The calcualtion of torsion for hotrolled structural steel sections is based on the following publications:
American Institute of Steel Construction
Steel Design Guide Series  9
"Torsional Analysis of Structural Steel Members"
by P.A. Seaburg and C.J. Carter (1997)
Theory
Torsional moments applied to open sections generate rotation and warping of the section.
Pure torsion generates shear stresses in the section, while warping generates shear and normal stresses.
For example, an Isection:
The program carries out the following design checks:
Shear Stresses:
Normal Stress:
For example,
AISC _ LRFD:
The program carries at the design checks at every 0.1L, i.e. 11 equidistant points along the span length.
The stressess are also checked at several points on the cross section:
ISections:
[Sections:
Cold Formed Sections:
The program checks the stresses at both ends of all segments and at any point of maximum or along the length of any segment.
Torsion and warping are ignored for all sections with a closed portion.
