For models with members, the modal analysis is influenced by the specifications for the member divisions. The settings for RFEM and RSTAB are managed in program-specific dialog boxes.
Mesh Settings (for RFEM)
The FE mesh affects the determination of the eigenvalues.
Masses are applied to the FE nodes. Members on which no FE nodes are created as a result of a division and which are not integrated into surfaces only receive one FE node at the member start and one at the member end. In the case of a diagonal mass matrix, the masses of these members are thus concentrated on the two end nodes. If these nodes are supported, it may not be possible to calculate eigenvalues.
To force the member division for such members, select the Activate member divisions for large deformation analysis, structural stability, dynamic analysis ... check box in the 'Members' tab of the Mesh Settings. For the example shown above, a total of eleven FE nodes are created on the 5.0 m long beam with a division of '10' for special member types. The masses now act not only on the supported nodes, but also along the member. Therefore, it is possible to determine mode shapes.
Member Divisions (for RSTAB)
The member divisions affect the determination of the eigenvalues.
The masses are applied at the division points. Members for which no member division is specified only have one division point at the member start and one at the member end. In the case of a diagonal mass matrix, the masses of these members are thus concentrated on the two end nodes. If these nodes are supported, it may not be possible to calculate eigenvalues (see image Incorrect Eigenvalues).
To force the member divisions, enter a corresponding number of division points in the Member Divisions for Calculation dialog box. This allows you to determine the mode shapes.
The largest value "N" of the member divisions is used for the modal analysis. The longest member of the model is divided by the value "N". This results in the maximum member division length. This length is used to determine the member divisions of all other members: The member lengths are divided by the division length and rounded up to the closest whole number.
