Torsional-Flexural Buckling Effects in Portal Frames

The effects of continuity on torsional-flexural buckling have been studied. In 1953, an approximate treatment by Salvadori proposed calculating the critical load for each component under the correct loading and minimum restraint conditions, the lowest value then being a lower bound on the frame's buckling load. This note suggests that, where normal bracing is provided, sufficiently accurate torsional-flexural buckling loads may be obtained for a modification of this early approach. Following normal finite element procedures, each member in the frame is divided into a number of line-elements joined only at the nodes. The stiffness matrix for the structure is then assembled from the stiffness matrices of all the elements. A numerical study shows that the careful use of results for isolated members leads to acceptable estimates of the frame buckling loads.