For torsional constant, analytical formulas are available for some standard steel sections as shown in this wiki page.
But what about arbitrary shapes? How can I obtain the formulas for torsional constants? If no such formulas are available, are there any first principles that I can use to deduce the approximate formulas or FEM calculation for torsional constant?
Note: a related question about the FEM formulation of torsional constant is being asked here.