In structural steel design, the Torsion Constant, J, represents the ability of the steel beam to resist torsion, i.e. twisting. It’s units are mm4 or inches4.
The bending resistance formula, in which the torsional constant is used, is:
θ = Angle of Twist
T = Applied Torque (N·m or lb·ft)
L = Length of Beam (mm or in)
J = Torsional Constant (mm4 or in4)
G = Modulus of Rigidity (GPa or psi)
Values of J for various shapes
Standard structural steel shapes, including the Torsional Constant, J, are found in the AISC Steel Construction Manual, section 1.