How does SAP2000 compute the moment of inertia of floor diaphragms about the vertical axis

[jsmatte]

Does anyone know how SAP2000 compute the moment of inertia of floor diaphragms about the vertical axis passing through the center of mass? I am writing a code to analyse different structural configurations and I am comparing the results with those obtained in SAP2000. For the mass matrix, I have the exact same masses for the translational degrees of freedom, but very different masses for the rotational degree of freedom about the vertical axis. The equation I use to compute the moment of inertia is m*(A^2 + B^2)/12, where m is the mass of the floor and A and B the dimensions of the floor.

Thanks,

 

[jsmatte]

I would also like to know what are the units of the output txt files... if anyone knows

 

[johnbridge231]

As for the units, they always refer to these chosen by the user at the lower right corner.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[IJR]

jsmatte

Please try to test the Define-Mass Sources menu. Do a simple model, enter masses manually in one model, then set up the same model but let SAP2000 generate the masses.

Check the difference in results of eigen-value analysis. That should show you how SAP does the mass computations

respects
ijr