The effect of soil-structure interaction on the optimum design of steel space frames is investigated using metaheuristic algorithms. Three-parameter elastic foundation model is used to incorporate soil-structure interaction. A computer program is developed in MATLAB interacting with SAP2000-OAPI for two way data flow in all optimization procedures. Optimum design of space frames is formulated according to LRFD-AISC (Load and Resistance Factor Design, American Institute of Steel Construction) specifications. The parameters of foundation model are obtained by using soil surface displacements. It is concluded that consideration of soil-structure interaction ends up with heavier frames, and method is applicable for practical purposes.