Inelastic Buckling of Unbraced Space Frames

Inelastic lateral buckling of an unbraced orthogonal space frame is studied analytically. The frame consists of four simply-supported columns and four beams, with two beams framing into each column top. All members are of hollow, rectangular cross section of elastic-plastic material. Uniform load is applied to each beam along with concentrated loads at the column tops to simulate multistory frames. Both the frame and loading have two planes of symmetry. The collapse load, assuming the frame to remain stable, and the lateral buckling load are determined. Single-story frames carried loads of at least 90% of the collapse load before buckling occurred. Many simulated three-story frames buckled laterally at loads considerably less than the collapse load. Comparison of results with the AISC requirements for columns in unbraced plane frames indicates that the requirements are conservative when the columns are subjected to biaxial moments. Equations for estimating the lateral buckling load for the frames studied are given.