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This page is devoted to frequently asked questions (FAQ) related to modal analysis. |
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Modal analysis
For a structure with cables, how should modal analysis proceed?
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Answer: To obtain meaningful results when performing modal analysis of a structure with cables, load cases in which dead and other loads are applied should use the option for Stiffness at the end of nonlinear load case. Modes will then be based on the stiffness of the cables under applied loading. To capture complete nonlinear dynamic behavior, nonlinear direct-integration time-history analysis is most effective.
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Analysis output
Why is my simply supported beam not returning a fundamental period?
Answer: To compute the modes of vibration, un-check the option to Lump Mass at Story Levels, available through Define > Mass Source.
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Error and warning messages
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Answer: This warning message indicates that numerical problems occurred during analysis due to the structure being unstable when solving for eigenvalues.
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What is the meaning of the warning message: Excessively large effective-damping ratios have been set to 0.999950?
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Answer: This warning message may occur when using proportional damping for which damping increases with large frequency. Ritz vectors would subsequently generate high-frequency modes with large damping. This round-off to 0.999950 is necessary for numerical formulation.
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What is the meaning of the warning message: Loads are applied to massless degrees of freedom?
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