RAM Frame: Dynamic Mass Offset

I have a model in which the periods and effective mass modal participation is different for each of these dynamic load cases:

1. Modal Analysis ( Dynamic with No Ecc)

2. Dynamic with + Eccentricity (5%)

3. Dynamic with - Eccentricity (5%)

Does RAM shift the center of mass to alternate points offset ( eccentric locations) from the original calculated center of mass and modifying the mass moment of inertia? 

Also, if that is the case, then per Section 12.9.5 of ASCE 7-10 we would not need to apply "Ax" i.e. Amplification of Accidental torsion. My question is why do you mention here in this post ( link below) about Ax?

 https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/2980/ram-frame-seismic-loads-faq

Please let me know. Thanks!

Parents
  • For a rigid diaphragm, dynamic analysis with eccentricity we shift the center of mass by the 5% or other user input eccentricity value.

    For the Equivalent Lateral Force procedure we shift the point that the load is applied. We don't automatically apply additional torsion for irregular structures with ELF. The user has to increase the eccentricity under Loads Masses when such is required by code. The wiki you cited is specifically about ELF.



Reply
  • For a rigid diaphragm, dynamic analysis with eccentricity we shift the center of mass by the 5% or other user input eccentricity value.

    For the Equivalent Lateral Force procedure we shift the point that the load is applied. We don't automatically apply additional torsion for irregular structures with ELF. The user has to increase the eccentricity under Loads Masses when such is required by code. The wiki you cited is specifically about ELF.



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