Hello everyone. I am modelling a 2D frame in Abaqus software and finding the structure's mode shape and natural frequency. I attached a photo of my structure. I think the first-mode shape of the structure is incorrect because the whole structure should move in one direction and vibrate in the first mode. I highlighted two members(1 and 2). These members looked like they were vibrating in the second mode even though the mode shape was 1st. I think this analysis is not correct. Can u please guide me on what I should do to get the correct answer...The bottom BC is fixed and doing 2D analysis.
Ram Kumar, the joints in the truss can have a significant effect on the mode shapes and natural frequencies of the structure. Joints can introduce flexibility and nonlinearity into the structure, which can alter the mode shapes and frequencies. Make sure the joints are accurately modeled with meaningful connectors.
Ram Kumar
In the output data of Abaqus modal analysis, the first few modes should not be considered. These mods have a small amplitude to be ignored. When choosing the first mode, pay attention to its amplitude. Check out the next modes.
"In the output data of Abaqus modal analysis, the first few modes should not be considered."
This statement not correct.
A modal analysis won't give you any information about the absolute amplitude. Its a relative measure between nodes in the structure. The first Eigenfrequencies are the dominant ones. If you have no boundary conditions or to less to avoid rigid body motion, you will get zero eigenfrequencies. They are related to the six rigid body modes.
To your question. It can be that the bending eigenfrequencies of your beams are smaller than the frequency of the whole structure. You have to compare your modes (MAC criterion) with experiments and frequencies.
If your experiment does not measure these effects, check the stiffnesses of the beams and the connection between them. If the rotational DOFs are not correctly coupled you will reduce massively your stiffness.
Alternatively you can model it with truss elements. In that case you won't get any bending frequencies of the beams.
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