A method for analyzing the elastic flexural-torsional buckling of rigid frames with I-section members is developed. This is used to study the behavior of a symmetrical rigid-jointed fixed base portal with a central beam load and two equal column loads. The continuity conditions at the rigid beam to column joints are simplified by assuming that end warping of the I-sections is elastically restrained. The interaction between column buckling (when only the columns are loaded) and flexural-torsional buckling under the beam load only is studied, and the critical load combinations are presented as load interaction diagrams. The results of experimental investigations of two small scale portal frames with zero warping restraints at the beam to column joints are in close agreement with the theoretical predictions.