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- Make a copy of the model, and add unit mass to ALL 6 DOF at every joint, Rotational inertial is necessary to pick up torsional modes for frame elements.
- Make sure the mass source is defined as from self-weigh and additional masses and not lumped at stories
- Run an Eigen analysis. Any solver will do for this, so use the fastest. A dozen modes is enough to detect problems
- Loss of accuracy more than 11 digits usually represents a rigid-body mode or mechanism, and the eigen modes usually pick these up as having large periods.
- It is also good to look for the report on the number of eigen-values below the shift, as this may pick up problems due to P-delta that may not be near zero-frequency (large period)
- The standard-solver is a good way to pick up problems like 6 to 10 digits lost due to large stiffness differences that cause inaccuracy but as indicated above this are not necessarily instabilities.
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