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This page is devoted to *frequently asked questions* (FAQ) related to [modal analysis|kb:Modal analysis].
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h1. Modal analysis


h2. For a structure with cables, how should modal analysis proceed?

*Extended Question:* I would like to perform a modal analysis of a structure with cables. Because cables are tension-only, the model is nonlinear, though modal analysis is theoretically elastic. Are there any guidelines for this situation?

*Answer:* To obtain meaningful results when performing [modal|kb:Modal analysis] analysis of a structure with [cables|kb:Cable], 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|kb:Nonlinear] dynamic behavior, nonlinear direct-integration [time-history|kb:Time-history analysis] analysis is most effective.

{related-incident:no=24152|comment=Modal analysis for structure with cables.}


h1. Analysis output


h2. Why is my simply supported beam not returning a fundamental period?

*Answer:* To compute the [modes|kb:Modal analysis] of vibration, un-check the option to Lump Mass at Story Levels, available through Define > Mass Source.

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*Related Incident:*
* {incident:no=47624|comment=Fundamental Period of Glass Panel}
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h1. Error and warning messages


h2. What is the meaning of the warning message: Eigenvalue was found out of sequence?

*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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*Related Incident:*
* {incident:no=35539|comment=See 6/10/2011 event}
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h2. What is the meaning of the warning message: Excessively large effective-damping ratios have been set to 0.999950?

*Extended Question:* When I run a time-history analysis using modes from the Ritz-vector modal load case, I receive a warning message which states: Excessively large effective-damping ratios have been set to 0.999950. This message does not occur when using Eigenvectors. Is there an explanation?

*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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*Related incident:*
* [27472: Ritz vs. Eigen Vectors|$3704576]
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h2. What is the meaning of the warning message: Loads are applied to massless degrees of freedom?

*Extended Question*: During Ritz-vector modal analysis, I received a warning message which states: Loads are applied to one or more massless degrees of freedom, corresponding Ritz modes may be unavailable or inaccurate for dynamics. What does this mean, can it be ignored, and if not, how should I revise the model?

*Answer*: This message indicates that load is applied to a [joint|kb:Joint] which either has no [mass|kb:Mass] or is [restrained|kb:Constraint]. Gravity load from structural self-weight creates a set of fixed-end forces which includes shear and moment. These forces will also impose load on massless degrees-of-freedom (DOF). In a structure with [frame|kb:Frame] or [shell|kb:Shell] objects, each joint has up to six DOF, including translations (UX, UY, UZ) and rotations (RX, RY, RZ). Mass from the material density of structural objects is assigned to translational DOF. Additional mass may be assigned to any DOF, including rotational DOF such that [rotational inertia|kb:Time-history output-acceleration accuracy] is captured.

Ritz-vector application during dynamic loading is most effective when all applied loads act on DOF with mass. While earthquake loads automatically act on DOF with mass, mass should be assigned to DOF subjected to other types of load, and to [nonlinear|kb:Nonlinear] NLLink elements, which are treated as external force generators.

It is not necessary for loaded DOF to have mass when Ritz vectors are generated for gravity and other static loading conditions applied during [time-history|kb:Time-history analysis] analysis because load is applied so slowly that dynamic behavior is not important. In this case, either the warning may be ignored, or a small mass may be assigned to DOF in question.

Additional information is available in the {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} [_Analysis Reference Manual_|doc:CSI Analysis Reference Manual] (Modal Analysis > Starting Load Vectors, page 331).


h2. After assigning shell joint offsets during Ritz-vector modal analysis, what is the meaning of the warning message: Loads are applied to massless degrees of freedom?

*Answer*: When [joint|kb:Joint] offsets are applied to [shell|kb:Shell] objects, translational forces may cause moments at joint locations. Because [mass|kb:Mass] is lumped at translational DOF (UX, UY, and UZ), the rotational DOF (RX, RY, RZ) which correlate with applied moments are massless, causing the warning message to generate. Unless the offsets are very large, the warning message is not significant, and may be ignored.

To edit the model such that the warning message does not occur, mass may be assigned to the rotational DOF of loaded joints to provide for rotational inertia. A reasonable mass value would be between ^1^/~10~ _md_ ^2^ and ^1^/~100~ _md_ ^2^, where _m_ is the translational mass located at the joint, and _d_ is the offset distance. A smaller value on the order of ^1^/~1000~ _md_ ^2^ may be used to simply avoid the massless condition.