Open topic with navigation

Validation of modal analysis: local modes

Dynamic analysis troubleshooting > modal analysis > local modes

Check for local modes

Local modes mobilize only a very small portion of the mass of the structure and are usually irrelevant for the overall seismic behaviour of the structure.

Such modes do not disturb the overall behaviour of the structure. They are just associated with very low values of relative modal mass.

They might become problematic, because they often have relatively low frequency values and appear before or between significant seismic modes, thus increasing the number of modes necessary to reach the required 90% of relative modal mass.

Typical case of local modes are listed in the section "Insignificant modes" in "Validation of modal analysis: relative modal masses".

In the following example, the antenna on the roof of a building shows local modes are have no impact on the seismic resisting system.

Mode	Frequency [Hz]	Wxi/Wxtot [%]	Wyi/Wytot [%]	Mode shape	Comments
1	1.31	34.9%	0.5%		fundamental mode in direction X
2	2.23	0.0%	0.0%		1st antenna mode irrelevant for main resisting system
3	2.23	0.0%	0.0%		2nd antenna mode irrelevant for main resisting system
4	2.38	1.3%	66.8%		fundamental mode in direction Y

Although it is, of course, possible to use such a model to obtain results for checking the antenna itself under seismic action, it is usually an approach that is very costly. Such entities should rather be evaluated and checked as non-structural parts.

Avoiding local modes

Most of the time, local modes are not needed for the design of the main seismic resisting system. Therefore, the model should be modified, so that such modes do not appear in the results. Alternatively, irrelevant modes can simply be ignored in modal superposition (see below: mode filtering). The latter, however, can require computing a large number of eigenmodes, depending on the geometry and properties of the structure.

Be aware, that avoiding local modes also removes from the results any related local results in the concerned entities. In case those entities need checking against seismic action, they must be checked using code provisions related to non-structural entities.

Non-structural parts

For instance: non-supporting walls, fixed technical equipments, windows...

• do not include non-structural parts as members in the model
• only insert their effect on the structure as loads and masses

Structural parts with negligible local behaviour

For instance: cables, bracings' diagonals, beams without explicitly modelled transverse stabilization...

In those cases, several approaches are possible:

• avoid local behaviour by moving the mass of such members to structural nodes that are directly connected to the main resisting system. Two possibilities:
  • make sure that the member is meshed as a single finite element, by setting appropriate mesh settings; in such a case, the mass of the member is automatically concentred on its end nodes, thus avoid any dynamic local behaviour; axial force only members can be used for this, as they are calculated as single finite elements by default
  • if the above is not possible, remove any mass applied on the member (use property modifiers to remove the self-mass) and input equivalent on the end nodes manually
• control local behaviour by adding appropriate stabilization in the model
  • for cables, take pre-tensioning into account (see initial stresses)
  • for beams, consider modelling transverse stabilizing entities such as diaphragms

Mode filtering

Alternatively to avoiding local modes (or irrelevant modes in general), "Seismic loading: Mode filtering" may be used.

It implies, that a sufficient number of modes is computed to reach the required total modal mass ratio, which might become very high in case there are many local modes.

Mode filtering allows, having calculated enough modes, to consider only modes that have the highest values of modal mass ratio during modal superposition. Negligible modes are thus automatically skipped, based on their low participation factor.

Next step: specific checks for IRS analysis

Open topic with navigation