I have having dificulty in determining the torsional capacity of a square HSS section. I am designing a shaft for 9.5 cubic yard mixer powered by a 80hp drive at 6 rpm output. I have calculated the applied torque to be 70,000 ft-lbs (95 kNm). I would like to determine the size of HSS required if I use both 319 Stainless (Fv = 103 MPa) and 350W Mild Steel (Fv = 210 MPA) [Fv = allowable shear stress]. I will be comparing both materials as both offer advantages. Is there a way to use the Torsional Constant [J] to determine allowable torque on a section? I have been able to calculate the required capacity for solid round, hollow round, and solid square sections. The method to determine the allowable torque on a hollow square shaft eludes me! It is my hypothesis that a 10x10x1/2" S.S. HSS will suffuce however I am not certain. Any advise or direction would be greatly appreciated!