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. Equation The bending resistance formula, in which the torsional constant is used, is: $$\theta = \frac{T\cdot L}{G\cdot J}$$ Where: θ = Angle of Twist T = Applied Torque (N·m or lb·ft) L = … [Read more...]