The ultimate shear strength of the steel is 300 MPa and a factor of safety of 4 is required for this application. The allowable shear stress for the tank is therefore:
We now have a numeric value for the allowable shear stress. Next, we must determine the maximum shear stress produced in the tank by the hoop and longitudinal stresses.
To understand the relationship between σlong, σhoop, and the maximum shear stress, consider Mohr's circle for the stress element shown.