Optimal design of cold-formed steel portal frames for stressed-skin action using genetic algorithm

•If stressed-skin action is included, the cost of frames can be reduced by 42–53%.•The wind load design cases were no longer critical.•If the frame is designed on the basis of rigid joints, cladding failure could occur.•It is important to take into account semi-rigidity and partial strength of joints.•Failure of the roofing could be an issue when the frame is relatively flexible.

This paper describes a stressed-skin diaphragm approach to the optimal design of the internal frame of a cold-formed steel portal framing system, in conjunction with the effect of semi-rigid joints. Both ultimate and serviceability limit states are considered. Wind load combinations are included. The designs are optimized using a real-coded niching genetic algorithm, in which both discrete and continuous decision variables are processed. For a building with two internal frames, it is shown that the material cost of the internal frame can be reduced by as much as 53%, compared with a design that ignores stressed-skin action.