Twisting for Z Section 

     The following is from page 105 of AISI S100-2007-C:

"For Z-sections, the principal axes are oblique to the web, a load applied in the plane of the web, resolved in the direction of the two axes, produces deflections along each of them.

By projecting these deflections onto the horizontal and vertical planes, it is found that a Z-beam loaded vertically in the plane of the web deflects not only vertically but also horizontally. If such deflection is permitted to occur then the loads, moving siderways with the beam, are no longer in the same plane with reactions in the ends.

In consequence, the loads produce a twisting moment about the line connecting the reactions."

If the loading in the plane of web is Q, the fictitious horizontal load to each flange (both in the same direction) should be P = Q I_xy / (2 I_x)