Singular configurations of structural systems

A singular configuration of a structural system is characterized by rank de ficiency of the equilibrium matrix and kinematic matrix (the rank is lower than both the number of degrees of freedom and the number of constraints). Such configurations exist only in systems that are not geometrically invari ant (underconstrained structural systems). Most interesting among them are systems with infinitesimal mobility which attracted attention of many promi nent researchers. This paper puts the entire issue in a different perspecti ve by addressing a critical, yet so far unexplored, aspect of singular conf igurations-their realizability. It turns out that the only generic, physica lly realizable type of a singular configuration is a system with first-orde r infinitesimal mobility, and even this cannot be constructed without induc ing prestress of finite magnitude. AII other singular configurations (unpre stressable first-order mechanisms; higher-order mechanisms; and singular co nfigurations of finite mechanisms) are unrealizable. Moreover, short of exa ct or symbolic calculation, they are also noncomputable and are just formal analytical constructs. (C) 1998 Elsevier Science Ltd. All rights reserved.