Jul 06
Nastran Beam FEM from Beam Model
Alexander Boucke provided the beam data information and I have been trying to get a Nastran model working - obtaining the same modal frequencies. I'm unsuccessful as yet.
Please add your comments here as to any progress you are making?
The resource to which this is referring is https://c3.nasa.gov/dashlink/resources/425/
The 2 frequencies which I provided were the 1st and 2nd bending mode
I had forgotten to include the spcfile that I have included in the FEM. I hesitate to post my matlab code which I use to generate the bdf files from the file provided by Alexander Boucke.
Email correspondence between Alexander and Carol has been uploaded (PDF,HTML). I have also uploaded more current email that includes correspondence with Jack Castro to understand beam implementation in Nastran vs Boucke
Alexander provided all the node locations in global coordinates - B (neutral axis), C (mass center), D (shear center) in global coordinates. Nastran requires distances N and M on the pbeam cards that are in the local element coordinate system for each beam. THe N distances would correspond to the distances between the neutral axis and shear center and the M distances between the mass center and the neutral axis.
7/14/2011 - my latest attempt with Ns and Ms gives HUGE differences in the modal frequencies and mode shapes.
We have identified an issue with the mass matrix. As soon as I added in any M terms to the PBEAM card the frequencies go way down. Having the N terms did change the frequencies but not a lot but the M terms made a big difference so the NSM is not being handled properly.
Cards were modified to be long format. The impact is minimal and doesn't get me any closer. I don't seem to have the problem with the M values having huge impact on frequencies anymore. I haven't identified why yet.
The latest FEM's have been uploaded. The model was converted to double precision (Nastran long format) to eliminate that as a source of error.
I also have uploaded spreadsheets with comparisons of mass for debugging.
I have uploaded a lot of other files as well that I have created that contain Beam properties, etc.