It is a combined stress problem. I did not include the bending on the full cross section for gravity load or lateral load, only for the torsional effect.
Mt stands for torsional moment. The applied torsional moment is 150*8 = 1200*#, but if applied at midspan, 600"# goes to each support. The rotation and shearing stresses for that condition is what I provided.
For a tee section or an angle or a cruciform section, there is no warping because all of the elements intersect at a common point, the shear center.
BA