A Convergence Technique for Determining the Elastic Critical Load of Rigidly Jointed Trusses

Introduction
Stifness Tests
As the load on a frame is increased the stiffnesses of the struts decrease and their carry-over factors increase. Thus a joint in the loaded frame rotates further, when a
testing moment is applied, than it would if the frame were unloaded. Moreover, bigger moments are carried over to adjacent joints. Eventually at the elastic critical load of the frame the stiffness of every joint is zero.

Arthur Bolton