A finite element method is proposed to determine the chord side wall failure load of welded RHS T-joints with axial compression branch. The width ratios β (=b1/b0) vary from 0.8 to 0.9, and the range of wall slenderness is 16.1≤h0/t0≤42.3. With strain hardening of the material not take into account, the validity of the FEM model is verified using existing experimental results. The failure of the joints is mainly due to the buckling or crippling of the chord side wall, which agree well with the experiment. The mean failure load determined by FEM is about 93% of the test and with a coefficient of vibration of 0.037. The ultimate strength so determined is also compared with the existing design formulae. The proposed method is more reliable. Further investigations are required to develop design formulae for the side wall failure of RHS joints with width ratio β≠1.0.