still at all times.

When buidling a simple FEA model (with SPCs on the feet) and running a SOL103 the first mode is present at 1.5 Hz. It is a bending mode with the top wiggling about and the feet fixed to the ground.

When solving a SOL103 to generate the MNF file, instead of adding the SPCs (since they can't be used in the MNF creation process) I used RBE2 elements for the nodes which were previously used in the SPCs and added the master nodes to an ASET in the model. To the best of my knowledge, this is how you are supposed to build the MNF files.

When viewing the modes of the MNF file, the first one appeared at about 2.9 Hz. What seemed completely off to me though is the fact that in this mode the top part of the table was still whie the feet were moving freely (still a bending mode). I also made sure I included the correct nodes in the ASET.

What I am asking is wether I have made a mistake in the way I tried to constrain the flexible body. Is there a different way to make sure that the feet are constrained when building the MNF file, so that its modal properties are correct?

As far as the Adams model is concerend, I did constrain them there (fixed joints to the ground) and the model is working properly. But the first mode of the MNF files should be at 1.5 Hz and not at 2.9 Hz.

Thanks,

Josef
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0 Josef K. over 2 years ago

Hello Bruno,

Reading my own post now I see that I did not state my question properly. Let me clarify.

My model is a table-like structure, with its "feet" bolted on the ground. This should mean that the nodes at the bottom of the feet should be completely still at all times.

When buidling a simple FEA model (with SPCs on the feet) and running a SOL103 the first mode is present at 1.5 Hz. It is a bending mode with the top wiggling about and the feet fixed to the ground.

When solving a SOL103 to generate the MNF file, instead of adding the SPCs (since they can't be used in the MNF creation process) I used RBE2 elements for the nodes which were previously used in the SPCs and added the master nodes to an ASET in the model. To the best of my knowledge, this is how you are supposed to build the MNF files.

When viewing the modes of the MNF file, the first one appeared at about 2.9 Hz. What seemed completely off to me though is the fact that in this mode the top part of the table was still whie the feet were moving freely (still a bending mode). I also made sure I included the correct nodes in the ASET.

What I am asking is wether I have made a mistake in the way I tried to constrain the flexible body. Is there a different way to make sure that the feet are constrained when building the MNF file, so that its modal properties are correct?

As far as the Adams model is concerend, I did constrain them there (fixed joints to the ground) and the model is working properly. But the first mode of the MNF files should be at 1.5 Hz and not at 2.9 Hz.

Thanks,

Josef
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