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Frequency Analysis

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Analysis > Frequency Analysis prompts you with the following dialog.  

 

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It allows you to set important options before performing frequency analysis on the model.  

 

The load combination for mass and stiffness must be specified.  If you want to input nodal mass and/or mass moment of inertia directly, you may do so from Create > Generate Loads > Additional Masses or Tables > Masses > Additional Masses.  The load combination is also used to form the correct stiffness matrix if the model response is not linear.

 

The number of modes is used to determine how many frequencies and mode shapes are to be computed.  This value must be less than the total number of mass degrees of freedom.  Practically speaking, only the lowest eigen modes are used for design purposes.

 

The number of iteration vectors q is normally set as the minimum of (2 * p, p + 8), where p is the number of modes requested [Ref. 1].  A higher convergence rate can be achieved by using more iteration vectors.  This may be necessary if some eigen modes are missing after the solution or if the solution becomes unstable.

 

The tolerance of eigenvalues is used to measure the convergence of eigenvalues during each successive solver iteration.  It is expressed as the following:

 

(i = 1, 2, … number of requested modes)

 

where k is the subspace iteration counter.

 

A maximum number of subspace iterations is used to prevent excessive solution time.  If the solver reaches this limit without convergence, the eigen results should not be trusted.

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