When I look at the Period & Modes Report, I see that the cumulative mass participation for rotation is always 0% no matter how many modes are included in the analysis?
If a diaphragm includes a two-way deck or it is defined as a semirigid diaphragm, it is meshed and represented with finite (shell) elements. Each node of the finite elements has a nodal (point) mass associated with it. In other words, the diaphragm mass is represented with a network of spatially distributed nodal masses. Note each node includes point mass defined in the global X and Y-directions but not include rotational mass moment of inertia. Because the array of masses does not include rotation, the mass participation for this degree of freedom will always be 0. For the same reason, the mass participation for the rotational degree of freedom will also be 0 when pseudo-flexible diaphragms are used. It should be known that this type of modeling still suffices for capturing all essential dynamic properties. In other words, the proposed solution accurately captures any dynamic actions related to rotational inertias or any twisting modes due to having center of rigidity and mass center
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