Modal Analysis Parameters

Use this dialog to define modal analysis parameters for new dynamic cases in the structure.

Access

To define the modal analysis parameters for new dynamic cases in the structure, do one of the following:

• Select a modal analysis case in the New Case dialog and click OK.
• Select a modal analysis case in the Analysis Type dialog and click Set Parameters.
• Select a modal analysis case in the Calculation Restart dialog and click Define Parameters.

Dialog elements

Note: You can perform a second type of modal analysis: Modal analysis recognizing static forces.

Parameters

Contains the available parameters for the modal analysis: number of modes, tolerance, number of iterations, acceleration of gravity.

Mass matrix

• Consistent:The consistent matrix with regard to the rotational degrees of freedom.
• Lumped with rotations: The diagonal matrix with regard to the rotational degrees of freedom.
• Lumped without rotations: The diagonal matrix without the rotational degrees of freedom.

Active mass directions

Specifies the direction X, Y, or Z in which the mass is activated, i.e. the eigenvibrations.

By default, the mass is active in all directions for the modal analysis.

Note: You may need to deselect a direction when performing the seismic analysis. If you deselect the mass in the Z direction, for the modal analysis that precedes the seismic case, it eliminates local vertical vibration modes for floor slabs. In the seismic analysis for such model you will consider only transversal vibrations. Another example is the Footfall analysis, where the active mass for the Z direction is a default setting. In this case you consider vertical vibrations and ignore transversal vibrations of the model.

Disregard density

Excludes the density of the structure element (ρ=0) during the dynamic analysis of the structure.

Sturm check

Sets the algorithm for detecting the skipped eigenvibrations. If a skipped mode is found, an iteration is run again.

Advanced parameters >>

Displays additional modal analysis parameters for a load case.

Click Advanced parameters >> to access additional modal analysis parameters for a load case:

Advanced parameters

Analysis mode

• Modal: Conducts a modal analysis of the structure. n vibration modes will be found, the iterations will be completed if a convergence condition is reached.
  

  Activation of Sturm Check, which allows for searching for the omitted pulsations, is possible.

• Seismic: During the modal structure analysis, n vibration modes will be selected (not necessarily in the appropriate order). As Eigenvibration modes are important for seismic analyses, those which are characterized by a large mass participation factor will be set.

  A Sturm Check is not possible for this structure analysis mode.

• Seismic pseudo modal: Conducts a Spectral and a seismic analysis using the pseudo modal method. This should be used only when applying classic analysis methods based on modal decomposition results in a very long structure analysis time.

Method

Select the eigenproblem solution method: Block subspace iteration, Subspace iteration, Block Lanczos algorithm, Lanczos algorithm or Base reduction.

Limits

Limits the number of defined structure vibration modes.

Note: The Percent of mass participation option uses the participation mass percentage condition to assign the number of modes.

Seismic analysis parameters

• Include damping in calculations (according to PS92): accounts for support with damping during the modal analysis. Defined damping coefficients are used in the seismic code PS92 formula. 5 section 6.2.3.4 of the French seismic code PS 92 (NF P 06-013) necessitates structure-ground interactions, and thus influence of the ground on a value of the structural damping of a structure.

Definition of eccentricities

Opens the Definition of Mass Eccentricities dialog which allows you to consider the mass eccentricity during the seismic analysis.

Note: If you access modal analysis parameters from the Calculation Restart dialog, Calculation restart - calculation of additional modes is available. Performed calculations of vibration modes are maintained after the calculation restart. Depending on your settings, only successive modes are calculated. Recalculation of all vibration modes will not happen, saving calculation time.