Block Format Keyword Defines the eigen modes and static modes computation for flexible bodies.


(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
grnd_ID grnd_bc Trarot Ifile            
Nmod Inorm Cutfreq Freqmin        
Nbloc Incv Niter Ipri Tol        


Field Contents SI Unit Example
eig_ID Mode identifier

(Integer, maximum 10 digits)

unit_ID Unit Identifier

(Integer, maximum 10 digits)

eig_title Mode title

(Character, maximum 100 characters)

grnd_ID Node group to which the modes will be computed
= 0
Modes are computed for the entire structure.


grnd_bc Node group to which specific eigen modes are applied 3
= 0
Free eigen modes are computed.
The node group defines a set of interface nodes.


Trarot Codes for translations and rotations

(6 Booleans)

Ifile Additional modes file flag5


Nmod Maximum number of modes to be computed

Default = 100 (Integer)

Inorm Eigenvector normalization method flag
= 0 (Default)
Eignvectors are normalized to the unit value of the generalized mass.
= 1
Eigenvectors are normalized to the unit value of the largest displacement in the analysis set.


Cutfreq Maximum eigen frequency
= 0
All Nmod eigen modes whose frequencies are higher than Freqmin are computed.
At most Nmod eigen modes whose frequencies lie in the frequency range Freqmin, Cutfreq are computed.


Freqmin Minimum eigen frequency7

Default = 0.001 Hz (Real)

Nbloc Number of eigen modes per block 9
The modes are computed per block of Nbloc eigen modes.
= 0
All eigen modes are computed at the same time.


Incv Factor to obtain the number of Lanczos basis vectors to use throughout the computation 10

Default = 2 (Integer)

Niter Maximum number of Arnoldi iterations

Default = 300 (Integer)

Ipri Printout level for ARPACK

Default = 0 (Integer)

Tol Relative accuracy to which eigen values are to be computed
= 0
The tolerance for eigenvalues accuracy is set to machine precision.

Default = 0.0 (Real)

Filename Additional modes file name

(Character, maximum 100 characters)



  1. This functionality is implemented for the purpose of the generation of flexible bodies. For detailed normal modes analysis of a model the use of the Bulk Data Format is strongly recommended.
  2. The use of the implicit option /IMPL/LINEAR in the Radioss Engine is required to compute normal modes.
  3. Boundary condition corresponding to the codes for translations and rotations are added to these nodes for the computation of eigen modes. Static modes, one for each additional blocked DOF, are computed.

    A static mode corresponds to the static response of the structure, all DOF of the set of interface nodes concerned by additional boundary conditions being blocked; except one taking the one value.

  4. The codes for translations and rotations follow the same rule as for the /BCS option.
  5. If Ifile ≠ 0: An additional file is provided containing pre-computed modes from a normal modes analysis, either experimental or numerical. These modes are used to reduce the dimension of the space in which eigenvalues are sought and thus enhance efficiency.

    If Ifile = 1, the additional file is given in a format defined in External Modes File.

  6. Multi-level condensation is no longer supported.
  7. The default (if set blank or zero) for Freqmin is 0.001 Hz. If a value other than zero is entered, that value defines a frequency in the unit system set for /EIG. The capability of computing rigid body modes is not fully implemented. It is recommended to either sufficiently constrain the model or to select a value for Freqmin that is high enough to eliminate all rigid body modes.
  8. Eigen modes are computed using ARPACK software (R. Lehoucq, K. Maschhoff, D. Sorensen, C. Yang).

    Figure 1.
  9. Better precision is achieved when only a small number of eigen modes are computed simultaneously.
  10. The number of Lanczos basis vectors to use through the course of the computation is given from the number of required eigenvalues per block (or total if Nbloc = 0) by the formula:

    NLanczos vectors = Nrequired eigenvalues * Incv.

  11. For the post-processing of modes shapes in HyperView, Radioss Starter input file (*000.rad) should be chosen in the Load Model panel and the first output animation file (*A001 which contains the first mode) in the Load Result panel.