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(Version 2015 and above)This version calculates story torsion based on the story shear difference between adjacent stories times Xacc and Yacc diaphragm eccentricity along X and Y directions, respectively. This method usually produces better and  less conservative results. In the case of semi-rigid diaphragm, each nodal force is multiplied times eccentricity based on diaphragm extent to find the torsional force at that specific node. 

ETABS V15  allows for moved floor  masses to be included in the analysis model. The way to achieve this is as follow:

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3- Define>modal cases and add new case, say ModalCase2, ModalCase3 etc. and make sure to choose radio button "use nonlinear case..." and select Lcase2, Lcase3...respectively. 

Reasons for considering accidental eccentricity

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