Torsional Constant For Rectangular Section.pdf

If you have an open section (e.g. I-beam, parallel flanged changed, equal angle i.e. with a closed flow of shear flows within the walls of a section) this formula will give you a good enough approximation: J = bt^3/3 where b is the always the longer side. e.g. for a rectangle J = breadth * depth^3/3 For an I-beam or indeed other section you can simply add all the bt^3/3 i.e. for an I-beam, J = (1/3)*(bTF*tTF^3+dweb*tweb^3 + bBF*tTF^3) It should be noted that the formula begins to break down where b isn't >> t. To get a better approximate there are various tables that give you a factor k to multiply the torsion constant with that’s a function of b/t Reference https://www.physicsforums.com/threads/st-venants-torsion-constant.763305/