Fully Constrained Design (FCD) is a new method for discrete sizing optimization of steel frames. Based on the optimality criteria approach, FCD features a distinct approach to constraint handling and the generation of new designs that makes the method scalable to large problems and applicable when the problem objective and/or constraint functions are discontinuous. FCD is tested on several truss structures and compares favorably to optimality criteria and heuristic methods in terms of solution quality and computational efficiency, respectively. A successful industry application of the method is presented to demonstrate the scalability of the method and its suitability for industry application.