Hello Everyone, I am after an expression for the torsion stiffness constant of a rectangular hollow section (RHS) with varying wall thickness. I have done some research into finding an expression that allows for four varying wall thicknesses but have not found anything as of yet. The only expressions that are published are of RHS's with two varying thicknesses (Roark's 7th ed, Wiki etc). If anyone could point me to a resource or present a proof I would greatly appreciate it. Attached is a sketch of the general shape (excuse the huge image). Thanks.