Both ETABS (v9 and later versions) and SAP2000 (v18 and later versions) automate the creation of accidental torsion loads when a nonzero eccentricity is applied in the response spectrum case. Because this approach is a static approximation, ASCE 7-10 requires amplification of the accidental torsion in accordance with section 12.8.4.3. The torsional moment is applied to all stories which have a diaphragm assignment (rigid or semi-rigid diaphragm). A torsional moment (Mz) is computed based on the seismic lateral story force obtained from RSA which is multiplied by the specified diaphragm eccentricity. The first approach uses the "Static Procedure". There are two basic approaches for consideration of accidental torsion in a response spectrum analysis (RSA).
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