The force Fi on shear line line i is given by
max (Fi+, Fi-)
where
Fi+ = Fdi + Fti+
Fi- = Fdi + Fti-
where
Fdi = F * Ki / S Ki,
where
Fti+ = T+ * Ki * di / (Jx + Jy)
Fti- = T- * Ki * di / (Jx + Jy)
where
J = S Ki * di a
T+ = (CL - CR + ae) * F
T- = (CL - CR - ae) * F
where
CR = S Ki * li / S Ki,
where
In the direction of loading, torsional forces add to the direct shear forces for some walls and subtract from the direct shear forces for other walls, depending on the direction of the torsional moment created by the difference in the center of load and rigidity.
Torsional analysis also creates torsional forces in shear lines orthogonal to the force direction. Unless we are designing for loads from both directions simultaneously, as we do for low-rise wind loads, the program discards the torsional forces in the orthogonal direction on the assumption they are smaller than the total force from torsional analysis of loads in that direction.