Minimum weight design of single-and multi-bay steel portal frames is presented. The design variables are the dimensions of prismatic built-up sections for beams and columns. Design constraints are considered as per ECP’ 01: shape, buckling, stresses, and deflection constraints. Both compact and non-compact sections are included in the formulation. Cases of loading comprise vertical and lateral loads. Analysis is done using Displacement Stiffness Method. An optimization technique based on the Method of Feasible Directions -through an implicit formulation– is adopted. Several examples are presented in order to assess the advantages of adopting optimization in structural steel design as compared to other classical design approaches