maximum allowable stress in mild steel
I belive what you need to know is the ultimate stress of the material (stress at failure or permanent deformation), then you can figure out the safety factor by: (ultimate stress)/(admissible stress - in your case 31.57 MPa - ). If your safty factor is 2 for example your rig will support twice the stress it was designed for before it fails. If this safety factor is to your satisfaction then you will know that the given stress is acceptable. Or you can select your safety factor before and limit the load or increase the section to satisfy it.
Hope this helps, Good luck