In the case of models with members, the modal analysis is affected by the specifications of member divisions. The settings for RFEM and RSTAB are managed in program-specific dialog boxes.
Mesh Settings (for RFEM)
The FE mesh has an impact on the determination of eigenvalues.
Masses are applied to the FE nodes. The members with no FE nodes created due to the division and the members not integrated into surfaces only have one FE node at the member start and one node at the member end. In the case of a diagonal mass matrix, the masses of these members are concentrated on both end nodes. If these nodes are supported, it may not be possible to calculate eigenvalues.
To force the member division for such members, check the Activate member divisions for straight members" option in the "Members" tab of the Mesh Settings. As you can see in the example shown above, eleven FE nodes are created on a beam with a length of 5.0 m and a target FE length of 0.5 m. Now, the masses do not act on the supported nodes only. Therefore, it is possible to determine mode shapes.
Member Divisions (for RSTAB)
The member divisions have an impact on the determination of eigenvalues.
Masses are applied to the division points. The members with no member division specified have only one division point at both the member start and the member end. In the case of a diagonal mass matrix, the masses of these members are concentrated on both end nodes. If these nodes are supported, it may not be possible to calculate eigenvalues (see the image Incorrect Eigenvalues).
In order to force the member divisions, use the Member Divisions for Calculation dialog box where you can enter the corresponding number of division points. It is thus possible to determine 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.
