Answer:
Accidental torsion is given in EN 1998‑1 Sect. 4.3., ASCE 7‑16 Sect. 12.8.4.2, and NBC 2015 Sect. 4.1.8.12 (4). The eccentricities are to be determined manually; in EN 1998‑1, they are given as 5% of the building length. In other international seismic standards, the principle of accidental torsion is regulated similarly, but the determination of the eccentricities may differ.For each equivalent load (at each FE node/internal node), a torsional moment is determined according to the following formula:
$\mathrm M\;=\;\left|{\mathrm F}_{\mathrm x}\;\ast\;{\mathrm e}_{\mathrm y}\right|\;+\;\left|{\mathrm F}_{\mathrm y}\;\ast\;{\mathrm e}_{\mathrm x}\right|$
The exported load cases are then output in duplicate number - with the torsional moment M rotating around the vertical axis, once in the positive and once in the negative direction of rotation.
