TORSIONAL FREQUENCIES-ROTACIONAL MASS-MODE SHAPE

[jesosan]

[ATOM] [ATOM]
In the simple structures i've modeled in STAAD i have tested the master/slave option to acomplish Guyan reduction or slab modeling with plate elements to simulate diafragm rigidity. In both cases results are the same: altough traslational frequencies are close to that calculated by SAP or ETABS and even by hand calculation, torsional frequencies don't coincide (not to mention related mode shape). Mass distribution is the same, (translational mass); i think that the difference can be originated by the rotacional mass and the associated floor mass moment of inertia which i don´t know how to set up in STAAD input file that in SAP is fully explicit.
Any comment or suggest relating this issue would be truly apreciated.

 

[struct1]

If you have a better analysis package available to you, why are you using STAAD? STAAD is OK for stiffness analysis. However, it often does a poor job at dynamics. Its solver is inferior.