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Issue: the cumulated modal mass ratio is less than 90%

Issue

In a modal analysis or seismic analysis, the cumulated modal mass ratio does not reach the code required 90%.

Increasing the number of modes does not help, as the total mass seems to max at a value that is significantly lower than the required value.

Meaning

This can be due mainly to two causes, which can be combined:

• The model contains immobilized or quasi-immobilized masses. This is usually due to relatively rigid parts of the structure that are connected to supports. The most common cases in buildings are foundation slabs and storeys below ground level.
• The modal mass is distributed across a large number of local modes. This usually due to masses assigned to members that are more flexible than the main horizontal resisting system. The most common cases are complex 3D frame or truss structures.

Solution

In general, simplify the model (see "Modelization for seismic analysis - Good practices"):

• Remove from the model masses that are not loading the main horizontal resisting system
• Avoid assigning masses along flexible members. Rather concentrate the corresponding masses at the nodes where such flexible members are connected to the main resisting system.

In case simplifying the model is not sufficient or not acceptable, alternative techniques are available to compensate for the lack of modal mass. See more details about Mass in analysis in "Seismic load case settings" and "Seismic loading: Mass in the seismic analysis")

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