*NOTE: This discussion applies only to SAP2000. ETABS automatically includes the accidental eccentricity specified in the response-spectrum load case.*

To consider **accidental torsion in a response-spectrum case**, eccentricity should be added as a separate static load case. The response-spectrum (SPEC) and eccentric (ECC) load cases should then be combined as SPEC + ECC and SPEC – ECC load combinations. Each level should have a rigid diaphragm, otherwise accidental eccentricity is not of significance.

Accidental torsion is applied through the following process:

- Define a load case named ECC (or other) of Type = Quake, Self-weight multiplier = 0, and Auto = None.

- Select any point on the diaphragm at each level.

- Select Assign > Joint/Point Loads > Force > Moment Global ZZ, then specify the appropriate value for torsional moment.

- Evaluate SPEC + ECC and SPEC – ECC load combinations. Note that the software already considers SPEC in both directions.

Accidental torsion may be considered without rigid diaphragms, though loading should be more distributed. Torsional moment is calculated at joint locations within a flexible diaphragm by first resolving the total lateral force in a given direction at each story level. This is done by summing the auto-seismic loads applied to joints within each story level. This force is then multiplied by the eccentricity to generate torsional moment (T). This torsion is then distributed to all joints at the story level in proportion to the joint mass as follows:

- Torsion at a given joint at a story level = T * (Mass at the joint / Total mass of all joints at the story level)

It may be convenient to use an Excel spreadsheet to calculate torsion at joint locations. Using interactive database editing, data may then be entered into SAP2000

Source: https://wiki.csiberkeley.com/display/kb/Accidental+torsion+in+response-spectrum+case

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