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Shell Bending Behavior

Shell Bending Behavior

Shell Bending Behavior

(OP)
I'm trying to do a modal analysis of a monocoque cylindrical structure using shell elements. However, I'm getting a ton of what appear to be low frequency panel modes and the bending modes do not match the frequencies calculated through another means. In order to try and figure out what was going on, I created two simplified models of a cylinder with constant thickness, one modeled with beam elements, the other with shell, and calculated by hand the free-free modes. Initially, neither Abaqus model matched the hand calculations, but they matched each other (about 20% below the hand calculated frequency). Changing the beam elements from (I believe) B31 to B33 resulted in the beam model's frequencies matching the hand calcs, but no matter what elements I use for the shell, I still get lots of panel modes and an incorrect bending mode frequency (tried S4R, S4, S8, S8R, S3, S3R, increased mesh density, etc). Has anyone encountered this before or has any idea what I might be doing wrong?

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