Versions Compared

Old Version 4

changes.mady.by.user Ondrej

Saved on

compared with

New Version Current

changes.mady.by.user Ondrej

Saved on

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.


{toc} h1. FAQ h2. How should I setup a modal analysis for a structure with cables? *Extended Question:* I am doing a modal analysis of a model that has cable elements in it. The model is intrinsically non-linear since it has tension-only cable elements. But the modal analysis is theoretically an elastic analysis. If you force SAP to do such an analysis, what will happen? In another words, what is the algorithm involved? *Answer:* To obtain meaningful results for modal analysis of a structure with cables, the modal analysis should be using stiffness at the end of nonlinear load case in which the dead loads (and possibly other types of loads of interest) are applied. Then the modes will be based on the stiffness of the cables under the applied loads. If you need to consider complete dynamic nonlinear behavior, you should consider running nonlinear time history analysis. {related-incident:no=24152|comment=Modal analysis for structure with cables.} h2. What are reasons for and consequences of a warning message that loads are applied to massless degrees of freedom in Ritz-vector modal analysis? *Extended Question*: When I run analysis, I get a warning message saying: Loads are applied to one or more massless degrees of freedom, corresponding Ritz modes may be unavailable or inaccurate for dynamics. When does this type of warning indicate? How can I modify my model to get rid of this message? Does that have to do with boundary conditions? If nothing is done to avoid this warning, what are the possible consequences? *Answer*: This message means that you are applying load in that load case to a joint that has no [mass|mass] , possibly at a joint that is restrained. In a structure with frame or shell elements, each joint has up to 6 [degrees of freedom|degree of freedom] (DOF): translations UX, UY, and UZ; and rotations RX, RY, and RZ. Masses which come from the elements (which, in turn, come from the material mass density) are always assigned only to the translational DOF. You can additionally assign joint masses at any of the 6 DOF of a joint. Gravity load on a frame element creates a set of fixed-end forces at the joints which include moments as well as shears. Hence gravity (self-weight) load in most structures will impose load (moments) on massless DOF. Ritz vectors used for dynamic loading work best if all applied loads act on degrees of freedom with mass. Earthquake loads automatically act on mass DOF. If you are applying other types of loads or are using nonlinear NLLink elements, which SAP2000 treats as external force generators, mass should be assigned to the loaded DOF. If you are generating Ritz vectors for the purpose of applying gravity and other static loads in a time-history analysis, then it is not necessary to have mass on the loaded DOF, since the load is applied so slowly that dynamical behavior is not important. So for this type of problems, this warning can be ignored. Alternatively, you could specify a very small nonzero mass for the DOFs in question. Also, please see the [CSI Analysis Reference Manual|doc:Analysis Reference Manual], chapter "Modal Analysis", section "Starting Load Vectors". h2. After assigning shell joint offsets, why I am getting warning messages that loads are applied to massless degrees of freedom in Ritz-vector modal analysis? When [joint offsets|joint offset] are used for the [shell element|Shell], translational forces may cause moments at the joints. However, the masses are still lumped at the joints, and do not include rotational inertia. These messages are to be expected. Unless the offsets are very large, they are not very important. To eliminate them, you can assign small rotational masses at the joints. A reasonable value would be equal to the translational mass as the joint, multiplied by the square of the offset distance, divided by 100 or 1000. h2. Why am I getting warning message about excessively large effective-damping ratios for modal time history analysis based on Ritz vectors? *Extended Question:* My model gives "Excessively Large Effective-Damping Ratios Have Been Set To 0.999950 ..." warning when I run time history using modes from Ritz Vector modal load case. When it uses Eigenvectors with all other parameters being the same, there is no such a warning. Is there any explanation for this warning message? *Answer:* This could be caused by using [proportional damping], for which the damping increases with large frequency, and Ritz vectors generate high-frequency modes that, hence, have large damping
Wiki Markup
Info

This page is devoted to frequently asked questions (FAQ) related to modal analysis.

On this page:

Table of Contents

Modal analysis

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 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.

Show if
groupeditors

Related Incidents:

  • Incident 24152: Modal analysis for structure with cables.

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.

Show if
groupeditors

Related Incidents:

  • Incident 47624: Fundamental Period of Glass Panel

Error and warning messages

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.

Show if
groupeditors

Related Incidents:

  • Incident 35539: See 6/10/2011 event

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.

Show if
groupeditors

Related incidents:

  • Incident 27472: Ritz vs. Eigen Vectors

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 which either has no mass or is restrained. 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 or 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 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 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 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 CSI Analysis Reference Manual (Modal Analysis > Starting Load Vectors, page 331).

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 offsets are applied to shell objects, translational forces may cause moments at joint locations. Because 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.