Comments on the understanding of the Virtual Work Method

Abstract

Students studying elementary structural analysis usually experience some confusion in understanding the concept of virtual work. Most of them think that the work is virtual because it results from a virtual force system, even though the adjective virtual in the term "virtual work" describes the work and not the forces. In this paper several comments are given on the concept of virtuality in the virtual work method. The discussion emphasize that the work is virtual rather than the applied unit load or displacement. Excerpts from various text books related to the virtual work method for the calculation of deflections using the unit load method, the derivation of Maxwell's Law of Reciprocal Deflections, and Betti's Law are all discussed. Derivations of these methods are carried out using two different approaches in which no virtual forces are introduced and emphasis is made on the fact that the work is virtual. The term "assumed loading case" is proposed to replace “virtual force system”. This paper aims to clarify the concept of virtuality to students learning the method, mainly to prevent the confusion which most students have.