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.
- A dynamic frequency analysis is linear only and therefore cannot
be performed if your model contains cable elements.
- Because it is linear, a dynamic frequency analysis treats tension-only
and compression-only members as normal members that can take tension
- P-D and
are not taken into account during a dynamic frequency analysis.
- 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.
- 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.
must perform a dynamic frequency analysis before you can perform a spectral,
harmonic or transient response analysis.