Loss of Torsional Stiffness Caused by Beam Loading

A beam under load can be more easily twisted or deflected laterally than when not under load. This loss of apparent stiffness is an example of the general principle that stiffness associated with a buckling displacement diminishes to zero as the buckling load is approached. This phenomenon was first reviewed and derived for beam‐columns and then derived for beam‐torsion members. It was found that stiffness decays linearly for beam‐columns and parabolically for beam‐torsion members. The results are important to designers of large flat roofs in which one beam must provide axial rotation restraint for the next beam in‐line. Simple design equations are recommended.