For calculating the allowable stress the code B31.3 is based on Max principal stress theory (Max Shear stress theory). According to which, the allowable Stress must be 0.5 time the Yield stress. But in code the allowable stress is 2/3 (0.667) of Yield stress. I am not able to understand why code takes an allowable stress value which is greater than value allowed by Max shear stress theory. Please explain and correct me if I am wrong. Thanks.