The following information comes from these pages of Roark's formulas for stress and strain: page 395: start of torsion chapter page 396: basic torsion formulas for circular cross-sections page 415: Table 10.1 starts page 427: Table 10.2 starts page 428: formulas specific to I-beams page 431: Table 10.3 starts Note that whereas the formula in the blue box can be used to calculate the stress in a circular shaft due to torsion, the formulas highlighted with red are needed to calculate the stress in a more complicated shape like an i-beam.