On the Strength Weakening Effect of Stiffening Ribs in the Design of Machine Components

Abstract

The bending stress in beams may often be reduced by adding material to the cross section. In some paradoxical cases, however, the bending stress increases by adding material from zones far away, or close to, the neutral axis. Similarly, the bending stress of rectilinear or curved beams may often be reduced by adding ribs to the initial beam section. However, such ribs may sometimes cause a both undesired and unexpected stress increase, although they still produce a beneficial stiffening effect. The aim of this paper is twofold: a) to examine this unexpected result within the context of the paradoxical behaviour of some known beam sections, and especially of a recently noted paradox; b) to provide a preliminary rule of thumb for the mechanical design of ribs sometimes added to the outer surface of an eye, with particular regard to the small end of a connecting rod.