Autodesk Robot Structural Analysis

[#1]

Hi guys,

I understand it's important to hit say 90% mass participation in the modal case to properly run seismic analysis. 

In order to define AS1170.4 in Robot you first need to define a modal case.  If I input a user defined base shear, Robot will try to match the CQC reaction to the user defined shear.  This seems to be regardless of the mass participation in the preceeding modal case. 

So - does this modal case need to hit 90% mass, or is the seismic case using the "correct" mass by scaling to hit a pre-defined value?

Cheers,

Tony

[#2 · Accepted solution]

Hi Tony,

modal case needs to hit 90% mass.

Using only the scaling to pre-defined base shear will result in overestimating the influence of some vibration modes while neglecting the influence of other vibration modes (not found by modal analysis because of not reaching 90% mass).

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