Simple Illustration of Frame Instability

TLDR: In this paper, a three-hinged portal frame is shown to be grossly unconservative and several approximate and exact analyses are presented, which show that the essence of correct stability analysis is proper accounting for load distribution within the frame under deformation.

Abstract: The problem of frame instability is not confined to tall and indeterminate frameworks. The problem is elucidated through an example of a three-hinged portal frame. The commonly accepted analysis of this frame is shown to be grossly unconservative. Several approximate and exact analyses are presented. They show that the essence of correct stability analysis is the proper accounting for load distribution within the frame under deformation. A simple test of this frame is described. Results confirm the analysis within the accuracy to be expected. It is concluded that there is a need for improved design aids to find the effective lengths of compression members in frames.