Abstract: There are two approximately formulas on welding connection: one is the strength calculation of ring anglewelding line in the action of bendingmoment; The other is the strength calculation of ring angle welding line in the action of torque. The derivation or the range of application of the approximately formulas in the all literatures was not given. The theory analysis on the approximately formulas, and the derivation in the paper made the range of application by accurately calculating and analyzing error of the approximately calculation was presented.

Key words: bending moment, torque, ring angle welding line, strength calculation, approximately formula