The effect of warping in the design of steel space frames having members of thin walled steel sections is significant. In this paper the optimum design problem of steel space frames is formulated according to the provisions of LRFD-AISC (Load and Resistance factor design of American Institute of Steel Construction) in which the effect of warping is also taken into account. Ant colony optimization technique is used to obtain the solution of the design problem. A number of space frame examples are designed by the algorithm developed in order to demonstrate the effect of warping in the optimum design. (C) 2011 Civil-Comp Ltd and Elsevier Ltd. All rights reserved.