Equations 4.2-4.4 in axial suggested by Cardoso (2014), were assessed then utilized in the derivation of design equations, to account for material orthotropy. Figs. 4.1– 4.5 present plots of the critical buckling stress, fcr, vs. the aspect ratios(h/b) for RHS sections. The orthotropic conditions are considered in five different cases of orthotropy. Three cases in which Ex/Ey=1, Ex/Ey=0.5, and Ex/Ey=0.33. In the first case, three different values of possion’s ratio were used. In the second and third cases, the possion’s ratios were fixed to zero. Shear modulus, Gxy, values of 1000, 4000, 7692 MPa. Were used. The curves are compared to those obtained using the finite strip method implemented using CUFSM. The equations for the critical local buckling coefficient, Kcr, which was used in the critical stress equation 4.1 of the orthotopic materials. Additionally, the critical buckling coefficients for the simply supported condition at plate inter sections recommended by current standards [17–19]. These were obtained by dividing Eqs. (1) and (2) by π2D11/bw 2, (Cadroso, 2014). As can be seen on Fig. 4.1-4.5, excellent agreement was achieved between the proposed by Cardoso (2014) expression (Eq. (4.4-4.6)) and the finite strip method, regardless of the aspect ratio. The coefficient of variation (COV) was 10% and the average was 1.0.