Dynamic frequency analysis

The SPACE GASS dynamic frequency analysis module is able to analyse multiple mass load cases consisting of self mass and/or user defined lumped masses in a single run. For each mass load case it calculates the natural frequency (eigenvalue), period, mode shape (eigenvector) and mass participation factors for any user defined number of vibration modes. The natural frequencies, periods, mode shapes and mass participation factors comprise the dynamic properties of the structure.


Important points

  1. A dynamic frequency analysis is linear only and therefore cannot be performed if your model contains cable elements.

  2. Because it is linear, a dynamic frequency analysis treats tension-only and compression-only members as normal members that can take tension or compression.

  3. P-D and P-d effects are not taken into account during a dynamic frequency analysis.

  4. If master-slave constraints are used then the accuracy of the dynamic frequency analysis depends on correct placement of the master nodes. Each master node should be positioned as close as possible to the center of mass of its slave nodes. Failure to do this could result in inaccurate dynamic frequency results. To find the center of mass of a group of slave nodes you could create a filter of those nodes and then produce a "Mass details report" for that filter. Note that this only affects dynamic frequency analysis and is not a requirement for static analysis.

  5. If your model contains gap or fuse members, or variable spring, plastic, friction or one-way restraints, because they are inherently non-linear in the way they behave, they are not really suited to a dynamic frequency analysis because it is linear. This will also have a flow-on effect to the spectral, harmonic and transient response analysis modules that use the dynamic frequency results. SPACE GASS will allow a dynamic frequency analysis, but all gap/fuse members and non-linear restraints will be treated in a simplified linear manner that may not give you the results you expect or want. Gap and fuse members will be treated like normal members with their tension and compression limits ignored. Plastic and friction restraints will be treated as fixed, while variable spring restraints will be treated as springs using a constant spring stiffness that corresponds with a zero deflection from its stiffness vs deflection table. One-way restraints will be treated a bi-directional.


image\ebx_-1773499217.gif You must perform a dynamic frequency analysis before you can perform a spectral, harmonic or transient response analysis.