When performing a dynamic calculation, I get the message: "The model has no free mass assigned. Therefore, dynamic calculation is not possible" or "Too many eigenvalue required." How can I calculate mode shapes?

Answer

It can be determined as many eigenvalues as there are degrees of freedom in the structure to which a mass is assigned. In order to determine the number of degrees of freedom, the number of nodal points of FE nodes must be multiplied by the active mass directions. The masses in fixed supports are not considered for this as they cannot freely oscillate.

If the "Divison for straight members" (RFEM) / "internal member division" (RSTAB) is not activated, it may happen, for example in the case of single-span beams, that the masses only exist in the supports and thus no eigenvalue can be determined. In such cases, activate the member division as shown in the figures.

Since the Lanczos solver in RF-DYNAM Pro cannot determine all existing eigenvalues, the "root of the characteristic polynomial" solving method can also provide remedy.

Keywords

eigenvalue degree of freedom dynamic calculation

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RSTAB Dynamic Analysis
DYNAM Pro - Natural Vibrations 8.xx

Add-on Module

Dynamic analysis of natural frequencies and mode shapes of member models

Price of First License
850.00 USD
RFEM Dynamic Analysis
RF-DYNAM Pro - Natural Vibrations  5.xx

Add-on Module

Dynamic analysis of natural frequencies and mode shapes of member, surface, and solid models

Price of First License
1,030.00 USD