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# Center Mass to Rigidity Eccentricity

## Center Mass to Rigidity Eccentricity

(OP)
When it comes to modeling the structure using semi-rigid diaphragms, does the eccentricity value make a difference? Since the floor diaphragm is sub-meshed into plates (given it's modeled as semi-rigid), the seismic and/or wind forces are NOT applied to the center of mass (as is the case of the rigid diaphragm), but rather distributed to individual nodes that compose the particular story diaphragm. Also, if semi-rigid diaphragms are used, the program does not display the center of rigidity following the 'center of rigidity' analysis (defined in Load Cases). If the loads are distributed spatially in this matter, the torsion on the structure is, in my belief, included based on stiffness matrix of the structure. The concept of center of mass (the location of concentrated lateral load) does not apply nor the location of the center of rigidity. However, if the eccentricities are defined explicitly in Loads>Masses, what is the program doing? (Assuming semi-rigid diaphragms are used). I am not seeing any differences in forces, stresses, reaction in my load cases where X +E and X –E are defined. Should I leave the eccentricity value at zero and let the building experience additional torsion based the spatial distribution of the lateral loads and the stiffness of the vertical elements?

### RE: Center Mass to Rigidity Eccentricity

It depends what version you are using.  With RAM Structural System version 14.02 the RAM Frame wind and seismic load generators were enhanced to deal with eccentric loads on semi-rigid diaphragms. Prior to that, eccentricity only mattered in rigid diaphragm applications.

Looking at the reactions or other results is a good way to confirm the behavior, you can also visualize the applied loads on the points of the meshed diaphragm using process – results – applied Story forces.
For more on Semi-Rigid diaphragm behavior, you might want to check out our Wiki on the subject here:

http://communities.bentley.com/Products/Structural/Structural_Analysis___Design/w/Structural_Analysis_and_Design__Wiki/ram-ss-semirigid-diaphragms.aspx

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