Inelastic Buckling of Steel Frames

A building frame that is not braced to prevent lateral displacement may buckle into a sidesway configuration when the critical condition is reached. For frames of practical proportions, this phenomenon usually occurs after the applied load has caused yielding in some part of the members. Thus, the maximum load-carrying capacity of such structures is determined by the inelastic buckling strength. A numerical procedure for determining the buckling load of partially yielded frames is developed and its use is illustrated with reference to a simple portal frame. The study represents the first attempt to determine theoretically the inelastic buckling strength of unbraced frames. Tests on steel models fabricated from a small wide-flange shape were conducted and the results show satisfactory agreement with the theoretical solutions. Based on the results of the theoretical and experimental studies, the validity of a currently (1965) used column design rule is examined and a new method is proposed.